Reusing Irrigation Water

8/12/2019 Reusing Irrigation Water

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Number 3 October 2000 ISSN 1 3265350

Communityand governmentworking together

to manage salinityin NSW

Draining & Reusing Irrigation Water

Good irrigation management can bedivided up into two parts: getting water

on when it is needed, and then gettingthe excess water off the paddock asquickly as possible.

Better irrigation management leads tohigher productivity, both in the shortand the long term; and reducedenvironmental impact.

Crops and pastures will benefit fromless moisture stress, the result ofappropriate irrigation supply andscheduling (Right Amount, Right Time)and reduced waterlogging (due toimproved drainage).

In the longer term, water and pumpingcosts can be lowered, waterlogging and

salinity problems minimised andwatertables controlled.

Waterlogging

A well structured soil has spacescontaining water and airbothnecessary for plant survival andgrowth (refer to Irrigation SalinityFacts: Principles of Surface Irrigation). Asoil at field capacity still contains asignificant number of air-filled voids.In a waterlogged soil the spaces inbetween the soil particles are filledwith water, so plant roots have noaccess to oxygen.


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Irrigation Salinity Facts2

Different plants react to this indifferent ways. Most agricultural plants(except rice) suffer as oxygen andnutrient uptake through the rootsslows. Rushes, docks and sedges thrive

and will out-compete more desirablespecies. In addition, beneficial soilmicrobes are destroyed, plant andanimal diseases spread, andmachinery and livestock damage theweakened soil structure.

Research has shown that summerpasture growth is reduced after onlysix hours of water ponding, and thatwater regularly lying on perennialpastures (white clover and ryegrass or

paspalum) for 2448 hours will reducegrowth by 25% over an irrigationseason, and lower the clover content.Rice is the exception, as it is naturallyan aquatic plant, so it tolerateswaterlogging.

Soil will remain waterlogged until theexcess water is removed by plant use,evaporation or drainage. If pondedwater remains on the soil surface,

Effects of waterlogging

nutrient uptake stops and theroots die

plants have no energy for growth beneficial soil microbes die

machinery and livestock candamage soil structure

plant and animal diseases occur water-loving weeds like rushes,

dock and sedge thrive pastures can be scalded by

standing water heating up on ahot day

some nutrients (especiallynitrogen) are leached downthrough the profile below theroots, where they are not used

A drainagereuse systemwill increase

wateravailable to

the farm by10-25%

film of water aroundsoil particles and in small pores

air spaces soil particleswater occupying

pore spaces

Figure 1: A well-structured soil has both air- and water-filled spaces (pores) in the profile (left). In a

waterlogged soil (right) the pores are filled with water,

depriving the roots of oxygen and slowing nutrientuptake and plant growth.

some will drain down through the soilprofile, adding to the local watertable.This seepage can cause risingwatertables and soil salinity, leading toenvironmental damage and productionlosses.

Waterlogging becomes an even greaterproblem in areas with shallowwatertables. Soils that becomewaterlogged following rainfall or

irrigation will stay saturated for longerperiods because there is lessopportunity for downward seepage andflushing of salts.

Rainfall is the major cause ofwaterlogging in the cooler months ofthe year, while poor irrigation is themajor contributor to waterlogging inspring, summer and autumn.

What can be done toavoid these problems?The key to improving irrigationefficiency (and reducing

waterlogging) is to improve paddocksurface drainage.

Reusing this drainage water will helpto improve overall irrigation efficiency.

Paddocks can be drained by surface orsub-surface systems.


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3Irrigation Salinity Facts


Surface drainage

Good surface drainage will only beachieved by having a layout withadequate slopes, suitable bay lengths

and a drainage system to removerunoff water from the bottom of thepaddock as quickly as possible (seeIrrigation Salinity Facts 2: Principles ofSurface Irrigation).

The drains should be able to takesurplus water away from everypaddock on the property. All paddocksshould be drained quickly, preferablywithin 24 hours. This will minimisewaterlogging losses and reduce the

amount of water seeping through tothe watertable.

Poor surface drainage is usually moreobvious on heavier soils. These soilstend to be flatter, as well as havingpoorer internal drainage (i.e. drainagedown through the soil). The effects ofpoor drainage are less obvious onlighter soils because excess water cansoak away if the watertable is deepenough. Efficient drainage of theselighter soils is very important in

preventing rising watertables.

