CSI ROAUST RALIA

CSIRO LAND and WATER

Assessing the regional risk ofsalinization over the Dalrymple Shire

Elisabeth Bui

Technical Report 26/97, September 1997

Assessing the regional risk of salinizationover the Dalrymple Shire

Elisabeth Bui

CSIRO Land and Water

Canberra, ACT 2600

Technical Report 26/97

September 1997

Final Report to the CSIRO Land and Water Care Program

on Salinity Control (Project nr 5).

CSIRO Land and Water

PO Box 1666

Canberra, ACT 2601

Abstract

The objective of the work presented here was to develop a GIS-based salinity risk

assessment for north Queensland extension agents responsible for evaluating tree-

clearing permit applications. The work proceeded (1) to identify potential intake areas

for recharge to groundwater and potential saline discharge areas; and (2) to identify

salinity hazard areas in the upper Burdekin river basin and estimate the risk of salini-

zation after tree clearing in these areas. The following risk management strategies are

suggested:

• To lessen the risk of salinization, recharge areas should not be cleared in water-

sheds where shallow unconfined aquifers are present and where soils with %TSS

>0.25 occur.

• Where intermediate recharge areas are cleared, the introduction of deep-rooted

improved pasture species may control recharge. However, as many of these are

leguminous, they may introduce a risk of soil acidification and nitrate leaching to

groundwater.

• More detailed hydrogeological information needs to be collected so that depths to

groundwater, regional and local flow directions, and aquifers can be identified with

more certainty.

• Drainage past the root zone and recharge rates need to be quantified over a wider

range of soils.

Preface

This report details work completed in 1995. Associated results have been presented

in:

Amerasena, P., Bui, E.N., Krogh, L., Mazza, G.M., Penny, R.S., and Whiting, J.

Recharge studies in the Dalrymple Shire: Results of drilling program 1992–1993.

CSIRO Division of Soils Technical Report 19/1996.

Bui, E.N., Cannon, M.G., Penny, R.S., Beech, A., and Riley, G. On the nature and

distribution of soluble salts in the upper Burdekin River basin. CSIRO Division of Soils

Technical Report 34/1996.

Bui, E.N., Smettem, K.R.J., Moran, C.J., and Williams, J. 1996. Use of soil survey

information to assess regional salinization risk using geographical information systems.

J. Environ. Qual. 25:433–439.

Williams, J., Bui, E.N., Gardner, E.A., Littleboy, M., and Probert, M. 1997. Tree

clearing and dryland salinity hazard in the upper Burdekin catchment in north

Queensland. Aust. J. Soil Res. 35:785-801.

1

The problem

Extensive clearing of native forests in southern and western Australia has altered the

water balance in those regions, leading to increased deep drainage and consequent

rises in water tables (Williamson, 1986). Any salt stored in soil above a rising water

table is re-mobilized and can be re-precipitated close to or at the ground surface if the

new water table is <2 m deep (Williamson, 1986). Soil salinity is a severe form of land

degradation that kills most plants and renders land unusable for agricultural

production.

Areas in temperate mediterranean climates with winter dominant rainfall have been

most severely affected by secondary (resulting from human activities) salinity. In the

open eucalypt woodlands of the seasonally contrasted tropics in northern Queensland,

rainfall is summer dominant. There, tree clearing is still used as a management option

to stimulate pasture growth and increase beef production (Burrows, 1991). However,

only a small area has been cleared to date, thus, if there is a significant risk of salinity

developing after tree clearing, it is still possible to instigate policies that could avert the

problem.

Definitions

A hazard is a source of potential harm. Risk is a quantitative measure of the frequency,

or the probability, of occurrence and the consequence of a hazardous event. Environ-

mental risk assessment involves the systematic analysis of available information to

identify hazards and to estimate risk to the environment. While risk assessment strives

to be an objective, scientific endeavour, it is not divorced from the policy aspects of

decision-making because it entails risk management. During risk management, it is

appropriate to consider how risks can be minized through alternative policies, regu-

ations, or management practices.

A salinity hazard is present if salt is stored in a landscape or if water tables are shallow.

Areas at risk of salinization after tree clearing are those where salt stored is likely to be

re-mobilized and re-deposited by rising groundwater tables. Thus, assessing the risk of

salinization requires an estimate of the likelihood and amount of recharge to ground-

water after tree clearing.

