Plantations,Farm Forestry

and Water

A Discussion Paper

Emmett O’Loughlinand

E.K. Sadanandan Nambiar

A report for theRIRDC/LWA/FWPRDC

Joint Venture Agroforestry Program

Water and Salinity Issues inAgroforestry No. 8

RIRDC Publication No. 01/137

ii

© 2001 Rural Industries Research and Development Corporation.All rights reserved.

ISBN 0 642 58357 9ISSN 1440-6845

Plantations, Farm Forestry and WaterPublication No. 01/137

The views expressed and the conclusions reached in this publication are those of the author and notnecessarily those of persons consulted. RIRDC shall not be responsible in any way whatsoever to any personwho relies in whole or in part on the contents of this report.

This publication is copyright. However, RIRDC encourages wide dissemination of its research, providing theCorporation is clearly acknowledged. For any other enquiries concerning reproduction, contact thePublications Manager on phone 02 6272 3186.

Researcher Contact DetailsDr E.K.S. NambiarCSIRO Forestry and Forest ProductsPO Box E4008KINGSTON ACT 2604

Phone: 02 6281 8311Fax: 02 6281 8312Email: Sadanandan.Nambiar@ffp.csiro.auWebsite: http://www.ffp.csiro.au/

RIRDC Contact DetailsRural Industries Research and Development CorporationLevel 1, AMA House42 Macquarie StreetBARTON ACT 2600PO Box 4776 KINGSTON ACT 2604

Phone: 02 6272 4539Fax: 02 6272 5877Email: rirdc@rirdc.gov.auWebsite: http://www.rirdc.gov.au

Published in November 2001Printed by Union Offset Printing, Canberra

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A mosaic of native forest, pine plantations and harvested areas in a landscape in ACT Forests.

Plantations and farm forestry in lower-rainfall areas are using an expanded range of species. Theironbarks in this plantation have the potential to produce high-value wood. Research and Developmentprograms will further improve the utility of a number of ‘non-traditional’ plantation species.

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ForewordSustainability entails balancing and accommodatingmultiple and sometimes conflicting values. Thefurther development of plantations and farmforestry in Australia will continue to face thechallenges of achieving economic and ecologicalbalance at farm, landscape and regional levels. Oneexample, already with us, is to effectively managethe interactions between tree plantations andhydrogeological processes.

Plantations for Australia – The 2020 Vision hasgiven a much-needed impetus to commercialforestry development. The continuing expansionhas already triggered significant changes in landuse in some regions. It is also widely recognizedthat well-planned and strategically locatedreforestation offers a major opportunity to redressthe hydrogeological imbalance that is the root causeof dryland salinity on farms. There is, however, aconcern that expansion of forests will diminish theamount of water available to other land uses such asirrigated agriculture. The perceived competition forwater between forest plantations and other uses ispotentially detrimental to the orderly progress ofand judicious investments in resource developmentfor multiple benefits.

Recognizing the need for a participatory approachby and dialogue between various interest groups,CSIRO Forestry and Forest Products, with thesupport of Agriculture, Fisheries and ForestryAustralia (AFFA) and the Joint VentureAgroforestry Program (JVAP) organized a nationalworkshop, Plantations, Farm Forestry and Wateron 20 – 21 July 2000. The aims of the workshopwere to:

• Review the scientific principles underlyinginteractions between communities of trees andhydrogeological processes at the landscapelevel;

• Examine the potential positive and negativeimpact of tree crops in the context of land use;

• Explore current and future scenarios of selectedecosystems (regions) as case studies; and

• Prepare a discussion paper based on the aboveanalysis to provide balanced information for allconcerned with land management anddevelopment of land use policies.

The workshop content focussed on regions whereindustrial and farm activities are advancing rapidly:southern and south-western Australia, where theextent of development and available informationprovide a reasonable basis for case analysis.

The papers presented at the workshop werereviewed and published as a proceedings13. Asynthesis of the papers, the views which emergedfrom structured discussion, and an executivesummary, are presented in this discussion paper.

This report, a new addition to RIRDC’s diverserange of over 700 research publications, forms partof our Joint Venture Agroforestry R&D Program,which aims to integrate sustainable and productiveagroforestry within Australian farming systems.

Most of our publications are available for viewing,downloading or purchasing online through ourwebsite:

• downloads atwww.rirdc.gov.au/reports/Index/htm

• Purchases at www.rirdc.gov.au/eshop

Peter CoreManaging DirectorRural Industries Research and DevelopmentCorporation

Dr Glen KileChiefCSIRO Forestry and Forest Products

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AcknowledgementsWe thank Alan Brown (CSIRO) for valuable helpin reviewing and editing this paper. Other CSIROcolleagues Richard Benyon, Rob Vertessy andTrevor Booth provided comments and someinformation. Comments by Sharon Davis (JVAP),Richard Stanton (Plantations for Australia) and PhilPritchard (AFFA) helped to improve thepresentation.

Photographs kindly supplied by CSIRO staff, IanCraig, Michelle Michael, Brian Myers andSadanandan Nambiar.

Anne Griffin (CSIRO) helped with the preparationof this paper.

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ContentsForeword iii

Acknowledgements v

Executive Summary and Conclusions vii

The context viiPlantations, farm forests and water viiKnowledge gaps xThe way forward xi

1. Background 1

2. Water Balance in Landscapes 3

2.1 Principles and processes affecting water flows 32.2 Surface water flows 32.3 Interception loss 42.4 Plant uptake and transpiration 42.5 Groundwater 52.6 Salt and water 5

3. Planted Forests and Water issues 7

3.1 Regional water issues 73.2 The Murray Darling Basin: southern high-rainfall region 73.3 The Green Triangle 93.4 The Macquarie Valley 103.5 Western Australia: south-western region 11

4. Effects of Afforestation on Water 15

4.1 Interactions between water and trees 154.2 Leaf area 154.3 Leaf transpiration 154.4 Tree rooting patterns 164.5 Annual water yields 174.6 Changes in seasonal flows 184.7 Impacts on peak flow runoff 194.8 Effective catchment areas 204.9 Time response of landscapes to change 20

5. Effects of Afforestation on Salt 22

5.1 Classification of catchments for salinity control 225.2 Alternative farming strategies 23

References 25

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Executive Summary and Conclusions

The contextAustralia is committed to ecologically sustainabledevelopment. Sustainability is a balancing actinvolving accommodation of and tradeoffs betweenmultiple and sometimes conflicting values. Inrecent years there has been a surge of investmentand activity in plantations, farm forestry, andgeneral revegetation in Australia. Incentives forthese have come from the expectation of botheconomic and environmental benefits, especially torural and regional Australia. The sharedcommitment of the community, industry and theState and Commonwealth Governments to thePlantations for Australia – The 2020 Vision10 hasgiven a much-needed impetus to commercialforestry development. This vision, if fulfilled,would more than double the present land base underplantation forestry. Revegetation with deep-rootedperennials on a large scale is being proposed by avariety of land management agencies as oneimportant option for saving the land and water fromincreasing salinisation. The purposes of theseprograms include wood production, mitigation ofland degradation, enhancing bio-diversity, carbonsequestration and bio-energy production. Theextent, structure, composition and life cycle ofplanted forests will therefore vary widely. The levelof these current and proposed activities isinfluenced by many factors, and varies substantiallybetween regions. Cumulatively, they have thepotential to bring about a large change inagricultural land use.

The development of plantations and farm forestryfaces the challenge of achieving economic andecological balance at farm, landscape and regionallevels. One major challenge, already with us, is themanagement of the interactions between treeplantations and hydrogeological processes toachieve sustainability. Well-planned andstrategically located reforestation can redress thehydrogeological imbalance that causes drylandsalinity on farms. Concerns have arisen, however,that in some situations expansion of plantationforestry on a large scale could diminishhydrogeological flows and threaten wateravailability or water quality for other uses. Theterm ‘farm forestry’ can refer to a continuum ofreforestation from small-scale alley farming andscattered woodlots on farms through to industrialscale plantations on farmland. Largescalereforestation can reduce water yields of catchments(surface and groundwater), and increase riversalinity by decreasing the dilution. In some areas of

Australia, industries and towns which depend onlimited resources of fresh water have limits or capsplaced on their use of the water. Competition forwater resources between plantation forestry andother uses can lead to disruption of industries,community conflict and is also detrimental to thethe process of resource development for themultiple benefits. Balanced land and water usepolicies, and effective discussion andcommunication between stakeholders and landmanagement agencies, and good quality biophysicaldata and predictions are necessary for attractinginvestment for a range of industries includingcommercial forestry and revegetation programsnecessary for combating land and waterdegradation. This is particularly so becauseinvestment in plantation and farm forestry foreconomic and/or environmental outcomes isinvariably long-term in nature. Debates on forestryand water often become adversarial, as though thereis a simple universal solution of the complex issuesthat deserve consideration. This is not the case –interactions are diverse, ecosystem-specific andrelated to goals of land management and land use.

The ‘Plantations, farm forestry and water’workshop held in July 2000 addressed the urgentneed for a balanced analysis of this sensitive issue.Diverse views and expectations were presented inthe context of the best scientific informationavailable. Issue reviewed included: biophysicalissues related to planted forests and water, thenature of impacts at the catchment scale, regionaland ecosystem-specific factors, commercial andenvironmental considerations, trees and their rolefor managing land and water degradation, and thetradeoffs needed to achieve balanced policies.

Plantations, farm forests andwaterTrees planted for commercial or environmentalpurposes may use more water than the crops orpastures they replace at the same site. The plannedexpansion of Australia’s commercial forestplantations by 2020 can therefore cause a reductionin surface water flows and recharge to groundwaterin some areas depending on the scale of planting.The tradeoffs between the multiple benefits accruedby forestry enterprises, and some reduction in totalwater available is the key issue.

The impact of forests on water will not be uniformacross all the areas planted: it will be stronglyecosystem-specific, being influenced by the natureof the water flows, landscape features, area and

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density of plantings, and management. Impacts onwater will be greatest in high-rainfall areas. Manyrecently-established plantations are in areas ofrelatively moderate rainfall (600-850 mm) wheretheir effects on water flows will be considerablyless than those of plantations which have beenestablished in high-rainfall areas. In such areas, thereduction in usable water resources (streamflow andrecharge) would typically be about 100 mm peryear (one megalitre per hectare of planted area)reducing to zero where the rain is less than 500 mmper year. Plantings in higher-rainfall zones (e.g. 850- 1500 mm) will result in greater reductions instreamflow. If extensive afforestation occurs inthese zones, it will reduce water yield from thesecatchments, which currently have high waterproductivity. The water allocated for downstreamusers will therefore be less secure.

New tree plantings will also be beneficial forlimiting the expansion of salinised land andimproving water quality in some areas. Often, theissues of water availability and land salinity occursimultaneously. The critical factor here is get theright balance - between rainfall, location and size ofthe plantings in the catchment context, local vs.regional impacts – and to move towards a holisticand balanced approach to land use.

Proportion of area plantedAbout 90% of the Pinus radiata (the species whichcomprises the highest proportion of the currentplantation estate in Australia) plantationsestablished in Australia in the past are on areas thatreceive less than 1000 mm of rainfall annually, andlarge areas exist within the 650-850 mm zone. Theeffects of these plantations on water flows will beconsiderably less than those of plantationsestablished in higher-rainfall areas.

The new plantation area of two million hectaresproposed in the 2020 Vision is also small in relationto the nation’s total medium rainfall catchmentarea. For example, in the Greater Green Triangle*

area where the plantation estate established since1860 is still expanding, the total area planted by2020 is likely to be about 10% of the total areapreviously cleared for agriculture.

Expansion of forest plantations will have effects onwater resources roughly in proportion to the level oflandscape afforestation that occurs. This will befurther influenced by the way the plantations aremanaged: for example their rotation length,thinning regime and inter-rotation fallow period,and site quality which determines growth rate.

* South-eastern South Australia and south-westernVictoria

The enhanced water use of a stand of trees canaccount for significantly more water abstractionfrom the soil and groundwater than annual cropsand pastures. In a catchment, the impact on waterresources is moderated by the area planted, its’location, the species used, the climate and the waythe plantations are managed.