Improving surface drainage

Techniques that can be used toimprove surface drainage include: making the slopes of paddocks

steeper than 1:800 so that wateris less likely to lie on the paddock.

making bays shorter so that waterspends less time draining fromthe top of the bay to the bottom.

keeping drains and toe-furrows

clean and well-maintained at alltimes to make sure that waterflows away quickly. This can beachieved by making drains thatare self-draining, and cleaningtoe-furrows if they are filled in atharvest time. Ponded water willencourage aquatic weed growth,which slows down the flow ofwater, lowering head andincreasing recharge.

using a rotary drainer to makespinner cuts from the waterloggedor low-lying areas to the drain atthe bottom of the paddock. This

may be necessary on paddocks thathave not been laser-levelled or areflatter than 1:800 (and should beincorporated down the centre allbays with slopes flatter than1:1200). Spinner cuts can alsoimprove drainage on bays withheavy pasture growth.

Practices to avoidThere are a number of practices to avoidwhen draining paddocks: drains should not be so deep that

they run through sub-surface sandbecause this will lead to waterseeping out of the drain down tothe watertable.

drainage lines should not cutthrough an existing high watertablebecause water from the watertablewill enter the drain. If thegroundwater is saline and drainage

is being recycled for irrigation, thequality of irrigation water will belowered.

excess irrigation water should notbe directed into district drains. Thiswater may contain unacceptablelevels of salts, nutrients orchemicals and should be recycledon-farm.

gravel pits should not be used fordrainage disposal. It may appear tobe an easy solution in the short

term, but gravel and sand pits areinvariably part of the localgroundwater system.

paddocks should not be drainedonto roadsides or into other pondedareas. This water seeps through tothe watertable and can causesevere damage to road foundations,costing shire councils (andratepayers) large amounts for roadmaintenance.

The key toimprovingirrigation

efficiency is

good drainage

Benefits of a drainage reuse system

increase water available on-farm by 1025% improved farm drainage greater flexibility in irrigation management

retention of nutrients & chemicals on-farm


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Sub-surface drainage

Sub-surface drainage involvesremoving water from the soil after ithas passed below the soil surface, andbefore it reaches the local watertable.Tile or mole drains and groundwaterpumps are the most common methodsof subsurface drainage.

Tile drainageTile drains are networks of small

drains (made out of terracotta orplastic pipes) collecting sub-surfacewater from all over a paddock (Figure2). These empty into a mainline whichleads to a collection sump. From herethe water is pumped out to a drainagechannel or reuse system.

Tile drainage is only effective wherelighter soil types allow good sub-surface water movement. The costs oflarge scale tile drainage can be very

high and it is generally used only inconjunction with high value crops.

Mole drainage

The internal drainage of some soiltypes may be improved by formingsmall enclosed channels in the soil at adepth of 50 or 60 cm. This can be doneby using a mole drainer.

The effectiveness of this techniquedepends on the stability of the soiltype. Mole drains will collapse quicklyif the soil is not suitable. Research

carried out by CSIRO at Griffith hasshown that mole drainage is a viablemanagement option for controllingwaterlogging and salinity in the rootzone of irrigated crops on self-mulching grey clays. Refer to IrrigationSalinity Facts: Using Mole Drainageformore information.

Groundwater pumping

In some areas it is possible to pump

water from highly permeable aquifers(beds of gravel or sand in the soil).

On-farm groundwater pumping usuallytakes water from shallow aquifersusing a spearpoint system.

The effectiveness of groundwaterpumping for controlling watertablesdepends on how much water ispumped, how quickly the aquifer fillsup again and the permeability and

thickness of the aquifer.

A thick aquifer of coarse sand or gravelwill usually mean that the water isdrawn from a large area, whereas anaquifer made up of a thinner band offiner sands will usually yield less waterand may be less effective in loweringthe watertable over a wide area.

The water pumped may be used forirrigation, depending on its salinity(see the Salt Action Information Sheet:Using Shallow Groundwater forIrrigation).

Irrigated grapevines

Groundwater

Mainline

Feeder

line

Water pumpedfrom sump

to drainage channelor retention basin

Sump

Figure 2:A tile drainage

system canintercept deepseepage from

high value cropssuch as grapes.