Objectives

The objective of the work presented here was to develop a GIS-based salinity risk

assessment for north Queensland extension agents responsible for evaluating tree-

2

clearing permit applications. The work proceeded (1) to identify potential intake areas

for recharge to groundwater and potential saline discharge areas; and (2) to identify

salinity hazard areas in the upper Burdekin river basin and estimate the risk of sal-

inization after tree clearing in these areas. Some risk management strategies are

suggested.

Study area

The upper Burdekin river basin roughly corresponds with the Dalrymple Shire which

occupies an area of 68 000 km2 (roughly the size of Tasmania) in North Queensland

(Fig. 1). The dominant land use is extensive beef cattle grazing. The climate is season-

ally contrasted and rainfall ranges from 500 to 1600 mm, with 80% of precipitation

occurring between November and April (De Corte et al., 1994). Although the potential

evapotranspiration is high (2000–2500 mm per year), the concentration of rain over a

5–6 month period leads to filling of the soil water store and localized waterlogging

every 3 years on average (Coventry and Williams, 1984). The landscape over most of

the shire is characterized by level to gently undulating plains with long slopes and low

gradients. Mesas, low ranges, plateaux, and valleys constitute the major relief ele-

ments. Soils in the region exhibit a complex pattern, but generally parent material and

geomorphic history are the dominant factors controlling the character and distribution of

soils (Isbell and Murtha, 1970). A common soil catena consists of red and yellow earths

(Stace et al., 1968) on uplands, sodic and related soils with abrupt textural contrast on

intermediate slopes in gently undulating land or below scarps on moderately sloping

land (<10%), and cracking clay soils, often sodic, with gilgai microrelief in the lower

positions (Gunn, 1967).

Methodology

Hazard Identification

The following criteria have been used to identify the presence of a salinity hazard and

evaluate the risk of salinization: climate; vegetation cover; position in the landscape;

depth to groundwater; rate of recharge to groundwater; and presence of salt in soil

above rising groundwater or salinity of groundwater (Williamson 1986; Shaw et al.

1986; Tickell 1994).

Event-tree analysis is a hazard identification technique that uses inductive reasoning

to translate different initiating events into possible outcomes. Applied to salinity risk

assessment where tree clearing is the initiating event, it can be diagrammed as in

Fig. 2. The event-tree analysis suggested that the following questions needed to be

answered:

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At what depth are groundwater tables? Are they likely to rise after tree clearing? If yes,

how much and how fast? If yes, are salts likely to be re-mobilized?

The lack of hydrologic data precluded a thorough hydrogeologic investigation. There-

fore, the study is based on the assumption that the shallow groundwater tables would

most likely rise in the areas where they occur downstream of potential recharge areas.

Thus, the central question is: where are the recharge areas? This assessment of the

risk of salinization for the region involves the integration of topographical, hydrological,

hydrogeological, and soils data.

The GIS database

The following were used to create the GIS database:

1. Digital elevation data on an 18" (about 500 m) grid and drainage at 1:1 000 000

were purchased from the Australian Surveying and Land Information Group (AUSLIG).

A digital elevation model with a 600 m grid cell size was constructed and used to

derive geomorphic parameters such as stream network, stream order, watersheds,

slope angle, and compound topographic index (cti). The cti is an indicator of soil

saturation or wetness and is defined as ln (As/ tan a) where As is the specific

catchment area above a given cell and tan a is the slope angle at that cell (Moore et

al., 1991).

2. A digitized copy of Groundwater Resources of Queensland map (1:2 500 000), and

point borehole data for depth, electrical conductivity (EC), and major ions (Queensland

Department of Primary Industries, Water Resources) were the main sources of hydrol-

ogy information. More borehole data were obtained from Queensland Department of

Lands records for the northern end of the shire. The bores were drilled over several

decades at different times of the year so that water levels derived from them at the time

of drilling are uncertain.

Depth to groundwater seams were obtained from the bore records and were subtracted

from topographic elevation to calculate the elevation of a groundwater or aquifer sur-

face. Plots of elevation at the aquifer surface against topographic elevation (Fig. 3a),

and of elevation of the potentiometric surface against aquifer elevation (Fig. 3b) sug-

gest that there are two populations, one of unconfined and one of confined aquifers,

because unconfined aquifers should express a 1:1 linear relationship between ele-

vation of aquifer and potentiometric surfaces. The unconfined bore data were used to

approximate a depth to groundwater surface.

