Managingwater betterThe agronomic, economic and environmental benefits of irrigation scheduling

Copies of this booklet can also be downloaded from the UK Irrigation Association website www.ukia.org

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This publication forms part of a project funded by Defra (Innovation andTechnology Network) and the Environment Agency. The project was undertakenby Cranfield University in association with RTCS Ltd. The authors of thispublication – Jerry Knox and Tim Hess (Cranfield University) and Melvyn Kay(RTCS Ltd) wish to make it clear that the content of this publication and the viewsexpressed are those of the authors and do not necessarily represent the views orpolicies of the organisations listed above.

Acknowledgements

Hess, T. M., Knox, J.W., and Kay, M.G. (2008). Managing water better.The agronomic, economic and environmental benefits of irrigation scheduling. An information booklet produced for Defra Innovation Network and theEnvironment Agency. Cranfield University.

Further information:

Booklet produced by:

Tim Hess Natural Resources DepartmentCranfield UniversityCranfieldBedfordshire MK43 0AL

T: 01234 750111 ext.2763E: t.hess@cranfield.ac.ukW: http://www.cranfield.ac.uk/sas/

aboutus/staff/hesst.jsp

Jerry KnoxCentre for Water ScienceCranfield UniversityCranfieldBedfordshire MK43 0AL

T: 01234 758365E: j.knox@cranfield.ac.ukW: http://www.cranfield.ac.uk/sas/

aboutus/staff/knoxj.jsp

Melvyn KayUK Irrigation Associationc/o Moorland House10 Hayway, Rushden Northants NN10 6AG

T: 01427 717627E: m.kay@ukia.orgW: www.ukia.org

Innovation NetworkWarwick HRIWellesbourneWarwick CV35 9EF

T: 02476 575073W: www.warwick.ac.uk/climatechange

Centre for Water ScienceCranfield UniversityCranfieldBedfordshire MK43 0AL

T: 01234 758365W: www.cranfield.ac.uk

Environment AgencyKingfisher HouseGoldhay Way, Orton GoldhayPeterborough PE23 5ZR

T: 08708 506506W: www.environment-agency.gov.uk

UK Irrigation Associationc/o Moorland House10 Hayway, Rushden Northants NN10 6AG

T: 01427 717627W: www.ukia.org

Supported by:

5343 - Managing Water Better 7/11/08 16:40 Page 1

longer extract it. Plants then wilt and are unable torecover even when more water is added. This ispermanent wilting point.

The critical soil water deficit is different for differentcrops and growth stages. Figure 1 shows a typicalirrigation schedule for potatoes during a drysummer. The critical deficit is low early in theseason and small, frequent irrigations are necessary

to prevent scab. A much larger deficit is thenallowable later in the season and larger, lessfrequent irrigations encourage bulking.

It is important to know what the critical soil deficitsare for the crops you grow on your local soil types.Good irrigation management ensures that thecritical soil water deficit is not exceeded at all thedifferent growth stages.

Survey of scheduling practices

Feeling the soil and walking the crop are essential forgood irrigation management, but you need tocombine this with more objective methods. A surveyof irrigation in England (2005) showed that 60% ofthe total irrigated area is now scheduled usingobjective (scientific) approaches (Figure 2). But manygrowers still rely solely on their judgement or intuition.

Scheduling – the basics

The objective of irrigation scheduling is tomaintain optimum soil water conditions for cropgrowth in order to meet crop yield and qualitytargets with minimum water wastage.

Under-irrigation is not desirable. It can causeplant water stress resulting in reduced yields.Poor timing of irrigation at key times can alsoimpact on crop quality. Potatoes, for example,can develop scab during tuber initiation andreduced tuber bulking later in the season. Thisimpacts on market grade, tuber quality, andultimately on contract price.

Some growers over-irrigate to make sure theircrops have enough water. But this too causesproblems – wastage of water, labour and energy,and fertiliser leaching. It can also aggravate soil erosion.

Getting it right

Start by thinking about your soil as a reservoir forstoring water. Different soils hold different amountsof water but not all of it is available to your plants.You need to be aware of four levels in your soilreservoir – saturation, field capacity, critical soilwater deficit, and permanent wilting point.When the soil reservoir is full your soil is saturatedand all the pores between the solid particles are

filled with water. Plants do not like this conditionbut natural drainage soon empties the larger poresand reduces the soil water level to field capacity –a much more favourable condition when plantroots can easily take water from the soil. Sandysoils lose a lot of water during drainage and at fieldcapacity may only be 10% water. Clays can usuallyhold much more water – up to 40%.