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Reusing drainage water

Making full use of drainage water is animportant part of water management.

A good reuse system will capture allpaddock runoff, with the ability tostore water for later use.

Storing and reusing drainage waterand rainfall has a number ofadvantages: Water allocation on the farm can

effectively be increased by 1025%, depending on the standardof irrigation management (throughsaving water otherwise lost as

drainage). Reuse gives greater flexibility inthe timing of irrigations. Watercan be applied to crops andpastures when they need it,rather than having to wait a fewdays for a water order to beprocessed, or having to deal withrationed flows at times of peakdemand. Also, groups of bays orfurrows can all be irrigated andshut off to suit the slowest,without loss of water from the

quicker bays. Irrigation management is easier

and more flexible. If the wheel isturned off before the irrigation isfinished, stored water can be usedto finish the watering. Likewise, ifthe watering is finished but thewheel wont be turned off foranother half a day, the excess canbe stored and reused later.

Reuse systems can providegreater supply flows so each

paddock can be watered faster,depending on farm channelcapacities and irrigation layoutallow it.

By keeping drainage water on thefarm, the pressure on districtdrainage schemes will be reduced,particularly after heavy rainfall.

When draining recently fertilisedpaddocks, up to 20% of thefertiliser applied can be flushedout with the drainage water. If notrecycled back onto paddocks,these nutrients are lost to thefarm, costing money. There are

also limits for nutrients andchemicals entering localwaterways.

If the farm does not have accessto district drainage, mishaps such

as rainfall immediately afterirrigation can be handled withoutthe farm becoming severelywaterlogged.

A reuse system can be used toirrigate otherwise uncommandedland, with the aid of pumps.

In rice cropping, the crop can bedrained promptly at the righttime, and the water reused on apasture or other crop. Thissimplifies the critical issue of

shut-off of irrigation.

Figures 3 and 4: Poor drainage managementwastes water, costing you money

and causing seepage through to the watertable.


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Principles of a good reuse system

The design and cost-effectiveness of reuse systemsvaries from farm to farm and from region to region.

Still, the following general principles are relevant tomost districts: Integrate all developments of the irrigation layout

so that the farm will work as a complete system.Follow a whole farm plan or IDMP.

Aim to be able to cope with average rainfall andirrigation runoff at the same time. To achieve this,drains need to be well-designed, having thecapacity to handle large flows.

The system should be simple and reliable to ensureease of operation and minimise operating costs.

Drains need to be well maintained and should

empty out quickly to minimise weed problems. Pump the recycled water as close to the mainsupply point as possible. This will maximise thearea over which the water can be used. This willalso make shandying poor quality water easier.

Bring all drains back to one pumpsite. Command as much of the farm as possible. The pumpsite should be easily accessed for

operation and maintenance.

Design the on-farm drainage recycling system to allowaccess to the district drainage scheme, if available.During heavy rainfall, runoff from landformed country

occurs very quickly. If the on-farm system cant handlethe water then district drainage can be used to disposeof the excess water, if more than 12.5mm of rain hasfallen in that rainfall event.

Quality of RecycledWaterThe salinity of drainage water can vary

depending on the salinity of the soiland the quality of the irrigation water.If the water from sub-surface drainageis of acceptable quality, it can bereused for irrigation. If not, disposal ofthis water can be a major problem.However, by recycling water andmanaging drainage quantities; smallervolumes of saline drainage water willbe produced, and so little will need tobe disposed of.

Be careful when irrigating saline soiland reusing the excess water: thedrainage water will have dissolvedsome salts from the paddock.

Regularly monitor the quality of reusewater to make sure it does not reach alevel which will damage crops andpastures. If salinity levels are above 0.8dS/m, consider shandying the recycledwater with channel water (see SalinityNote 7: Water Salinity Guidelines).Hand-held salinity meters are available

to quickly test water on-farm andshould always be used to test drainagewater before it is reused.

When reusing drainage water becareful not to damage one crop withthe chemicals from another. Forexample, drainage from a maize cropthat has been treated with Atrazineshould not be reused on soybeansbecause the water may becontaminated with the herbicide which

will damage the sensitive soybeans.

This may necessitate careful design ofthe supply and recycling systems socontaminated water can be divertedaway from sensitive crops such as rice.

Due to the draining of chemicals andsalts into the reuse system, the storageshould not be used as a supply fordomestic use.