3. Available soil surveys included:

(a) the digital Atlas of Australian Soils (1:2 000 000) (Northcote et al., 1960–68), avail-

able from the National Resources Information Centre (NRIC) and consisting of digitized

maps with the legend symbol as the only attribute attached to any polygon;

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(b) hard copies of the soils map covering the upper Burdekin basin published at 1:

1 000 000 by the Australian Department of National Development (Isbell and Murtha,

1970); and

(c) about 2000 point observations and measurements of pH and electrical conductivity

(EC1:5) on profiles from the survey sites from the on-going Dalrymple Shire Collabor-

ative Land Resources Survey. The map from the Dalrymple Shire Collaborative Land

Resources Survey will be available at a scale of 1:250 000 (Rogers et al., in press).

Digital elevation, bore hole, and soil attribute data have been manipulated using TIN

and GRID modules in ARC/INFO. More detail on the procedures involved in interp-

olating raw data to create surfaces is given in Locsey (1994). The risk assessment

work proceeded with a cell size of 600 m.

Watersheds were used as the basic land management unit in the hazard and risk

assessment for the following reasons:

• because “the natural and cultural fabrics of a watershed are manifestations of the

interactions of biophysical processes, including the hydrological cycle, and socio-

economic activities" (Thapa and Weber, 1995);

• what happens upstream impinges on what happens downstream;

• because the Queensland state government has adopted an "integrated catchment

management" approach to environmental management and property planning;

• to integrate errors in the positional accuracy of saline soils or shallow water tables

over a large area.

Because the average size of properties is around 15 000 ha and a workable catchment

group consists of about 10 properties, a desirable watershed size is 150 000 ha; this

size corresponds generally to a third-order watershed at the scale used in the project

(600m grid cell size). The digital elevation model was used to generate a stream net-

work closely resembling the AUSLIG 1:1 000 000 drainage, and to delineate watershed

boundaries. The junction of third- and fourth-order streams with higher-order ones was

used as the pour point for the watersheds. A total of about 50 watersheds was

obtained.

Inferring Hydrological Organization from Soil Surveys

In the absence of sufficient hydrological and soil physical data, the digital Atlas of

Australian Soils (1:2 000 000) (Northcote et al., 1960–68) was used to infer catchment

behaviour. Soil map unit descriptions include information on soil-landform

relationships, parent material, soil thickness, depth to bedrock as well as dominant and

sub-dominant soil types.

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To approximate recharge areas in the Dalrymple Shire, the digital Atlas of Soils map

units were assigned codes for the attributes permeability (high, medium, or low) and

drainage (well, moderate, or poor) on the basis of the dominant principal profile form,

soil thickness, underlying bedrock, and topographic position described for the map

units (Appendix). Because there was no attribute table containing the soil map unit

information linked to the digital Atlas of Soils, this was done qualitatively and manually

using the following heuristic rule as an example: if the dominant soil in the map unit

has a deep profile, with gradual clay increase (e.g., a Gn2.1 in the Northcote (1971)

scheme) and occurs on a plateau, it will be highly permeable and well-drained. The

tacit assumption is that the soils occur in a stable landscape. Soil map units with

permeability code of 'high' and drainage code of 'well' were assumed to correspond to

potential intake areas for recharge to groundwater.

Topographic lows are areas where weathering and erosion products accumulate and

can form authigenic, often smectitic, clay minerals, especially in regions with low slope

gradients. Potential discharge areas were assumed to correspond to soil map units that

occur in low spots on the landscape and that have low permeability and poor drainage.

Spatial statistics

Point observations with measurements of electrical conductivity (EC1:5) on 1721 pro-

files from survey sites in the region (Fig. 4) were used to calculate weighted profile

means for % total soluble salt (TSS) as follows. The EC1:5 value for a horizon was

multiplied by 0.336 to convert it to %TSS (Shaw et al., 1986). The %TSS of a sampled

horizon was multiplied by the horizon thickness and the sum of these products was

divided by the total thickness of the sampled profile (Oertel and Giles 1963). Soils with

an EC1:5 >0.7 dS m–1 can be considered saline; this corresponds to a %TSS >0.25.

The % TSS data were used to test the hypothesis that the spatial association between

saline soils and discharge and recharge areas is not random. Of 1614 sites sampled

within the Dalrymple Shire, 79 were considered saline (%TSS >0.25). All sites were

further than 250 m apart, therefore, a 250 m grid was used for the test. Each grid point

(pixel) was given two labels: distance to nearest discharge and distance to nearest

recharge area. First, the spatial distribution of all profiles was tested for randomness by

locating 1614 points randomly over the grid. A hit was recorded for the i-th distance

class each time a random point was located at that distance. Exactly 1000 iterations

were performed and the mean number of hits for each distance class was recorded to

provide the distribution function of distance for complete randomness. This was

repeated using 79 points to compare the occurrence of points within discharge and

recharge areas. The distances from each of the sampled sites to the nearest recharge

and discharge areas were recorded. Not all distance classes have the same

frequencies; therefore, more hits are expected in the classes that have the greater

6

frequencies. To normalize this effect, test results are expressed as the number of hits

per unit area.