Some arguments about the negativehydrogeological effects of forestry are coloured bythe apprehension of ‘wall to wall forests’ replacingtraditional agriculture, and those plantationsconstantly having full canopies. This concern is an-over simplification. In most areas new forests areestablished as discrete and fragmented treecommunities across the terrain. Even whereplantations occur contiguously, they exist at anygiven time as a mosaic of age classes, site qualitiesand stand densities, giving rise to a range ofimpacts on water fluxes.

When establishing new plantations in areas wherewater availability is a critical issue, the planning ofland use should take into account overall ecosystemprocesses and economic imperatives.

Effects on flow for downstream waterusersPlantations will have the maximum effect onsurface water flows if they are established in high-rainfall areas, especially if these areas are sourcesof water for downstream water users. If plantationexpansion is mainly centred around moderaterainfall zones (600-850 mm) as it has beenhistorically, its effect on water resources fordownstream users will be far less than that ofplantings in higher-rainfall zones. Expansion ofplantations into lower-rainfall areas will probablyhave negligible effect on downstream waterresources. At the local scale, new plantations oncleared land will alter dry-weather flows inunregulated rivers, causing them to revert to theflow patterns they had before the land was cleared.

The introduction of tree species which are not localto an area may reduce dry-weather flows to levelsbelow those experienced historically. This can beexpected to occur if the planting covers asignificant proportion of the catchment area and thenew plantations are managed to achievesignificantly higher leaf area than that of theoriginal native vegetation. Widely-spacedagroforestry plantings and appropriate plantingdesigns are important management tools availablein this context.

Where plantations are established in parts of acatchment that are hydrogeologically isolated fromstreams most of the time, the impacts onstreamflow will be negligible or very little.

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Effects of speciesIndustrial-scale plantings of hardwoods andsoftwoods have approximately the same overalleffects on surface and groundwater resources incomparison with cleared land. Different treespecies, however, show patterns of water use,growth and survival that vary significantly withclimate and watertable depth. Their capacity togrow and maintain leaf area, ability to grow rootsthrough harsh soil conditions, salinity tolerance andother physiological attributes can give rise todifferent outcomes in relation to site water balance.Matching tree species with sites is necessary toensure the viability of tree planting programs,especially where trees are planted in new and oftenharsh environments.

Differences may also occur if tree crops aremanaged for different end uses. For example, Pinusradiata plantations are managed for pulp andsawlog production over a cycle of 25-35 years with3-4 thinnings, whereas most of the currentE. globulus plantations are grown for chipwoodproduction and hence on a rotation of 10-15 years.

Effect on floodsTree plantations of all types reduce peak runoffrates during flood-producing storms. Comparedwith agriculture they moderate peak flows. Thesmaller peak flows lessen flood damage, landscapeerosion and river siltation by trapping sediment.Better river water quality is maintained by lowerturbidity and sediment loads. These are clearbeneficial effects to both landowners anddownstream water users. River flows duringsmaller storms are also diminished, resulting inreduced flushing – possibly to the detriment ofaquatic habitat.

Changes through timeThe impact of afforestation on streamflow will varythroughout the life of managed plantations as leafareas change. When plantations are thinned orharvested the impacts will diminish, such that thelong-term average effects on water resources willbe less significant. The nature of the tree harvestingcycles in catchments and the location of the trees inrelation to terrain features also alters thehydrogeological response time. The time taken forstreamflow to re-adjust after a land-use change ismuch shorter than the time needed for land orstream salinity to change.

Effect on watertables and ground waterWatertables and underground water resources willbe affected by the changed recharge patterns underplantations, through enhanced interception ofrainfall and increased transpiration. Planted tree

species that draw water directly from the watertablecan achieve higher growth rates using this extrawater, but can depress the level of the groundwater.However, the ability of trees to use ground water inany significant amount seem strongly dependent onthe depth to watertable. The ensuing effect is likelyto be local, not extending beyond the plantationarea. In many situations this is recognised as abeneficial approach for maintaining theproductivity of limited areas of salinised landsuffering from degradation. Where the potentialreduction of deep and high quality groundwater(e.g. in the Green Triangle Region) is of concern,the important considerations should include thenature of the land in which trees are planted, itsoverall hydrogeological features and the extractionof the ground water by other users.

Trees for salinity controlPatterns of land and stream salinity (especially inWestern Australia) are still changing as a result ofclearing native forest and woodlands during thepast century. Positive effects of afforestation can beidentified in some landscapes suffering fromdryland salinisation. Where the causes of drylandsalinity (e.g. in the upper Murray-Darling Basincatchments) can be linked to local groundwaterbehaviour, strategically located woodlots andplantations can check or slow its spread, andeventually rehabilitate damaged areas. The salinityof river flows can also be reduced. These benefitscould be manifested in a few years in catchmentswith local groundwater systems. But the efficacy ofplantations depends on the use of improvedmethods based on hydrologic knowledge andanalysis, a better understanding of how differenttree species can be grown and managed in their newenvironments, and the extent and strategicplacement of tree plantings in the terrain.

Rehabilitation of landscapes where groundwater isdominated by regional scale processes isproblematic (i.e. Western Australia). These arelikely to require more extensive afforestation atscales that result in major switches in land use fromagriculture to forestry. If such changes in land useoccur, the time taken for benefits to be apparent islikely to be several decades.

At both regional and local scales, the sustainabilityof plantations in salt-affected areas will be an issue.This bears directly on the viability of forest-basedindustries established there and an understanding ofthe tradeoffs between economic and environmentalbenefits.

Growth and management of standsAn over-riding issue is the commercial viability ofthe new plantations themselves. First, this requires

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development of new infrastructure and markets forthe tree products. Second, the tree species used inplantations must survive and grow at rates thatjustify the investment. Even when trees are plantedprimarily for environmental benefits, where thetrees drive the change in ecosystem processes andtowards better land use practices, it is essential thatthe trees are managed well and their potentialproductivity in a given environment is realised.Otherwise, the expected role of trees in drivingcarbon, water and mineral flows, and enhancingbiodiversity, will not be fulfilled. In stark contrastto the ongoing attention given to other farmingactivities, many environmental plantings of trees onfarms receive scant attention to managementbeyond planting, leading to lost opportunities forobtaining a product from the tree crop. Thissituation, important for both economic andenvironmental outcomes, is slowly but steadilychanging.

Knowledge gapsThere is a body of knowledge available now withwhich we can improve land management practicesby further development of plantations and farmforestry. The level of this information variesbetween catchments and regions, as does the extentto which available information is being used in landplanning process.

As we progress through the implementation of theVision 2020 it is also clear that, despite significantachievement, the knowledge needed to maximisebenefits from the Vision plan is not adequateconsidering the environmental and economicmosaic within which plantation forestry is beingpromoted. New information is needed on thebiophysical factors that affect tree growth, theirwater use, and the diverse and ecosystem-specifichydrogeological effects when trees are establishedon formerly cleared land. Current debate on forestsand water is based on a narrow classification of sitewater availability in terms of ‘low, medium or high’rainfall. This takes no account of other criticalfactors including evaporation, landscape featuresand terrain, properties of the regolith and other landcapability features. Rainfall alone is an inadequatepredictor of ecosystem processes and function,given the complexities discussed in this paper. Anincreasing number of field observations show thatthe simplistic and common assumptions about treegrowth in the so-called ‘low-rainfall zone’, basedon meagre and often unreliable information onrainfall, require significant revision.

Continuing efforts are required to close thesignificant knowledge gaps which exist so that theeffects of large-scale new tree plantings on waterresources can be more fully assessed and current

controversies and misconceptions addressed. Thesegaps, not in order of priority, include:

• The relative water use of various tree species indifferent climates, their productivity and theirability to draw upon groundwater located atvarious depths.

• The temporal patterns of water use (andgroundwater levels) by managed treeplantations throughout rotations.

• Tools for managing productivity and droughtrisks in new forests (and species) in areas proneto drought.

• The survival rate of various tree species in low-rainfall and/or saline soil water environmentsand their ability to grow roots through salinisedsoil profiles.

• The changes in dry-weather flows fromafforested versus cleared catchments.

• The hydrogeological attributes of catchments,and the ‘effective’ catchment area. This shouldlead to a system of classifying landscapes interms of their interactions with water resources,ecosystem function, susceptibility todegradation and potential for rehabilitation.

• Effects of isolated, small units of farm forestsdistributed throughout diverse terrain in alandscape on the overall water balance of thecatchment, compared to the effects ofcontiguous plantation forestry in the landscape.

• Methods to locate salt in landscapes, and assessits mobility under various land uses.

• Decision support systems for locating,designing and managing trees integrated withoverall land use.

• Further verification of the foresthydrogeological models and improved capacityto account for the dynamics of forestmanagement in space and time.

• Improved models for aggregating local effectsto larger scales at the catchment and regionallevel.

• Tools to evaluate tradeoffs.• Policy and legal frameworks to fairly and

reliably allocate rain, surface and groundwaterto a range of users and to the environment.

There is a need for researchers to fully describe theclimatic, topographic, soil, geologic andgroundwater attributes of their experimental sites ina consistent manner, and to contribute thesedescriptions to the data bank needed for classifyingcatchments. Second, experimental sites need to bemonitored for a duration long enough to detect theeffects of the imposed land use change. Gatheringand interpreting this information will take 10 ormore years. This is a short time compared with thetimescale of the commercial tree plantationindustry, and the planning horizons for changingland use in Australia.

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The way forwardThere are clearly no simple and universal criteriafor debating and analysing the potential impacts ofplantation forestry, farm forestry, and otherrevegetation programs on water flows and wateraccess in our vast country. The overwhelming pointthat has emerged is the need to recognise thediversity of ecosystem-specific processes andimpacts that are relevant to local and regionaleconomic, environmental and social contexts. Itfollows that it is imperative that the forestry andwater debates be guided by balanced analysisincluding science, economics, policy, politics andcommunity needs. In all regions where major landand water use changes are envisaged, the tradeoffsof water availability and quality for local anddownstream users need to be assessed. Similaranalysis of tradeoffs is essential where impacts ofland use on groundwater and its use are criticalissues.

Plantation establishment is just one land and water-use change that is currently occurring in Australia.Others include increased use of irrigation, new cashcrops which replace grazing, and several otheractivities, which use water at an increasing rate. Insuch a complex situation an examination of theimpacts of forestry on land and water in isolation isunlikely to provide sound and lasting policies andsolutions. The analysis of tradeoffs needs to beinclusive of economic and environmental values ofgreatest importance to a region. The methods forassessing the impacts are available, but knowledgeof the key factors needed to apply the methods tospecific sites is meagre. They clearly vary betweenlocations and regions.

Alley farming with eucalypts and silky oaks mixed with grazing in Billabong Creek Catchment, NewSouth Wales.

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1. BackgroundAustralia’s rainfall and streamflows are highlyerratic. Compared with other continents,streamflows are also small. Clearing of forest andwoodland for agriculture during the last century hasgreatly increased the volume of water movingthrough catchments, and much of this water hasbeen allocated for irrigation. In many regions,commitments to existing water users consume alarge part of the available water, leaving little forrequirements of new enterprises. One majorenterprise, already expanding rapidly in someregions, is tree planting on cleared land.Reforestation of cleared land is perceived to reducestreamflows and therefore intensify the competitionfor water. On the other hand, strategically locatedtree plantations can help reverse land and waterquality degradation, especially where drylandsalinity is a severe threat to farmlands. We need tounderstand and recognize the impact that expandedareas of tree plantations will have on water and landresources.

Recent initiatives aimed at natural resourcemanagement in Australia will affect ruralproductivity and the land and water resources thecommunity depends upon. These initiatives include:

• Plantations for Australia: the 2020 Vision,which promotes the case for a major expansionof commercially productive forestplantations10;

• The policy development document Our VitalResources – National Action Plan for Salinityand Water Quality that outlines the actionneeded to ensure that our water and landmanagement practices will sustain productiveand profitable uses, as well as our naturalenvironment1;

• Basin Salinity Management Strategy – 2001-2015 in which the Murray Darling BasinCommission outlines its plan to attract capitalinvestments into forestry and vegetationmanagement schemes12; and

• National Investment in Rural Landscape 2000,an investment strategy proposed by NationalFarmers Federation and AustralianConservation Foundation. This strategy callsfor a large-scale investment program includingreforestation of farming land for economic andenvironmental recovery of rural Australia9.