Figure 5:A farm storage built as part of a reuse system allows

drainage water to be recycled for later use.


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Building a Drainage andReuse System

Planning

Good on-farm water managementrequires farm drains at the end ofevery bay collecting water to a centralpoint, an outlet to district drains and afacility to allow the storage and reuseof irrigation and rainfall runoff.

Planning is best done on a whole farmbasis, so the layout is integrated andefficient. Remember planning on paper

is less costly than moving dirt.

Your reuse system should: recycle water over as great an

area of the farm as iseconomically possible to maximiseflexibility. Seasonal drainageshould be able to be used on sameseason crops and pastures.

be able to cope with all irrigationrunoff, and the first 12.5mm ofrainfall.

be simple and reliable to make

sure that it is easy to operate. be designed to drain out well to

minimise weed problems. be capable of handling storm

runoff flows, which often exceedirrigation drainage flows.

Direct the recycled water to as close tothe wheel as possible. This willmaximise the flexibility of the systembecause you will be able to use yourrecycled water everywhere channel

water is used. It will also makeshandying slightly saline water easier(see Salinity Note 7: Water SalinityGuidelines).

Ideally the whole property should becommanded to give maximumflexibility. However, some areas mayhave to be left out because it isuneconomic to command them.

Equipping the system

StorageThe storage should be large enough tohold all the drainage water from eachirrigation and from unexpected rainfall.It is preferable to collect water bygravity rather than having to pump itinto storages. The more storageavailable within the drainage lines themore flexible the system will be.

Recycling dams must be placed on

suitable soils (sufficient depth of claybeneath them) to minimise loss ofstored water through the dam bed intothe groundwater.

Points to be considered when workingout the volume required for storage: What is the size of the area that

will be irrigated with this water? How much water is to be applied? Is the water to be harvested run-

off only, or from other sources,such as creeks, neighbours or

roadsides? Is it possible to put overflow into

the regional drainage system? Does it meet the requirements of

the EPA and irrigationauthorities?

For more information, refer toIrrigation Salinity Facts 5:On-farmStorages and the WaterWisepublications.

PumpsA reuse system will need to include away of getting the water back into theirrigation system. This involves using apump of some sort.

Usually the most economic pump siteis some point between the supplychannel and drainage channel. If astorage dam is included in the reusesystem the pump should be located atthe storage site. The pump site shouldbe easily accessible, making it moreconvenient to use and maintain.


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This information has been produced as part of Salt Action, the community

and government initiative for managing salinity in NSW.

Authors:Richard Swinton, NSW Agriculture, GraftonSimon Gibbs, Salt Action, NSW Agriculture, ForbesAllan Grogan, Works Publishing, Exmouth

Design, editing, artwork and layout:Simon Gibbs, Salt Action, NSW Agriculture, Forbes

With thanks to:David Rosenbaum, formerly NSW Agriculture, DeniliquinChris Rolfe, NSW Agriculture, Wollongbar

For more information, contact your local Irrigation Officeror Salinity Officer at NSW Agriculture:Condobolin (02) 6895 1007 Forbes (02) 6850 2922Dareton (03) 5027 4409 Griffith (02) 6960 1300Deniliquin (03) 5881 9999 Yanco (02) 6951 2611Macquarie Valley Landcare, Trangie (02) 6888 7778

NSW AgriculturePrinted on recycled paper by ActivePrint

Diesel

Advantages

Can be located anywhere Flow rates can be easily altered

Disadvantages

High cost to purchase High maintenance requirement Short life Transmission costs can be

expensive for higher horse power

Electric

Advantages

Cheaper to purchase pump Reliable Long life Easy to automate

Disadvantages

Very high cost involved in gettingpower to site

Fixed flow rates

Comparison of power supplies for reuse system pumps

The pump and power unit should be

selected to be as efficient as possiblefor the task required. This willsignificantly reduce operating costs.Poorly selected or mismatchedequipment can double or even triplethe cost of recycling your irrigationwater. A less efficient pump mayrequire a much larger and moreexpensive power unit.

The type of power unit used to drive

the pump (electric or diesel) dependson individual circumstances. Someadvantages and disadvantages ofdifferent pumps are detailed above. Formore information, see IrrigationSalinity Facts: Reuse Pump Systems.

Documents

Reusing Irrigation Water