Risk assessment

Model

A multi-criteria evaluation procedure was developed to combine the hazard criteria and

arrive at a relative risk assessment map showing areas at high, medium, or low risk of

salinization in the upper Burdekin catchment. The following decision rules were imple-

mented:

On any given catchment, if a recharge area is present and %TSS >0.25 and depth

to groundwater <6 m, then the risk of salinization after tree clearing is high.

On any given catchment, if a recharge area is present and %TSS >0.25 and depth

to groundwater >6 m but <20 m, then the risk of salinization after tree clearing is

medium.

On any given catchment, if a recharge area is present and %TSS <0.25 and depth

to groundwater <6 m, then the risk of salinization after tree clearing is medium.

On any given catchment, if a recharge area is present and %TSS <0.25 and depth

to groundwater >20 m, then the risk of salinization after tree clearing is low.

Where are potential recharge and discharge areas?

In the catena described by Gunn (1966) in Queensland, relief and lateral water flow

are responsible for geochemical differentiations reflected in soil type. Highly weathered

soils enriched in quartz sand and sesquioxides occur on uplands, sodic soils are found

on intermediate slope positions, and smectitic soils are in topographic lows. Dissol-

ution, translocation and chemical reactions between weathering products within the

landscape lead to the observed pattern of soils. These processes have operated since

the Tertiary and are analogous to those described by Bocquier (1971) in Chad where

the environment is similar. Thus, the assumption underlying the interpretation of soil

map unit descriptions that the landscape is at equilibrium is reasonable in terms of

pedogenetic models.

To validate the pedological model used to approximate recharge and discharge areas,

the spatial distribution of EC or TSS data were compared to potential recharge and dis-

charge units (Fig. 5). Table 1 summarizes the comparison of location of points within

recharge and discharge areas. Recharge areas should be leached and have low

salinity and discharge areas should have high EC or TSS. That is indeed the case.

Only 2 out of 104 sampled points falling within recharge areas were saline (fewer than

expected if random). Saline points were more frequent than expected in discharge

areas (15 out of 127 points).

7

Table 1. Number of points falling within recharge and discharge areas for random

simulations and measured profiles.

Number of points at distance zero Recharge Discharge

All profiles 104 127

Random 1614 points 232 72

Saline profiles 2 15

Random 79 points 11.3 3.6

Whereas the total point distribution (Fig. 4) is not random with respect to distance from

recharge and discharge areas, the saline points appear closer to discharge areas and

further from recharge areas (Fig. 6). Saline points are more frequent than random

points within 30 km of a discharge area, randomly distributed within 20–25 km of a

recharge area, and less frequent than random at distances >25 km of a recharge area.

Saline discharge areas are often geologically controlled, by stratigraphic relationships

or by structures such as faults or dykes, however spatial statistical results suggest that

the pedological model for inferring catchment behaviour appears to have merit.

Where are the risk areas?

A salinity hazard is present if salt is stored in a landscape or if water tables are

shallow. Thus, Figs. 7 and 8 correspond to the contribution to salinity hazard from soils

and unconfined aquifers, respectively. We can assume that:

• watersheds with high salinity hazard are those with shallow groundwater and salinesoils.

• those with medium salinity hazard are those that have deeper groundwater andsaline soils; and those that have shallow, non-saline groundwater and non-salinesoils.

• those with low salinity hazard have deep groundwater and non-saline soils.

Risk is the probability that a hazard will become a problem. Areas at risk of salinization

after tree clearing are those where salt stored is likely to be re-mobilized and re-

deposited by rising groundwater tables. Thus, assessing the risk of salinization

requires an estimate of the likelihood and amount of recharge to groundwater after tree

clearing.

Field work in vegetated areas suggest that recharge of ~17 mm/yr occurs on red earths

(Coventry and Williams, 1984). Water balance simulations using field-measured soil

properties showed that tree clearing on likely intake areas can increase deep drainage

up to 10 times (Williams et al., 1993). Moreover, recharge, albeit slower, is expected to

occur over the whole landscape.

8

Tree clearing in southern Australia has led to 100 fold increases in recharge to ground-

water with land salinization problems developing after 50–100 years (Allison et al.,

1990). While the soil-climate interaction in the wet/dry tropics is similar to that in the

winter-dominant rainfall areas of Australia in that tree clearing increases deep

drainage, the recharge rates are lower. Thus salinity problems may take longer to

develop in northern Australia but the risk still exists.