In addition several State Governments have landdegradation mitigation strategies that haverevegetation and reforestation as a key componentand seek to promote investment and co-investment

in relevant programs (e.g. WA Salinity Strategy;Vegetation Banks).

All these initiatives call for significant revegetationand/or reforestation activity for commercial andenvironmental outcomes in large areas of thelandscape that had previously been underagriculture. In most cases, the large-scalerevegetation will advance only if commerciallyfavourable returns are likely – to either small orlarge private investors.

The Plantation 2020 Vision initiative that emergedin July 1997 sets out a framework to treble thenation’s (then) forest plantation area by the year2020. To achieve this goal, it will be necessary toestablish new plantings of 80 000 ha per year (afourfold increase in the new planting rate in theearly 1990s), adding 2.2 million ha to the 1996plantation area of 1.1 million ha. After the plan wasannounced the planting area increased from about55 000 ha in 1997 to 65 000 ha in 1998 and 95 000ha in 1999 (National Plantation Inventory 2000).

The National Action Plan for Salinity and WaterQuality initiative was announced by the PrimeMinister in October 2000. AFFA estimates that theannual costs for land and water degradation are upto $3.5 billion, and given current trends, within 20years Adelaide’s drinking water will fail WorldHealth Organisation standards in two days out offive. The Murray-Darling Basin Commission hasproduced draft strategies for Integrated CatchmentManagement and Basin Salinity that explicitlyrecognise the role of tree planting to arrest land andwater degradation. As well, the National FarmersFederation and the Australian ConservationFoundation espouse ‘a much larger role for trees inrural landscapes in the form of ... forest and forestindustries, with commercial plantations and agro-forestry, ranging to revegetation with indigenousvegetation’.

All initiatives have common ground in that theyaddress significant environmental, economic andsocial issues that threaten rural areas. They alsorecognize substantial non-market benefits includingrural employment, improved farm profitability, landand water salinity abatement and carbonsequestration. These benefits may accrue over along period.

Cumulatively, the potential impact of theseinitiatives and policies on land use in Australiawould be very large indeed. A major unknown inthis is the potential effect that the projectedplantings will have on water resources, includingsurface runoff, streamflow and groundwater. It is

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Even before the introduction of agriculture,humans have changed the balance offorest and grassland over many millennia,mostly through the widespread use of fire.The occurrence of fire from natural causessuch as lightning also constantly disturbedthe state of the natural vegetation. It ismisleading, therefore, to surmise that thelandscape and its water resources were instatic equilibrium before Europeansettlement occurred in Australia. However,mechanization of agriculture vastlyaccelerated the removal of native forestand woodland, and replacement bypasture and annual crops, in the past 80years. This, together with the rabbit pestand more frequent fires, has sharplyincreased the rate of change of landcover. The consequences are the majorcause of accelerated land and waterresource degradation facing us now. Wemust not assume that the landscape andits water resources have now achieved anew state of static equilibrium.

already clear that the perceived competition forwater between plantations, farm forestry and otherusers would be detrimental to the orderly progressand investment in resource development envisagedin various initiatives. It is well established that thepatterns and amounts of water use differ markedlybetween different vegetation types, for a number ofphysical and biological reasons. For example,annual crops have different seasonal waterrequirements from perennials, and deeply rootingtrees can access a greater depth of soil water thanshallow-rooted pastures. While there are estimatesavailable that indicate the magnitude of theseeffects, they need to be more thoroughly assessedand verified to predict the extent of decline inavailable water resources which may occur as aresult of enhanced tree planting in different areas atvarious times.

The principal water issues related to the expansionof tree planting on cleared agricultural land are:

• What will be the impact on the currentlyavailable water resources in rivers? Howmight these resources change regardless ofwhether or not trees are planted?

• How will dry-weather flows in unregulatedstreams be affected?

• What effects will tree plantations have onwatertables and underground waterresources?

• How can plantations be used to arrestrising stream salinity and the spread ofsalinised areas in farmlands?

• How long before benefits from treeplanting are seen?

• How do we plan the best use of treeplantations at farm to regional scale?

• What are the key issues for considerationas tradeoffs for sustainable resourcedevelopments?

This discussion paper addresses these issues. It isbased on reviews and discussions held at theworkshop in July 200013. The approach taken isbased on our understanding of how trees, land andwater interact in landscapes and how, in someenvironments, this interaction affects salinity. Insome cases, tree plantings are without doubt

beneficial, where land salinisation can be arrestedor at least delayed. In other cases, there is potentialfor diminished streamflows in dry weather, orlowered groundwater levels. It is clear thatplantations will have impacts that differ acrossdifferent ecosystems and climatic regions, and thata tradeoff in available water resources may occur ifthe effects of the plantings are not properlyconsidered. We present, in summary form, theinformation already known that is relevant to theplantations and water issue. It applies thatknowledge to a few key regions in Australia wherecritical land or water resource issues exist, andidentifies areas where more information or analysesare required.

Many of the current debates on water, plantationsand farm forestry assume that forests impact onwater resources in a universal way. We hope thispaper will allow such assumptions and opinions tobe placed in proper perspective.

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Stormflow and baseflow

Stormflow ( ) is the runoff that appearsduring and shortly after rain. Baseflow (- -)maintains flow in the stream when rain isnot falling; it first passes through the soiland emerges as groundwater flow throughthe streambed and banks. Minorstormflows and baseflow are important formaintaining the ecological health ofstreams.

2. Water Balance in LandscapesThe impact of historical land use changes cannot bequantified for certain because we have no directmeasurements of streamflow behaviour or salinitybefore land clearing and agriculture becamewidespread. The study of water balance is acomparatively recent science, so the onlyinformation we have about the early hydrologicalenvironment comes from explorers’ notebooks andnarratives passed down from the indigenouspopulation and the new settlers. But given theextent of known clearing that has taken place (e.g.some 15 billion trees in the Murray Darling Basinalone), we can surmise that even in unregulatedrivers, annual flows and their seasonal distributionsare quite different now compared with 150 yearsago. Total flow volumes are now greater than theywere before European settlement, and streams thatdried up in summer are now likely to maintainsome flow for longer periods.

The trend to re-establish trees on cleared farmlandwill act to return streamflows back towards theirpre-clearing state. This will be a local effectconfined to the areas that are revegetated. Thus,annual water yields may decline, and dry-weatherflows may disappear. On the other hand, saltdischarge into streams and expanding drylandsalinity may be abated. The magnitude andeconomic consequences of these impacts can beevaluated only through assessments of thehydrologic changes which occur after plantationsare established, and where they are established.

In the past several decades, much of the scientificcomprehension essential for these hydrologicevaluations has been developed. It has beennecessary to gather basic experimental data for theappropriate tree species in the varying soil andclimate conditions across the continent. The finalsteps now involve assembling this information intoa framework that can be used to predict thehydrologic impact under various scenarios. Asmore data become available, the accuracy andreliability of these predictions can be improved.However, we are already faced with choosingbetween options for land use, and the decisions thathave to be made have some urgency. Application ofexisting knowledge is therefore the key, eventhough significant gaps remain in it.

2.1 Principles and processesaffecting water flowsIn this section we outline how water, topography,soil and vegetation interact in the landscape. Itemphasises the factors and processes that changewhen tree plantations are established in land underpastures or annual crops.

2.2 Surface water flowsStreamflow usually has two components: stormflowand baseflow (see box). Major stormflow runoffoccurs only when the rainfall intensity is greaterthan the soil’s ability to receive it as infiltration, orwhen the soils are saturated. This type of runoff iscommon in mountainous or monsoon-proneregions. Minor stormflows usually occur whenlower-intensity rain runs off from parts of thecatchment that are more or less permanently wet,generating the many ‘freshes’ that occur in streamswhenever rain exceeds a few tens of millimetres.

Flood flows resulting from storms can be collectedin reservoirs and released later as regulatedstreamflow. But unless the storages are very large,

4

Left-over rainfallBoth streamflow and groundwaterrecharge are residual quantities, that is,the leftover rainfall that is not evaporatedfrom the soil or the plants. So any changein plant evaporation has a highlyleveraged effect on incremental waterresources. For example, in a regionwhere annual rainfall and evaporationvalues are 1000 and 950 mmrespectively, a decrease in evaporation ofonly 50 mm brought about by forestclearing could well double streamflowfrom the region. Conversely, a completeand permanent conversion of cleared landto forest can reduce streamflow to zero. Ifthe planted forest is managed as amosaic of different age classes withperiodic thinning and harvesting,streamflow will fluctuate between theseextremes, depending on the temporalextent of forest cover.

Australian native vegetationThe Australian native vegetationcommunities in most climatic environments(rainfall less than 1000 mm) have evolvedsuch that they exploit essentially all of theavailable soil water, leaving little for residualstreamflow. Thus, most of the river flow inthe Murray Darling Basin comes from smallparts of its catchment where the amount ofrain (or snow) far exceeds the vegetation’sability to use it.

most floodwaters flow into the sea, and contributelittle to the useable water resource. The impact ofplantations on such flows is quite beneficial,through the moderating effect they have in reducingflood peaks, replenishing soil water and preventingerosion. Forests generally yield better quality waterthan cultivated land, and reduce the rate of siltationin reservoirs. Plantations and farm forests havenegligible effects on the quantity of water that canbe captured by reservoirs during these high flows.

Apart from these intermittent high flows,streamflow is determined primarily by how muchwater is stored in the soils of a catchment. Drainagefrom the soil supplies water to the watertable(recharge), which flows downslope towards thestream, appearing as baseflow. The wetter and morepermeable the soil, the higher the recharge. Largerrecharge rates result in stronger flows from springsand larger seepage areas in lower hillslopepositions. These maintain flows during dry weather,and provide the wet soil areas where surface runoffis generated during small or moderate storms. Anyfactor that diminishes soil water therefore has adirect effect on runoff volume during storms, andbaseflow between storms. Vegetation acts to reducesoil water in two ways: by interception and by planttranspiration.

2.3 Interception lossRainfall interception loss on bare soil is zero. Inforests, interception loss is the amount of rain thatwets the canopy and evaporates from there. Thisreduces the amount of rain that reaches the soil.Tree canopies can retain several millimeters of rain,

and the total evaporation during prolonged rain canexceed this amount many times over. On an annualbasis, the interception loss from pine plantationscan be 20%-30% of rainfall, while for eucalypts theloss is 10%-20%. These losses are determinedmainly by the size of the leaf area. In plantationsleaf area is higher and more persistent than inpastures or annual crops, and evaporation ofintercepted water from tree canopies is more rapidthan from low grasses because of greater energyexchange with the atmosphere. Grasses and lowannual crops therefore intercept only a fraction ofthe water intercepted by tree canopies.

2.4 Plant uptake andtranspirationThe loss of water vapour through microscopicopenings on leaf surfaces (stomata) as part of the

process of photosynthesis is called transpiration. Ifsupply of soil water is plentiful and as long as thevegetation is not dormant, transpiration proceeds ata rate that depends on the energy available fromsolar radiation and heat from the atmosphere. Whenwater in the soil is partly depleted, physiologicalresponse by the plant constrains the rate oftranspiration, eventually reducing it to near zerowhen the roots have no access to water. Becausestomata do not close completely, plant desiccationand death can occur when water stored in the planttissues is exhausted. The way plants adapt to theselimiting processes differs between vegetation types(e.g. pastures vs. trees) and between different treespecies (e.g. pines vs. eucalypts), and is a criticalfactor in how plants use soil water.

Tree roots explore a greater depth of soil than dothose of annual crops or pastures, and have agreater ability to extract moisture from soil at lowsoil water content. Evergreen trees transpirethroughout the year. Thus, trees can transpire agreater total volume of soil water than shallow-rooted plants.