If we assume that the watersheds with the shallow unconfined water tables will respond

the fastest, then the qualitative ranking of risk areas is equivalent to the hazard areas.

These are shown in Fig. 9. However, it is impossible to validate the regional salinity

risk assessment at this stage short of wholesale tree clearing over the upper Burdekin

basin.

Uncertainty

Uncertainty in the risk assessment results from model uncertainty, (i.e., are the right

decision rules being implemented?), and from data uncertainty, (i.e, positional and

measurement accuracy). The advantage of using a catchment as a land management

unit is that high salt content does not have to be present in the same grid cell as

shallow groundwater. Using catchments recognizes the fact that rising groundwater in

one cell might affect a neighbouring grid cell with a high salt content. Thus positional

data uncertainty is spread over a number of cells in a catchment and reduced for the

catchment overall. The highest level of uncertainty occurs at the edges of a catchment

if those edge grid cells are the saline ones with shallow groundwater that lead to a

'high risk' classification.

Model uncertainty arises from the assumptions made to identify potential recharge

areas and in the selection of hazard criteria. Spatial statistics provided some validation

of the pedological model used to select potential recharge areas. An attempt to verify

the outcome of the risk assessment was made by introducing cti in the model. Where

the cti is greater than the median, soils should be saturated and water should sit in the

catchment rather than drain away as surface water. Catchments with >30% of their

area with cti >12 are shown in Fig. 10. The catchments designated at risk by the initial

model all have ≥30% of their area with cti >12, thus supporting the assessment that

they would very likely become salinized if trees were cleared.

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Recommendations for risk management

To lessen the risk of salinization, recharge areas should not be cleared in watersheds

where shallow unconfined aquifers are present and where soils with %TSS >0.25

occur.

Where intermediate recharge areas are cleared, the introduction of deep-rooted im-

proved pasture species may control recharge. However, as many of these are legumin-

ous, they may introduce a risk of soil acidification and nitrate leaching to groundwater.

More detailed hydrogeological information needs to be collected so that depths to

groundwater, regional and local flow directions, and aquifers can be identified with

more certainty.

Drainage past the root zone and recharge rates need to be quantified over a wider

range of soils.

10

References

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1990. Land clearance and river salinisation in the western Murray basin,

Australia. J. Hydrol. 119:1–20.

Bocquier, G. 1971. Genese et evolution de deux toposequences de sols tropicaux du

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Burrows, W. H. 1991. Tree clearing in perspective-a question of balance. Search

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Coventry, R. J., and J. Williams. 1984. Quantitative relationships between morphology

and current soil hydrology in some alfisols in semiarid tropical Australia.

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Rogers, L.G., Cannon, M.G., and Barry, E.V. (in press). Land Resources of the

Dalrymple Shire. Queensland Dept. of Natural Resources.

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landscape, soil, and water salinity – processes and management options. Part A.

In Landscape, soil and water salinity, Proc. Burdekin Regional Salinity Workshop,

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Washington, DC.

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54/94D. Water Resources Division, Power and Water Authority of the Northern

Territory, Darwin, NT, Australia.

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and dryland salinity control in the upper Burdekin catchment of North

Queensland. p. 135-145. In Proc. Nat. Conf. Land Management for Dryland

Salinity Control, Bendigo, Victoria, Australia. Sept. 28 – Oct. 1, 1993.

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and Revegetation Research 5: 181–196.

Fig. 1. Location of the Dalrymple Shire in Australia.

Fig. 2. Event-tree analysis for assessing risk of salinization after tree clearing.

Fig. 3. Groundwater relationahips: )a) elevation of aquifer surface against topographicelevation and (b) elevation of potentiometric surface against topographic elevation.

Fig. 4. Distribution of sampled sites in the Dalrymple Shire.

Fig. 5. Distribution of estimated recharge and discharge areas in the Dalrymple Shire withoverlay of saline sites. Saline soils are found on recharge areas (2 out of 90).

Fig. 6. Probability density functions of distances from (a) discharge and (b) rechargeareas of measured and random point processes.

Fig. 7. Distribution of saline soils in the Dalrymple Shire.

Fig. 8. Depth to groundwater from borehole data.

Fig. 9. Salinity risk ranking of catchments in the Dalrymple Shire using soils salinity anddepth to groundwater as the principal hazard identification criteria.

Fig. 10. Incorporating cti in the salinity risk assessment for the Dalrymple Shire.