Interception and transpiration reduce soil watercontent and recharge to watertables. The largest

5

Recharge varies across landscapes

Near ridge-tops with thin soils andfractured bedrock, percolating waterquickly passes through the root zone. Inmid slope positions, more water isabsorbed by the deeper soils there,leaving less to drain to the watertable. Infootslopes, groundwater flow tends toemerge in springs and seepages, sorecharge there is zero.

reduction occurs with tall, leafy evergreen plantswith deep roots. Vegetation change can have ahighly leveraged effect on streamflows: the effectcan be positive or negative, depending on the natureof land use change and the environment where itoccurs.

The influence of vegetation on water resources isgreater than other effects, with the possibleexception of climate change.

2.5 GroundwaterThe critical factor that influences groundwater isrecharge; that is, the percolating water that passesthrough the root zone. Groundwater resourcesstored below watertables are similar to surfacewaters in that they have inflows, outflows andstorages. But the similarity ends there. Inflow togroundwater (recharge) is almost impossible tomeasure reliably, flow rates through thegroundwater conducting system must be inferredindirectly, the properties of the conducting mediumare usually highly variable, and their boundaries arealmost always unknown. Separate groundwatersystems may also interact on a local or regionalscale. However, groundwater resources are affectedby land use changes.

Recharge to groundwater is not uniform acrosslandscapes (see box). The hillslope location whererecharge is overtaken by discharge is the ‘hingepoint’. Above this point, strategies for groundwatercontrol imply manipulation of recharge. Below thehinge point, extraction or interception ofgroundwater is the only means available formanipulating the groundwater level. Estimating therate of travel and time of residence of groundwaterbefore it passes from the upper recharge zone intothe lower discharge zone is therefore important forits management – both for accessing the water forconsumptive use, and for controlling its level.

The rate of travel of water through undergroundaquifers is slow, ranging from millimeters per dayin clays to metres per day in gravelly aquifers. Theflatter the topographic relief, the slower the rate oftravel. Where the travel rate is low, there isopportunity for roots to extract moisture from thewatertable. If this rate of moisture extractionmatches the accumulated groundwater flow fromupslope, then the hillslope below the vegetationdoes not receive any groundwater recharge flowingfrom the upslope area. Thus, in sloping terrain, it issometimes possible to totally intercept shallowgroundwater flow before it reaches a downslopeseepage area. Conversely, if groundwater issupplied to seepage areas through pathways that aredeeper than the root zone (e.g. via regionalaquifers), then roots are unable to affect thegroundwater flow by intercepting it. In this case,the suggested strategy for manipulating

groundwater discharge has been to reduce rechargeat its source, that is, by reducing recharge atlocations where the regional groundwater isreplenished – which may be a great distance fromthe discharge areas.

In relatively flat areas, the rate of groundwatertravel may be so slow that the effects of vegetationon groundwater resources are wholly local. This isbecause the vertical water fluxes associated withtree plantations overwhelm the lateral flow rates ofslowly moving groundwater.

2.6 Salt and waterSalts may have accumulated in soils from wind-blown sea sprays, from the weathering of rocks, orfrom ancient marine sedimentary deposits. They aretransported by groundwater and theirconcentrations in soil water rise where waterevaporates or is transpired. Thus, soils in the rootzone of vegetation tend to accumulate highconcentrations of salts in a characteristic bulge.Saline seepages at the surface display this bulge tothe extreme, where salt crystals cover the landsurface during dry periods. Water moving throughor across these zones in wetter periods carries salt-laden water into streams.

The water recharge in hillslopes can mobilise saltsin different ways. Under high recharge rates, freshrainwater leaches salt from the soil profile to thewatertable, eventually purging the profile locally.Soils can therefore be salt-free, but the underlyinggroundwater may be the recipient of the salt, whichmoves downslope towards saline seepages andstreams. On the other hand, locally reducedrecharge (under a tree plantation for example) can

6

be the cause of local salt accumulation in the rootzone if saline inflows continue from upslope.

Salt storages in the profile are quite dynamic,moving vertically and laterally through the soil andlandscape, in short or long timescales. Theirmanipulation requires good understanding of thesalt stores and their location, and how they aremobilised by recharge, groundwater movement andwater use by vegetation.

Groundwater is simply a detained storage for waterthat passes through the soil profile. It is not staticafter it joins the watertable, but moves under

gravity towards seepages and streams. Whether it isused as a resource or manipulated for salinitycontrol, it can be managed only if we know how itresponds to changed recharge inputs and the rate atwhich it travels through various subsurfacepathways. The time taken for its journey can beonly a few days from streamside areas, or it can bemany decades in extensive regional groundwatersystems. The amount of water intercepted or takenup by the vegetation determines how much of itremains as a stored resource, and how much canlead to landscape degradation through salinedischarge

Two-year-old radiata pine plantation on farm at 700mm rainfall in Billabong Creek Catchment,New South Wales.

7

3. Planted Forests and Water IssuesThe development of plantations and farm forestry isfaced with the challenges of achieving economicand environmental outcomes at farm, landscape andregional levels. One major challenge ismanagement of the interactions between water,plantations and land use to achieve sustainability inthe broad sense.

Australia’s total plantation area in 1999 was1 337 300 ha, made up of 71% pines and 29%hardwood (mostly eucalypts). The 2020 Visiontarget is 3.3 million ha. Almost 95 000 ha wereplanted in 1999. Of this, 89% was eucalypts. Thisaltered balance between eucalypts and pineplantations does not imply that existing pine areaswill be replaced by eucalypts, but rather that greaterareas of eucalypts will be established on clearedfarmland that presently carries only very smallareas of remnant forest. Most of the plantationexpansion is foreseen in Western Australia, SouthAustralia, Victoria and NSW.

Expanded plantation areas can impact on theproportion of rainfall that eventually appears asstreamflow or groundwater. The sum of thesesources (stores) comprises our water resources.They are unevenly distributed, and are affected indifferent ways by vegetation change. The effects onwater resources of the total expanded plantationarea is therefore not the critical issue. The issue iswhere the plantations will occur, and what size theywill be in relation to the affected landscapes and thenature of the water resources. In some cases, theeffects will be confined to the farm scale; in others,the effects may have an impact on water yield orquality of water available downstream. The diversenature of these impacts needs to be firmlyestablished and understood in any analysis aboutthe relative benefits or disbenefits of afforestation.

The actual effects of tree plantations on the waterbalance and availability of water resources dependon regional weather attributes, vegetation cover,plantation management and rotation length. Anyintegrated approach to the management of waterresources in a specific climatic region must takeinto account all uses and practices which impingeon water availability and demand. In addition towater use by new plantations, for example, existingagricultural practices and infrastructure arerelevant: pumping of stored water; flood irrigationand leaking irrigation channels that can be wastefulof water; while the overall impact of farm dams onwater yield is unknown.

3.1 Regional water issuesWe now consider case studies of representativeregional perspectives and issues where industrialforestry and farm forestry are advancing rapidly.The aim is to highlight the diverse regional factorsdriving plantation establishment and land usechanges, their environmental attributes, and thelikely impacts of plantation forestry on land andwater resources. The water and plantation issues inthe four regions described below are examples ofscenarios which occur elsewhere in the country.These regions also have rainfall and commercialinfrastructure necessary to support a viable forestplantation industry. The analysis illustrates thespectrum of tradeoffs that must be considered whileembarking on large-scale plantation development inany region. The tradeoffs become complex wherethere is competition for access to water resources,where river salinity is increasing, where agriculturalland is being degraded by dryland salinity, andwhere environmental flows need to be restored.

Land use patterns are changing in these regions forother reasons, and their effects on water resourcesshould also be critically assessed.

3.2 The Murray Darling Basin:southern high-rainfall regionIn the north-eastern catchments of Victoria withinthe Murray Darling Basin (Fig. 1), a forestplantation base sustains a viable and valuableindustry. The existing plantation area is 39 000 ha(almost all softwood), on land which originally wasmostly native forest. Based on criteria of suitablesoils and rainfall – minimum 650 mm per year – anadditional 400 000 to 450 000 ha of cleared privateland has been identified as suitable for theestablishment of commercial plantations16.

The north-eastern river catchments in Victoriacontribute 38% of the total water for the MurrayDarling Basin System, although they comprise only2% of the total System area. Consequently, thenorth-east is a strategic and very important sourceof water in the basin. A significant increase in theforest plantation base could impact on the Basin’soverall water yield.

These catchments also discharge a high salt loadinto the Murray River as a result of clearing thenative vegetation over the past century.

8

Figure 1. South-eastern Australia; the numbers at the top indicate the value of isohyets in millimetres peryear

This salt load currently contributes about 31 EC(Electrical Conductivity) units annually to the riversalinity at Morgan, South Australia. If no salinityintervention programs are implemented, it isexpected that the salt load will increase by 1%annually over the next 30 years, raising the riversalinity at Morgan by a further 10 EC units. Withinthe north-eastern catchments themselves theestimated rate of spread of saline discharge areas is5.8% per annum without intervention, resulting inan additional 117 000 ha of salt-affected land overthe next 30 years14.

In New South Wales, an analogous situation occursin the Middle and Upper Murrumbidgee Basin, thecatchment upstream from Wagga Wagga (Fig. 1).In this basin, mean annual rainfall varies from 500mm in the west to 2500 mm in the southernmountain regions. The median annual runoff from

the entire basin (26 863 km2) is 185 mm. In thehigh-rainfall sub-catchments, mean annual runoffexceeds 800 mm. Again, this portion of theMurrumbidgee Basin is a major source of waterresources for downstream water users. The regionalready supports extensive tree plantation areas inthe higher rainfall regions (mostly softwoods). Ahypothetical scenario20, where an assumed landcover of grassland was completely afforested bypines, has predicted that local declines of runoff ofup to 500 mm are possible in the wettest parts ofthe catchment. Smaller declines would occurelsewhere, but the loss in runoff would exceed 100mm over 75% of the catchment where grassland isreplaced by trees. The productivity of treeplantations, in general, increases if the trees do notsuffer from water stress, other variables beingequal. Therefore, future expansion of plantations

QueenslandSouth Australia

Victoria

Sydney

Brisbane

Melbourne

Adelaide Mildura

Horsham

Orbost

Dubbo

CoffsHarbour

Port Augusta

. ..

..

.

...

..

.Canberra

Murray- Darling

Charleville

Bourke

.New South Wales

200 600400300 900

1200

Murray R.

Murrumbidgee R.

Lachlan R.

Darling R.

9

could exert pressure to access locations in thehigher rainfall zones. The wetter the area whereplantations are established, the greater will be theimpact on residual water resources. However, inpractice, new plantations would rarely replace landunder pasture completely; rather plantations arelikely to be established in small units occupyingvarying proportions of individual farms and thelandscape.

3.3 The Green TriangleThe Green Triangle region includes the south-eastof South Australia and the south-west of Victoria,an area of 55 000 km2 (Fig. 1). All but 7% of thearea has been cleared of native vegetation, and landuse is now dominated by rain-fed agriculture(4 600 000 ha). An estimated 83 000 ha is irrigated– mostly pasture, but including an increasing areaof vineyards. There is a well-established softwoodforest plantation industry covering 150 000 ha, 3%of the area. The wood and paper products

manufacturing sector, with a gross annual output ofmore than one billion dollars, supports a dynamicvalue-adding forest industry vital to the regionaleconomy.

The region’s annual rainfall varies from 420 mm inthe north to a little more than 800 mm in the south,so much of the area is semi-arid. Most of the raininfiltrates the permeable soils, with the result thatthe region’s water resources are primarily drawnfrom underground aquifers. Recharge to the near-surface unconfined aquifer occurs in winter andspring, and is estimated to be only 10-15 mm in thenorth, increasing to more than 150 mm in the south.Extraction of water from the aquifer is regulated inseparate management zones, such that the volumeextracted does not exceed the long-term averageannual recharge in each zone. In 40% of thesezones, the permissible annual volumes forextraction account for the full estimated rechargevolumes. The total annual water volume allocatedfor extraction in all zones is about 1 000 000 ML7.

Harvested logs in a 35-year-old radiata pineplantation near Mount Gambier, SouthAustralia

A thinned stand of radiata pine nearMount Gambier, South Australia

10

Until 1990, tree plantation areas were expanding ata modest rate of 1500 - 2000 ha per year of radiatapine. This is changing. Currently, there is a shifttowards Tasmanian blue gum (Eucalyptus globulus)plantations, which are being established at 20 000to 40 000 ha per year on rain-fed farmland. If thismomentum continues, in less than ten years theblue gum estate will exceed the pine estate whichhas been established gradually since the 1880s.

The changed nature of the vegetation cover willaffect the quantity of rain that passes through thecanopy, transpiration and hence the amount of rainwhich infiltrates the soil. The interception loss of afully closed radiata pine canopy is about 20% to30% of annual rainfall while for blue gum this maybe 15% to 20% of the annual rainfall. Theinterception loss from pasture is smaller, but notzero. While accurate data for the interception lossare not yet available, the best approximation willfor the annual interception loss is about 150 mm peryear for pines and 110 mm per year for blue gum,in a 700 mm annual rainfall zone. It should benoted that the difference in aquifer rechargebetween trees and pastures in this environment isnot entirely due to the difference in recharge butalso due to the difference in transpiration. In theworst case, with all plantations unthinned and of thesame age, the net reduction in recharge may be lessthan 100 000 ML, or about 10% of the currentannual extraction rate. However, the loss will bereduced to a fraction of this amount becauseplanting, thinning and harvesting operations wouldoccur progressively and at any time there will be amosaic of canopy cover and age class of plantationsin the region. Thus, the impact of forestry on waterwould be moderated by the proportion of the areawhere the tree canopy is absent or incomplete.

The net effect of stand thinning and harvesting ongroundwater is not known for certain. A recentstudy of blue gum sites in the Green Trianglesuggests that thinning changes the balance betweeninterception and transpiration more than it changeswater use of the tree stand. It appears that theremaining trees are able to transpire more water(presumably because their roots have lesscompetition), even though the interception loss isdiminished by partial removal of the stand canopy.

A second factor of concern in the Green Trianglearea is whether blue gums are more capable ofexploiting aquifer water than pines. There isevidence to show that pine plantations do notextract water from the aquifer where they areestablished on sites with a deep soil profile. Therelative water uptake rates by pines and eucalyptsfrom groundwater is likely to depend on how deepthe watertable is, rather than on large differencesbetween tree species. This question needs to beresolved, especially in regions where groundwateris the major water resource and where plantations

are being established in areas with varying depth towatertable.

The water issues in the Green Triangle region areclearly different from these in the high-rainfallareas of the Murray Darling Basin. The latter area isa net exporter of water, a provider of waterresources for downstream users. The issues thererelate primarily to whether an expanded plantationforestry will significantly diminish the waterflowing from the region. In the Green Triangle, theissue is the perceived threat that expandedplantation forests pose to the water resources forother competing land users (including irrigators) ofthe region. Most rain that falls there is storedunderground and used locally in a competitive andstrictly regulated way. No large artificial storagesexist to buffer the water demands from one year tothe next. Some water flows from the region to thesea during wet periods. Local recharge is consumedclose to where rain falls. Expanding plantationforests would significantly affect recharge to theaquifers and this tradeoff should be evaluated inrelation to the economic value of the industry andthe needs and value of other water users.

3.4 The Macquarie ValleyThe Macquarie River is the largest river in theCentral West of New South Wales (Fig. 1). Its flowis highly regulated by the water storages of LakeBurrendong and Lake Windemere, and dischargesinto the Ramsar*-listed Macquarie Marshes.Downstream from the Marshes, the Macquarie andCastlereagh Rivers join before flowing into theDarling River. The eastern quarter of the Valley hasan annual rainfall of 600-800 mm, and becomesprogressively drier towards the west. TheMacquarie Basin supports a diversity of rain-fedand irrigated agricultural enterprises, providing11% of the gross value of New South Walesagricultural output. Plantation forestry in the higherrainfall areas of the Valley is a significant industry.Expanded and innovative plantation forestry is seenas a future land use option and an economic driverfor the region, potentially valuable for checking theincreasing salt loads in the river and for controllingdryland salinity at the farm level, particularly in themiddle half of the Valley where rainfall is 400-650mm15.

Salinity in the river is increasing as a result of landmanagement practices over the past century. Arecent study into the costs of salinity in theMacquarie Valley estimated that the current costs

* The Convention on Wetlands, signed in Ramsar,Iran, in 1971, is an intergovernmental treaty whichprovides the framework for national action andinternational cooperation for the conservation andwise use of wetlands and their resources.

11

are about $30 million per year8. The Murray-Darling Salinity Audit in 199911 predicted that in a‘business as usual’ scenario, end-of-valley salinitywould exceed 800 EC by the year 2020, and1500 EC by 2050. Where the river enters theMacquarie Marshes, salinity would rise to 2110 ECby 2100, well above the threshold of 1500 EC forhealthy wetland ecosystems.

Government agencies have developed salinitycontrol strategies with objectives that will protectwithin-valley land and water resources, aquatic andterrestrial nature conservation values, indigenouscultural heritage and built infrastructure, as well asthe shared water resources of the Murray andDarling Rivers. The strategies recognise theimportance of including market-based approachesto revegetation and plantation forestry.

The role of forestry for salinity control in theMacquarie Valley would be to reduce recharge togroundwater, and thereby prevent the furtherexpansion of saline discharge areas lower down inthe landscape. This strategy accepts the premisethat tree plantations, if correctly placed in thelandscape, can have the maximum effect ongroundwater recharge and minimum effect onsurface water yield.

A recent investigation has assessed the economicand salinity outcomes of salinity interventionoptions in the Valley. Preliminary results indicatethat if 35% of the cleared land in the upperMacquarie is converted from grazing to plantationforestry, then after 100 years of 30-year plantationrotations, salt loads into Lake Burrendong wouldstill increase by about 45%, and annual water yieldfrom the catchment surface water yield woulddecrease by 200 gigalitres - about 5%15. It is alsoestimated that the ‘business as usual’ scenario couldlead to end-of-valley salt load doubling by 2020and trebling by 2100. The Valley’s comparativelylow rainfall means that the commercial incentivesfor tree plantation establishment are somewhat lessthan in higher rainfall areas. The attractiveness oftree plantations therefore depends on their public aswell as private benefits. The public benefit, in thiscase, accrues to the in-valley as well as downstreamwater users in the form of reduced river salinity.The environmental benefits to the MacquarieMarshes, by preserving them, cannot be quantifiedreadily, but they must be valued as essentiallypriceless. The private benefit accrues to thelandowners and industries who sell and process thewood products and whose lands are protected fromthe ravages of dryland salting.

The local economic viability of plantations in theMacquarie Valley is therefore only one factor thatshould influence their future establishment. In viewof anticipated increases in river salinity and itsdisbenefits, ‘business as usual’ is not an option.

Farm forestry clearly has a place in the valley tomitigate the encroaching effects of salinisation forthe overall public benefit and as a driver for themuch needed economic and employment growth inthe region.

Maintaining current water flows for irrigation andthe river environment appears to be incompatiblewith reducing the salinity problem. If treeplantations are used to minimize salt mobilizationfrom farmland, then river flows (and salt loads) willbe reduced. However, we are less certain about theeffect on salt concentrations in river waters. Thesemay rise during low-flow periods and may affectin-valley water users.

Broadscale revegetation (afforestation) at strategiclocations in the landscape appears to provideopportunity for the use of trees to manage salinitywith relatively modest reduction in streamflow overtime. The nature and management of forestedcatchments is critical to achieve a desired outcome.The modelling already done for a future scenarioshows that we require a better understanding ofhydrogeology at local scales, and better knowledgeof tree water use, growth and survival whereplantations could be established. Modelling of riverflows should examine not only the changed saltloads, but also the in-valley variation in river saltconcentration.

3.5 Western Australia: south-western regionThe south-western region of Western Australiaincludes 18 million ha of agricultural land wherethe annual rainfall varies from 1400 mm in thesouth-western corner to around 250 mm at the limitof agriculture (Fig. 2). The rainfall pattern isstrongly seasonal, with wet winters and drysummers. Expanding areas of dryland salinity andsalinisation of water supplies are major problemsthat threaten sustainable agriculture in the region.The problems have been emerging since the 1920s,but only since the 1970s has the full extent andseriousness of the situation been realised. Currently,production from 1.6 million ha of agricultural landis affected by salinity. Although salinity has alwaysbeen a major feature of the landscapes in the lowerrainfall zones (<600 mm per year) prior toagriculture, salinised areas are increasing rapidly asa result of land clearing in the past century (seebox).

The issues confronting the region involve both thesalinity of surface waters from higher rainfallcatchments, and the effectiveness of tree plantationsfor controlling dryland salinity. The processes thatcause salts to move into surface waters or intosaline seepage areas differ across the area, anddepend on local geology, topography and water

12

balance. In all areas, the soil regolith contains astore of salts, increasing from 100 t ha-1 in the westto 10 000 t ha-1 inland. Under natural vegetation,this salt store is largely retained below the root zoneand is relatively immobile, but the changeover toshallow rooted, annual agricultural plants has

resulted in drainage through the soil profile of up to200 mm per year, averaging about 25 mm per yearover the region. This causes watertables to rise andmobilise the dissolved salt. Revegetation with treesand perennials is seen as the most important tool forrestoring the hydrologic balance.

Figure 2. South-western region of Western Australia; the numbers indicate the value of isohyets inmillimetres per year

Expanding salinised areas in Western AustraliaAreas affected by salinity inWestern Australia are stillincreasing. Predictions indicatethat in a ‘do nothing’ scenario,the area increases to 6.2 millionha by about the year 2075. Ifsufficient revegetation isestablished to reduce rechargeby 50%, the area salinised by2075 is 4.5 million ha, 25% lowerthan the ‘do nothing’ case. Ifnothing is done, the 4.5 millionha become salinised by the year2035. This means thatafforestation could delay theeventual salinisation by 40 years,but only partly limits its eventualextent.

4.5

6.2

Salinity Projections

A

rea

affe

cted

(%)

Year

10

30

35

20

01990 2010 2030 2050 2070 2090

40 years

Area

(M h

a)

Do nothing

50% reductionin recharge

Q1: Change in eventual extent? 25%Q2: How much time do we buy? 40 years

Esperance

Albany

Pemberton *

Perth

Katanning

Northam

Kalgoorlie

Norseman

Leonora

MorawaGeraldton

200

300

400

600

9001200

..

.

.

. .

..

.

.

.

13

The impact of increased recharge to groundwater isfelt at different scales, depending on the nature ofthe aquifers. Some are seasonal and of local extent,others are regional. Aquifers also differ in responsetime, so the effect of a change in recharge rate maytake months or many years to appear.Correspondingly, the remedial effect of treeplanting on salt discharge shows the samevariability, and may be impossible to predictwithout adequate knowledge of aquifer propertiesand the imposed changes in water balance.

For these reasons, Western Australia has supportedresearch over the past 30 years into the processesthat drive salinisation, and the effectiveness of treeplantation strategies. The research is necessarilylocation specific, because the hydrologic balanceacross the region varies greatly with the climate,and the growth and survival of plantations dependson local environmental factors.

At the same time, legislation has been enacted tocontrol or eliminate new land clearing in watersupply catchments, and many areas already clearedhave been revegetated. For example, in the Helenacatchment the result4 has been a current reduction instream salinity 15 mg L-1 y-1 [23 EC units per year].Furthermore, the delay between land clearing andthe change in stream salinity appears to be 10 to 12years, suggesting a similar time scale may beneeded before the effects of revegetation on streamsalinity are seen.

Tasmanian blue gums (E. globulus) are beingplanted on farmland in the wetter areas. With about153 000 ha established in the last 10 years, there isnow a major eucalypt plantation resource in thearea. Although the initial impetus for thisdevelopment was the expected economic andenvironmental (salinity mitigation) benefit, inreality most of the areas currently under blue gumplantations are not on land affected by dryland

salinity. Blue gums were chosen in preference tothe softwoods traditionally planted in the south-west, not only for the quality of their woodchipproducts but also because of their high biomassproductivity and the ability of their deep roots toextract water from the saturated groundwater zone.Although there are large areas of successful forestryin the region, balancing productivity and droughtrisks is an important emerging issue in this drought-prone region, where dry summers are the norm. Theblue gum plantations therefore have been confinedso far to the >600 mm annual rainfall zone. Morerecently, major initiatives are in progress to developa commercially viable revegetation programthrough the deployment of mallee eucalypts andPinus pinaster in drier areas (<600 mm annualrainfall)4.

It was initially speculated that dryland salinity inthe >600 mm rainfall zone could be controlled iftree plantations were established on 20%-30% of afarm’s area. The realities of farm economics willrequire that these plantings produce a commercialreturn in their own right either through tree-basedproducts or by the provision of environmentalservices (credits). Further development andevaluation of tree species that satisfy hydrological,commercial and environmental criteria is needed.And in hindsight, experimental evidence andmodelling shows that the hydrology of differentlandscape types determines whether tree plantingcan achieve a desired salinity abatement outcome.In particular, choice of the optimum farmingmethods for salinity control must rely on a goodunderstanding of the subsurface flow systemsinvolved, especially whether they are local orregional in extent.

14

Oil mallee in farm forestry in low-rainfall, salinity prone area in Western Australia

A productive stand of Pinus pinaster inWestern Australia

15

4. Effects of Afforestation on WaterOur knowledge of the effects of afforestation onwater resources is usually site specific. Invariably,we are expected to extrapolate observations to othersites where this type of information is not available.This can be done reasonably well if we know howthe processes that determine water balance varyacross different landscapes, climates and vegetationtypes. There is good understanding of the physicalbehaviour of catchments, and how their waterbalance is affected by soils, topography andclimate. This needs to be complemented byknowledge of how different plantation types, treespecies, climate, management practices and waterinteract over the long term.

In this section, we examine several of the key tree-environment interactions that influence water use,and summarise the evidence on how afforestationaffects water resources.

4.1 Interactions between waterand treesSignificant differences exist between water usepatterns of the tree species most favoured forplantations (see box). The key variables that affectwater use by trees are leaf area, the rate of water

transpiration through leaves and tree rootingpatterns.

4.2 Leaf areaWater use by trees depends on leaf area and the rateof water vapour transfer from leaves. In mostAustralian environments, native forests develop andretain leaf areas that are in dynamic equilibriumwith long-term mean annual rainfall (a roughsurrogate of the amount of available water), andmost rainfall is transpired. This relationship seemsto be independent of vegetation types, at leastamong Australian eucalypts.

4.3 Leaf transpirationThe rate of transpiration per unit of leaf area isdetermined by the difference in water vapourpressure between the inside and the outside of theleaf, and by leaf conductance to water vapour.Conductance to water vapour is controlled byopening and closing of the stomata.

Trees and water useA convenient basis for comparingpatterns of water use is toexpress actual water use of plantcanopies as a proportion of themaximum possible evaporationrate, which depends on availableenergy and wind convection. Thisproportion is referred to as thecrop factor (k). It varies with treespecies and the availablemoisture in the plant root zone.The dependence of k on soilwater is shown for stands ofdifferent tree species21.

This illustrates first, that when soil moisture is high, some tree stands like blue gums (E. globulus)and actively growing crops transpire water at the maximum possible rate, corresponding to k=1.Pinus radiata and mallee species transpire water at about 80% and 55% of the maximum raterespectively. Second, when the relative soil moisture drops below a threshold value, water usedeclines, until it approaches zero when the soil is totally dried out. The threshold where this occursalso appears to vary from one tree species to another.

Relative soil water store

0.0 0.2 0.4 0.6 0.8 1.0

Rel

ativ

e ev

apot

rans

pira

tion

rate

0.0

0.2

0.4

0.6

0.8

1.0

1.2

E. globulus plantations (LAI > 4) and ag. communities

E. maculata and P. radiata (LAI=3)

Mixed Eucalyptus tree belt

Mallee (LAI=0.25)

16

A high rate of stomatal conductance results in highrates of water use. For a given stomatalconductance, the maximum rate of transpirationoccurs when the leaves are surrounded by dry air.Stomata however are also highly sensitive to thedryness of the surrounding air (measured by thevapour pressure deficit, or VPD), so that as VPDincreases, stomata close to prevent excessive loss ofwater through the leaves. This stomatal closure inresponse to dry air means that maximum rates oftranspiration usually occur at a relatively low VPD.As VPD increases from zero, transpirationincreases very quickly before leveling off at aplateau, or even decreasing at very high VPD.

Under well-watered conditions, there can besubstantial differences between species in therelationship between stomatal conductance andVPD. For example, at low VPD, stomatalconductance can be up to four times larger for E.globulus than for P. radiata, but in very dry air, thestomata of blue gum leaves close more and inhibitwater loss to a greater degree than do those ofpines.

The physiological behaviour of plantation speciestherefore affects water use in a way that is highlyclimate dependent. In an environment with hot, drywinds – assuming plentiful soil moisture – pineswould be expected to use more water thaneucalypts. In humid environments, the conversemay be true.

As the soil dries out, stomatal conductance changesin a complex fashion. If water supply to the roots islimited because the soil is dry, the leaf stomatamust close as the VPD increases or the leaveswould wilt because the rate of water supply to theleaves would not be able to keep up with the rate ofwater loss. In some species it is possible that theroots send chemical signals to the leaves when thesoil is dry, enhancing the stomatal response toVPD.

Water loss also has the effect of cooling the leaves.In hot environments during summer there is a riskthat if the stomata close too much, leaftemperatures will increase to lethal levels. At siteswhere trees are stressed by dry soils during drought,eucalypts often respond by shedding leaves, therebylowering their water use and making the limitedwater available to a smaller leaf surface area, sothat leaf temperatures can be reduced below lethallevels.

In the longer term, species selection occurs, andtrees that maintain larger leaf areas during summer(e.g. E. globulus, E. nitens, E. grandis, Corymbiamaculata) do not survive in these conditions.Survival and growth of plantation speciesintroduced to areas where soil moisture ispersistently depleted, either by drought or theplantation itself, will be a problem. Selection of

provenances within species may also be importantin improving the potential for good survival andgrowth of species planted in these areas.

4.4 Tree rooting patternsRoot architecture differs between tree species, andthis can cause differences in water use rates (seebox). Some Eucalyptus species are also known towithdraw soil water from very deep in the soilprofile, and to take up fresh and moderately salinegroundwater at rates of up to 3 mm per day.

Some tree species are likely to be more effective forcontrolling downslope saline seepages than others.However, other physical factors (e.g. slope,hillslope length, soil permeability) also affectwatertables and seepage zones. Thus, the efficacyof plantations for saline discharge control must betreated as a site-specific question.

Generally, if plantations have rooting patternscapable of extracting water from sources additionalto rainfall (i.e. groundwater), they are likely tosustain a higher leaf area. Up to a limit, this shouldresult in greater productivity and greater water use,at least in the same species. The greater leaf areawill not be sustainable if such plantations havedepleted all the stored soil water, if flows ofgroundwater to the plantation are lower than theexcess of evapotranspiration over rainfall, or ifgroundwater uptake results in a continuing rise insalinity in the root zone.

Water extraction by tree rootsAt a site near Deniliquin, groundwater userates of Corymbia maculata (spotted gum) andE. grandis (flooded gum) plantations werecompared. The spotted gum produced a muchgreater root length density in the capillaryfringe above a shallow, fresh watertable thanthe flooded gum, and transpired water at closeto the maximum possible rate17. With far fewerroots in the capillary fringe, flooded gum hadless access to groundwater and transpired 400mm per year below the maximum possiblerate17. Despite this, the flooded gum had agreater stem volume per hectare at age 4,probably because the spotted gum wasallocating more growth into roots rather thanstems.

In Western Australia, blue gums depleted soilwater to depths up to 9 m, but their effect onlowering groundwater was localised to sometens of metres beyond the edge of theplantation.

17

Evapotranspiration in forests and grasslandsA relationship between meanannual ET (and hence runoff) tomean annual rainfall in Victoriancatchments with differentproportions of grassland and forestcover has been developed6. It isbased on long-term annualrainfall/runoff data for 19 largecatchments, with mean annualrainfalls ranging between 500 and2500 mm. The relationshipdemonstrated that there were cleardifferences between ET rates forgrassland and eucalypt forestcatchments. This was illustratedwith a pair of curves that denotedthe differences along a rainfallgradient.

According to these curves, a fully forested catchment evaporates 40 and 215 mm more per yearthan a fully grassed catchment with mean annual rainfalls of 600 and 1300 mm, respectively. Theclimatic zones where Australian plantations have been established historically, and where futureplantations are envisaged, have 500-1200 mm of rainfall annually19.

400

0

800

1200

1600

500 1000 1500 2000 2500

Mean annual rainfall (mm)

Mea

n an

nual

eva

potra

nspi

ratio

n (m

m)

HSRZhang Forest

Grass

A few generalisations can be made, based onphysiological observations and supported by fieldstudies of water use. These are:

For well-watered soil conditions• E. globulus plantations transpires water at close

to the maximum possible rate.• P. radiata plantations transpire water about

70% of the maximum possible rate.• When the air is very dry, P. radiata transpires

more water than E. globulus.

For drying soil conditions• The risk of death in E. globulus grown under

otherwise well-watered conditions is high indrought-prone environments.

• Other eucalypt species (e.g. E. nitens, mallees)show different patterns of water use in dryingsoil, and probably different sensitivity todrought.

For all conditions• Trees of all species intercept more rainfall than

crops or pastures. But interception by crop andpasture is not zero, as some have believed.

• Deeper roots of all tree species can extractmore water from the soil than those of crops orpastures and thus also reduce recharge.

• There is significant genetic variation betweentree species in their ability to use groundwater.

All these differences should be recognised andaccounted for to provide a better basis for matchingspecies with landscape features for sustainablereforestation programs.

4.5 Annual water yieldsAnnual water yield is the sum of surface andgroundwater outflow from a catchment. By usingannual data, we can neglect the seasonal differencesin rainfall and groundwater storage, and comparethe longer-term differences in water yield caused byvarious land uses. Water yield itself is highlyvariable from year to year, basically because ofdifferences in annual rainfall. The major water lossis evapotranspiration (ET), the sum of evaporationfrom plants (including both transpiration andevaporation of intercepted water) and evaporationfrom the soil surface. ET is far more uniform thanrainfall from year to year in a given plantcommunity. The biggest change in catchment waterbalance resulting from tree plantations is theireffect on evapotranspiration, so the followingdiscussion concentrates on this aspect.

In southern Australia, mean annual ET is usuallyless than 650 mm in grasslands, but can approach1300 mm in forests. ET tends to increase with meanannual rainfall (see box).

18

Studies done so far have not discriminated betweendifferent types of forest cover, or the changingdynamics of forest canopies where plantations areprogressively thinned and harvested. A managedplantation has, on average, smaller and changingleaf area than an untended forest, so its ET wouldbe correspondingly smaller than its unmanagedcounterpart.

Plantations established on cleared land take time todevelop, and during the establishment period thewater yield changes gradually as canopies and rootsdevelop (see box). Even before tree canopieschange the character of the vegetation cover, landpreparation for tree planting (e.g. deep ripping,contour furrowing) can also retain more runoff onhillslopes and reduce streamflow during rain.

Pines intercept about 10% more rainfall than donative eucalypt forests, which is enough to accountfor the higher water use by pines observed inAustralian studies. But experience in South Africashows that water use by intensively managedeucalypt plantations exceeds that of pines.Differences in climate, soils, tree species or insectpredation in South Africa may account for thiscontrary result. There are insufficient Australiandata to compare the water use of intensivelymanaged pine and eucalypts plantations in the sameenvironment.

Using the data that we have, estimates have beenmade of the reduction in mean annual water yield

that would result from complete afforestation ofgrassland with pine or eucalypt plantations. Thereductions in mean annual water yield are greatestin high-rainfall areas, and pine afforestation has agreater impact than native eucalypt forests. Forareas with 800 mm mean annual rainfall, meanwater yield may drop by 165 mm under eucalyptsand up to 220 mm under pines. In particularly wetyears, yield reductions may exceed these meanannual changes. These estimates, however, do nottake into account the dynamic nature of the siteoccupancy of plantation estates in a givencatchment or landscape.

4.6 Changes in seasonal flowsSo far, we have described the impacts of treeplantations on annual water yields from catchments.In river basins where large reservoirs have beenconstructed, the seasonal flows are buffered by theriver storages, and water released for downstreamusers is highly regulated. But where rivers are notregulated, seasonal flows are more relevant thanannual mean flows. In particular, dry-weather flowsand their persistence concern catchment managers.In dry conditions when ET continues in the absenceof rainfall, soil water is depleted rapidly; seepagesthat sustain baseflow dry up, and streamflowceases.

Water yield from a developing pine plantationChanges in water yield in adeveloping pine plantationhave been measured at a high-rainfall site near Tumut, NSW.Water yield from the pinecatchment, compared with anadjacent grassland catchment,decreased as the canopy androots developed. The declinein water yield increasedsteadily for 10 years, and iscontinuing. The majorreduction that occurred in 1996during a comparatively wetyear (1100 mm rainfall versusthe mean rainfall of 850 mm)illustrates the sensitivity ofyield changes to rainfall19.

-350

-300

-250

-200

-150

-100

-50

0

50

89 90 91 92 93 94 95 96 97 98 99

Year

Mean = 182 mm

Cha

nge

in a

nnua

l run

off (

mm

)

19

Changes in dry-weather flows

One of the best local examplesof altered dry-weather flows hasemerged from the Tumut (NSW)catchment experiments, whichcompared daily flows fromadjacent pasture and pinecatchments. The daily runofffrom the two catchments isshown, expressed as percentageof time that a certain flow isexceeded. Changes occurred inthe duration of daily flows of allmagnitudes. Most significantly,the afforested catchment nowceases to flow for almost 40% ofthe time. In comparison, thepasture catchment continues toflow all year round19.

Under these conditions, the influence that enhancedET from tree plantations exerts on residualstreamflow can be high.

In spite of the importance of dry-period flows towater users and aquatic habitat in smaller riverbasins, there has been little information gathered toassess the effect of tree plantations. Seasonal flowsfrom newly afforested catchments will probablyrevert to a semblance of their behaviour before theland was cleared (see box).

Because of the leverage effect where rainfall andET are closely matched, it is misleading to transferresults from one site to another where [rain minusET] is different. Before land clearing waswidespread, this value was very close to zero. Sinceclearing, ET has been reduced, so streamflowspersist. But [rain minus ET] fluctuates erraticallybetween positive and negative values from wet todry years, even on a seasonal basis, and thusstreams often dry up. Pine and eucalypt plantationsexhibit different seasonal patterns of water use, sotheir impacts on seasonal flows – particularly dry-weather flows – will also differ.

These effects on dry-weather flows can beestimated with appropriate analyses. The datacollection for this kind of investigation requires tenor more years, and the investigations are costly.Even then, the information gained is site-specificand applicable only to the imposed land use change.This should not preclude field experiments toresolve these questions. Nevertheless, resourceagencies and researchers have accumulated largedata banks on streamflow; these deserve to beanalysed to obtain information on the behaviour ofdry-weather flows in relation to land use changes.

A complementary approach is to use ‘best bet’analyses to predict changes in water yield and itsseasonal behaviour based on models of interactionsbetween vegetation, soils and climate. Thedevelopment and credibility of such models haveimproved rapidly in Australia, and has beenunderpinned by detailed studies of tree water use indifferent climate and edaphic (soil) conditions. It isclearly necessary to continue development andapplication of these modelling tools at sites wheretree plantations may be established.

4.7 Impacts on peak flowrunoffReductions in peak flow can be expected whengrassland is converted to plantation forest forseveral reasons including:

• Interception losses in tree canopies reducethe quantity of rainfall reaching theground;

• Soil moisture is depleted to greater depthsin plantations, so they can absorb morerain before the soil becomes saturated andsurface runoff occurs;

• Soil structure under forests allows easierinfiltration than in grassland, particularlywhere livestock have compacted thesurface soil; and

• In forests, smaller areas of saturated soilsexist adjacent to streams, thus reducing thevolume of runoff from these areas.

0.01

0.001

0.1

1

10

0 20 40 60 80 100

Pasture

Percent of time that runoff occurs

Dai

ly ru

noff

(mm

)

Pines (5-10 years)

20

New Zealand experience shows that flood peaksdecrease by about 60% after grassland is convertedto pine plantations. These results are typical ofexperience with storm runoff after land use ischanged from pasture to forest. The benefits forflood damage and landscape erosion are obvious –by reducing flood flows, maintaining land stabilityand soil cover, preventing river silting andpreserving water quality. However, during veryminor flood flows, the changes may conflict withdownstream water needs, such as the environmentalflows needed to sustain aquatic ecosystems, or riverflushing to improve water quality.

Flood flows from plantations are more similar tothose from native forests than from cleared land.Therefore, afforestation probably results in a returnto pre-clearing river flow regimes in catchmentswhere there are no large artificial storages.However, if flows in these streams are alreadyhighly allocated for riparian users, then conflictsbetween agricultural and environmental waterneeds can be expected.

4.8 Effective catchment areasSome parts of landscapes may become isolatedperiodically, in a hydrological sense, fromdischarge areas or streams. This occurs when upperparts of hillslopes dry out, and there is nodownslope water movement from these areas. Then,the streams only receive subsurface flow fromnearby parts of hillslopes. These streamside areasconstitute the effective catchment area for thestream. Its size varies seasonally with rainfall andevaporative demand, and depends on topographyand soil permeability.

The effective catchment area is always smaller thanthe area inside the watershed. The concept onlyapplies where water movement in the landscapefollows local topography and not where streams arefed by regional groundwater systems. This hasimplications for the effects of afforestation inlandscapes. If a land use change occurs outside theeffective catchment area, then the change has nohydrological effect. If the land use change is insidethe effective catchment, then it will affectstreamflow.

Identification and measurement of effectivecatchment areas at locations where tree plantationsare planned should be an essential planning toolwhere water availability is an issue. Specifically, ifthe plantations are located mostly outside theeffective catchment area, then the competition issuediminishes in importance. The effects of plantationsor any other land use change in parts of a catchmentthat rarely contribute to streamflow – especially in

dry weather – can be disregarded as far asstreamflows are concerned.

The basic methods for applying the concept tocatchments with local groundwater flow systemshave been developed, but work remains to be donebefore a useable planning tool is available.

4.9 Time response oflandscapes to changeA change in hydrologic balance may be manifestedas a rapid response in streamflow, or as a gradualchange over many decades. The response time inany situation is determined by a combination of thelength of the vegetation growth cycle and how longit takes for the landscape to readjust to the change.Thus, an ‘instantaneous’ change to vegetationcaused by wildfire in a small catchment with highlypermeable soils, for example, usually results in animmediate increase in streamflow over thefollowing few days or weeks. In a larger catchmentwith less permeable soils, it may take months forthe streamflow to adjust.

Recovery of the vegetation (after disturbance suchas fire, harvesting, clearing) has its own timescaledepending on how it is managed (replanting etc.).In forests, this may be several decades, asillustrated by the stunning example of decreasedwater yield that occurred in Melbourne’s watersupply catchments after old-growth forest wasdestroyed by fire in 1939, and replaced by naturalregrowth of the same forest species. Now, 60 yearsafter the fire, annual water yields are still severalhundred millimeters less than the pre-fire yield.Water resources in other major catchments are alsoprobably readjusting as a result of past clearing. Forstreamflow responses, the effects are probably fullymanifested by now, because the present crops havesmall hydrological timescales (months) comparedwith the native vegetation they replaced (decades).The same may not be true for salts that have alsobeen mobilised. Because of the mechanismsinvolved, salts move more slowly through thelandscape, and it is unlikely that equilibrium hasbeen reached with the altered vegetation. However,it is important to realise that the situation istransient rather than static, and we need to knowwhere we are ‘on the curve’. The implications forthis are discussed in the following section.

The time taken to feel the full effects ofdeforestation or reforestation on streamflow willtherefore be decades where rotations are long. Forsalts, it may well be longer, because we areintervening in landscapes where salt movement isalready undergoing long-term change. Managingwater and land resources in an environment ofexpanding forests will require better understanding

21

of the timescales over which the impacts will befelt. Plantation expansion itself occurs at varying

time scales in different environments, withoverlapping cycles of harvesting and replanting.

Break of slope plantations of blue gum in Honeysuckle Creek Catchment in Victoria

22

Saline hazard areas for treeplantationsAbout 86 million ha of agricultural landare subject to salinity hazard in Australia.This estimate is based on the extent oflands that are likely to contain salts thatcan be mobilised together with surplusrainfall (rain minus ET) of less than 125mm per year. Of this area, 5.5 million hais judged to have some potential forcommercial tree plantations. This requiresan annual rainfall greater than 600 mm,with less than 5 consecutive months ofrainfall less than 40 mm18.

5. Effects of Afforestation on SaltIn general, fully stocked stands of trees will usemore water than other kind of vegetation. Whererainfall is less than evaporation, the leaf areas thatdevelop use essentially all the available water. Animportant corollary is that in low to moderaterainfall zones (less than about 800 mm per year),salts accumulated in the subsoil because there wasinsufficient percolation of water to purge this salt.

Now, where rainfall is less than 800 mm, highrecharge under crops or pastures compared withtrees is responsible for the increase of salinedischarge areas. Thus, after vegetation clearing foragriculture, catchments with higher rainfall tend toyield more fresh water, while drier catchments tendto salinise.

Australia-wide, the Bureau of Rural Sciences(BRS) has estimated that there are about 5.5 millionha of land subject to salinity hazard that also havepotential for commercial tree plantations (see box).Most of this is cleared privately owned land in theeastern States. But factors such as infrastructure,market, return on investment and cultural attitudesmay mean that only 10%-20% of this land is likelyto be actually available for plantation development,unless these impediments are removed. From aforest industry perspective, even a fraction of thisland area is a significant potential resource.

Whilst it is a useful initial study, the BRS analysisused an assessment of plantation potential carriedout ten years ago2, which assumed areas withannual rainfall of less than 600 mm may beunsuitable for commercial plantation forestry. Thisassumption was reasonable then, but in the last fewyears there has been a great increase in interest indeveloping new systems suitable for low-rainfall(400-600 mm) zones. For example, oil mallee and

maritime pine plantations are already beingestablished in low-rainfall areas of WesternAustralia to provide both forestry products and alsocontribute to ameliorating salinity problems. On thebasis of the 600 mm lower limit the BRS analysissuggests there are virtually no areas in WesternAustralia where plantation forestry can contributeto dryland salinity mitigation. This is clearlyincorrect and the area of land across the wholenation subject to salinity with potential forcommercial tree plantations would be dramaticallyincreased if areas in the 400-600 mm rainfall zonewere included in the BRS analysis. In manyenvironments annual rainfall alone is an insufficientcriterion to demarcate the land use potential forfarm forestry based enterprises.

The current salt stores in catchments have takenmany millennia to accumulate so we can assumethat they are not increasing, at least at timescales ofhundreds of years. What is important is the locationof salt stores in landscapes, the rate of mobilizationof this salt, where it moves to and why.

The rate of discharge of salt into a saline seepagearea (and then to the stream) depends only on thegroundwater processes that carry salt to thedischarge areas. If plantations do not influence thisrate of salt delivery, then the effects of trees on saltwithin the catchment will be small. If the rate doeschange but the total salt store does not, trees willaffect only the timing of salt delivery and not thevolume delivered. If part of the salt store can beimmobilised, then salt loads will be reducedpermanently. However, this also implies a reductionin water throughput, so water yields becomesmaller and, due to reduced dilution, saltconcentrations may remain unaffected.

This suggests that the approach to revegetatingdrier catchments to arrest saline discharge might beto restore the original leaf area. Clearly, this is notan option if a region’s agricultural productivityusing present cropping patterns is to be maintained.The more realistic question to be studied andanswered is whether or not strategic tree plantationson smaller areas can prevent further salinisation, atthe same time preserving overall income forlandowners and the region as a whole: that is, theintegration of trees as a part of the farming system.

5.1 Classification ofcatchments for salinity controlAs discussed earlier, the effect of altered rechargerates on saline groundwater systems can be local or

23

regional. The time to response (change) also differsgreatly. The key diagnostic properties of acatchment with respect to its hydrologic response totree planting are5:

• The discharge capacity of the aquifer;• The nature of the recharge process: diffuse vs

localised recharge, its spatial distribution, andthe rainfall environment;

• The size of the groundwater system; and• The salinity of the soil and the groundwater.

If we know enough about these factors at a site,then the preferred strategy for tree use can beselected. In particular, we can estimate whether theresponse to revegetation will be rapid or slow, anddetermine the optimum size and location forplantings.

First steps have been taken to generalise theattributes of landscapes and their groundwatersystems and to devise a classification system forcatchments that is useful for controlling themobilization of stored salt3. The classificationemphasises that the hydrogeological processescausing land salination at a site must be correctlyidentified; it also lists the data needed tocharacterise a site’s susceptibility to salination, andwhat is needed to determine whether salination islikely to occur. This type of information isnecessary to allow catchment managers to identifythe optimum strategies for tree planting in variouslandscape types, both for salinity control and forsustained land productivity.

Local groundwater systems in landscapes of higherrelief extending over a few kilometres are likely tobe more amenable to local revegetation strategies.There, shallow groundwater flow may beintercepted and transpired before it reaches thesaline discharge areas. If the groundwater is fresh,localised plantings in hillslopes offer opportunitiesto control salinity. The best positions for suchplantings can be found by applying hydrologicmodels to individual catchments. Although thesemodels require local data on climate, soil propertiesand topography as well as technical expertise, theyare useful planning tools for farm forestry andbroad scale plantations. However, the bulk ofcatchments at risk in Western Australia are too flat,have low transmissivity, or are too salty for smalllocal plantings to be effective; these requirerevegetation over most of the landscape. There,only about 10% of salt affected landscapes appearto be suitable for local tree plantings.

The areas where regional aquifers dominategroundwater behaviour pose severe challenges tosalinity management. In these groundwatersystems, there is little opportunity for trees tointercept groundwater once it reaches the regionalaquifer. There, the requirement of plantations for

salinity control is to reduce recharge to a ratecomparable to that under the original nativevegetation. Moreover, recharge must be reducedover extensive areas rather than at strategiclocations in hillslopes. This requires levels ofafforestation that approach the original vegetationcover.

The afforestation area needed in WA for salinitycontrol is much larger than can be supported, inpractice, by commercial forestry criteria. Much ofthe plantings are therefore likely to benoncommercial, unless new markets for the treeproducts can be developed. Investigations of suchproducts and markets are being vigorously pursued.

Revegetation of saline discharge areas deservesspecial attention. Trees (of salt-tolerant species)planted in saline discharge areas can lower localwatertables for a time, but they do not enhancegroundwater discharge from upslope. That iscontrolled by soil properties and local topography.Salt tends to accumulate in the rooting zone ofplanted trees, which eventually suffer or die. Thearea affected by the saline seepage and the long-term salt loads to rivers are probably unchanged.

5.2 Alternative farmingstrategiesThe ability of E. globulus to use soil water at highrates and depress watertables is being explored inthe context of ‘Phase Farming with Trees’ (PFT).The concept envisages short-rotation (3-5 years)tree crops that rapidly deplete the water in the soilprofile; in the years following tree harvest,agricultural crops can be grown until the increasedrecharge raises the watertable again4. Modelling hasshown that PFT could be carried out with an overallrotation period of 15 years, but it would be viableonly for certain soil types. The accumulation ofsalts in the profile may also cause problems. Again,widespread adoption of PFT would be influencedby the commercial value of the short-rotation treeproducts, which could include biomass for energyproduction, and would entail major shifts infarming traditions.

In normal short-rotation plantings that do notinvolve the PFT strategy, E. globulus plantations onfarmland may experience water stress during thesecond and following rotations. This is because thefirst crop, often managed to get high growth rates,would have used (depleted) the water stored in thesoil when the site was under pasture. In the absenceof a sufficient fallow period to allow re-charge ofthe profile, the subsequent tree crop (also managedto obtain rapid growth and therefore high leaf area)may run into water stress earlier in the life cycle. Insome environments tree deaths are known to occur.Thus the knowledge of this long-term interplay

24

between available water, site environment andmanagement strategies is necessary for balancingproductivity goals and drought risks.

‘Alley farming’, where tree belts are interspersedwith strips of land for annual crops, is anotherstrategy being practised in the south-west. The aimis to locate and space the tree belts in hillslopessuch that subsurface water percolating downslope isintercepted before it reaches the saline seepages. Inessence, the tree belts transpire water at a rate thatmatches the increased recharge under the annualcrops. Optimum outcomes from this approachrequire good understanding of site attributes andlocal hydrology; in particular, alley farming islikely to be most successful where watertables arecontrolled by local topographic features rather thanby extensive regional aquifers.

The magnitude of the dryland salinity problem andthe scale of the revegetation needed to have animpact on it are very large in Western Australia andelsewhere. Greater efforts in farm forestry arenecessary if there is to be a prospect of making asignificant impact on salinity at a large scale inAustralia. To be effective, it will need to bepractised over larger areas, both at farm andregional scale. Significant benefits are possible,

however, at a farm and catchment scale and theseare encouraging further expansion. In several cases,the beneficial effects of afforestation will beapparent more to downstream landowners andwater users than to the properties where plantationsare established. Planting programs need to be bettertargeted by incorporating, for example, knowledgeof the effective catchment areas in landscapesdiscussed earlier. Where it can be practised toadvantage, farm forestry will need to use newplanning tools to select the locations in catchmentsthat optimise on-farm tree and agriculturalproductivity. It is also essential that planted treesare tended and managed well to fully realize theirgrowth potential.

It is therefore likely that the incentive forreforestation will be strongly influenced by theintrinsic value of the products at the farm gate andthe value of the environmental services it canpotentially provide beyond the farm gate. Theissues of incentives are particularly evident in thelower rainfall regions where dryland salinity is amajor threat. In these cases, the questions to beanswered relate to the productivity andsustainability across rotations of trees growing insaline environments, and the availability of tools tomeasure their environmental value.

25

References

1AFFA (2000) Our Vital Resources – National ActionPlan for Salinity and Water Qualityhttp://www.affa.gov.au:80/docs/nrm/actionplan/index.html

2Booth, T.H. and Jovanovic, T. (1991) Identification ofland capable for private plantation development.Report of the National Plantations AdvisoryCommittee, Appendix B, 1-92.

3Coram, J. (ed.) (1998) National Classification ofCatchments for Land and River Salinity Control.Water and Salinity Issues in Agroforestry No. 3,RIRDC Publication No. 98/78;. 37, 16, 6 p.

4Harper, R.J., Mauger, G., Robinson, N., McGrath, J.F.,Smettem, K.R.J., Bartle, J.R. and George, R.J. (2001)Manipulating catchment water balance usingplantation and farm forestry: case studies from South-Western Australia. In: Nambiar, E.K.S. and Brown,A.G. (eds). Plantations, Farm Forestry and Water:Proceedings of a national workshop. 20-21 July2000. RIRDC Publication No. 01/20, Canberra, 44-50.

5Hatton, T. and George, R. (2001) The role ofafforestation in managing dryland salinity. In:Nambiar, E.K.S. and Brown, A.G. (eds). Plantations,Farm Forestry and Water: Proceedings of a nationalworkshop. 20-21 July 2000. RIRDC Publication No.01/20. p. 28-35.

6Holmes, J.W. and Sinclair, J.A. (1986) Water yield fromsome afforested catchments in Victoria. Hydrologyand Water Resources Symposium, Griffith University,Brisbane, 25-27 November 1986. The Institution ofEngineers, Australia, Preprints of Papers, 214-218.

7Hopton, H., Schmidt, L., Stadter, F. and Dunkley, C.(2001) Forestry, land use change and watermanagement: a Green Triangle perspective. In:Nambiar, E.K.S. and Brown, A.G. (eds). Plantations,Farm Forestry and Water: Proceedings of a nationalworkshop. 20-21 July 2000. RIRDC Publication No.01/20. p. 36-43.

8Ivey – ATP (2000) The cost of dryland salinity toagricultural landholders in selected Victorian andNew South Wales catchments: Interim Report – Part2. A report prepared for the Murray-Darling BasinCommission, Canberra.

9Madden, B., Hayes, G. and Duggan, K. (2000) NationalInvestment in Rural Landscapes: an investmentstrategy for NFF and ACF with the assistance ofLWRRDC. 44 p. http://www.nff.org.au/

10MCFFA (1997) Plantations for Australia: The 2020Vision. Implementation Committee, MinisterialCouncil on Forestry, Fisheries and Aquaculture,Department of Primary industries and Energy,Canberra. 88 p.

11Murray-Darling Basin Ministerial Council (1999). TheSalinity Audit: A 100-year perspective. Murray-Darling Basin Commission, Canberra. 39 p.

12Murray-Darling Basin Ministerial Council (2000)(Draft) Basin Salinity Management Strategy – 2001-

2015. The Council, Canberra. 36 p.http://www.mdbc.gov.au/naturalresources/policies_strategies/projectscreens/pdf/salinity_man_strat.pdf

13Nambiar, E.K.S. and Brown, A.G. (eds) (2001)Plantations, Farm Forestry and Water: Proceedingsof a national workshop, 20-21 July 2000, Melbourne.Water and Salinity Issues in Agroforestry No. 7;RIRDC Publication No. 01/20. 62 p.

14North East Catchment Management Authority(NECMA) (1999) Draft North East Salinity Strategy.NECMA, Wodonga.

15Powell, J. (2001) Revegetation for salinity managementin the Macquarie Basin: a case study in the Murray-Darling Basin. In: Nambiar, E.K.S. and Brown, A.G.(eds). Plantations, Farm Forestry and Water:Proceedings of a national workshop. 20-21 July2000. RIRDC Publication No. 01/20. p. 51-54.

16Riddiford, J. (2001) Potential water issues associatedwith farm forestry expansion in high-rainfall areas ofthe southern Murray-Darling Basin. In: Nambiar,E.K.S. and Brown, A.G. (eds). Plantations, FarmForestry and Water: Proceedings of a nationalworkshop. 20-21 July 2000. RIRDC Publication No.01/20. 55-57.

17Theiveyanathan, T., Benyon, R., Polglase, P., Myers, B.and El-Gendy, R. (2000) Irrigation management fortree plantations in the southern Murray-DarlingBasin. In: Connellan, G.J. (ed.). Water – Essential forLife. Proceedings of the Irrigation Association ofAustralia 2000 National Conference, Melbourne, 23-25 May 2000. p. 52-61.

18Tickle, P., Keenan, R., Walker, J. and Barson, M.(2001). Can commercial tree planting help drylandsalinity mitigation and meet greenhouse gasabatement objectives? Bureau of Rural SciencesResearch Report. Canberra. (In press).

19Vertessy, R.A. (2001) Impacts of plantation forestry oncatchment runoff. In: Nambiar, E.K.S. and Brown,A.G. (eds). Plantations, Farm Forestry and Wate:.Proceedings of a national workshop. 20-21 July2000. RIRDC Publication No. 01/20. 9-19.

20Vertessy, R.A. and Bessard, Y. (1999) Anticipating thenegative hydrologic effects of plantation expansion:results from a GIS-based analysis on theMurrumbidgee Basin. In: Croke, J. and Lane, P.(eds). Forest Management for the Protection ofWater Quality and Quantity. Proceedings of the 2nd

Erosion in Forests Meeting, Warburton, 4-6 May1999, Cooperative Research Centre for CatchmentHydrology, Report 99/6, pp. 69-73.

21White, D.A., Beadle, C.L., Battaglia, M., Benyon, R.G.,Dunin, F.X. and Medhurst, J.L. (2001) Aphysiological basis for management of water use bytree crops. In: Nambiar, E.K.S. and Brown, A.G.(eds). Plantations, Farm Forestry and Water:Proceedings of a national workshop. Publication No.01/20, RIRDC Publication No. 01/20. p. 20-27.

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