As plant roots continue to take water from the soilreservoir they find it increasingly difficult to extract it

from the smaller soil pores. Eventually a criticalsoil water deficit is reached when the rootscannot extract water fast enough, the plantsbegin to stress, and crop yield and quality areaffected. At this point the soil water reservoirneeds irrigation or rainfall to fill it back up to fieldcapacity. If water is not added at this point theplants can still take some water. But as the soilreservoir empties the remaining water is held sotightly in the smallest soil pores that roots can no

For Stewart Alcock,Technical Manager atG and BB Houlbrooke,Ledbury (Hereford), thebenefits of schedulingfor strawberries areclear. His mainobjective is to “addvalue” – and beingable to meet cropwater requirements

with small, frequent water applications using trickle irrigation is where schedulingpays dividends.

The farm specialises in growing Grade 1strawberries under polytunnels for the majorsupermarkets which rely on English fruit frommid-May to mid-Oct. Since 1993 they have useda commercial scheduling service (Agri-Tech NP) in order to match crop water demand with supply on 100 acres of main crop and everbearer varieties.

At critical growth stages it is important not toover-water otherwise the fruit becomesexcessively soft, and difficult to pick and pack.

In the past, they have relied on neutron probes tomonitor changes in soil water, but this year theytested IrriWise™ – a scheduling system that usestensiometers buried at different depths. Soil watersuction data are automatically relayed back to thefarm office. This ‘real time’ soil water statusmeant that irrigation could be fine-tuned fordifferent growth stages and weather conditions.This first year, the tensiometer data werecompared with neutron probe data in order tobuild confidence in the new system. A DavisVantage™ weather station keeps a daily checkon evapotranspiration (ET) rates.

Strawberries are short season, shallow rooting,and sensitive to drought – especially underpolytunnels where water stress can quickly buildup even at moderate outside temperatures.Stewart says “The benefits of getting irrigationright are substantial. Grade 1 strawberries can beworth £3,000/t. But if you get the irrigationwrong, then Grade 2 strawberries are not worthpicking! For me, knowing daily crop water needsand keeping a close eye on changes in soil watermakes my life much easier – when it comes towater, strawberries prefer to ‘snack’ little andoften rather than ‘binge’!”

Why schedule?

Managing irrigation better is about optimising your soil watermanagement practices to improve crop yield and quality. It willalso improve your irrigation efficiency – something you willneed to demonstrate to renew your abstraction licence andmeet supermarket grower protocols.

Optimising your soil and water management practices is about applying the rightamount of water at the right place and at the right time. To do this you will needto continually make two important irrigation decisions – when to irrigate and howmuch water to apply to keep the soil in the right condition for your crops. This isirrigation scheduling.

Managing water better02 03Managing water better

Soi

lwat

erco

nten

t

Saturation

Field capacity

Critical soil water deficit for bulking

Critical soil water deficit forscab control

Critical soil water deficit

IrrigationsRainfall

Permanentwilting point

Water balance calculation

In-field soil

moisture measurement

Operator judgement

Other

Case Study – Good scheduling pays dividends on soft fruit

Figure 2 – Irrigation scheduling practicesin England (% of total irrigated area)

Figure 1 – Managing the soil water reservoir

Why schedule?

5343 - Managing Water Better 7/11/08 16:40 Page 3

The Options

Growers have a range of scheduling techniquesto choose from – each having its ownadvantages and disadvantages. The mostappropriate method for you will depend on thecrop you are growing, the accuracy you require,and the cost of the equipment and services.

Operator judgement

Although many farmers still rely on their ownjudgement, kicking the soil is an unreliable way ofassessing soil moisture. A much better way is tofeel the soil within the root zone. Charts areavailable to help you assess the amount of soilwater available, but this technique requires a lot ofexperience to use it well. The results are unlikelyto be reliable enough for producing premium

quality crops. However, it can be a useful fieldspot-check to see how other more sophisticatedscheduling methods are performing.

Water balance calculation

The water balance approach is a means ofassessing the soil water deficit from a balance ofwater going into and out of the soil. It works like abank account. Rainfall and irrigation are inputs intothe account and crop evapotranspiration (ET) is theoutput. Day by day the soil water deficit gradually builds up (much like your overdraft) and reaches acritical point to signal the start of the next irrigation.

How to schedule

Without proper scheduling it is difficult to keep track of what is happening in the field, particularly when the summer weatheris so variable. There are many different ways to scheduleirrigation. So you need to decide which method is mostappropriate for you taking into account your crops, soils andirrigation equipment.

The water balance relies on good localmeasurements of rainfall, irrigation andevapotranspiration (ET). Most farmers routinelymeasure rainfall but few measure ET as this ismore difficult. Daily ET data can be obtained fromthe Met Office. But some farmers are beginning tomeasure ET themselves using automatic weatherstations and more directly using atmometers (suchas the ETgage™). Whatever the source of data,ET values will need adjusting to account for croptype and stage of growth.

The price of simplicity is often a loss of accuracy.The water balance approach is only as good asthe input data. It works on the garbage in –garbage out principle. Errors can accumulateover the season and so it is wise to cross-checkusing another method – even if this is onlysampling and feeling the soil.

Water balance calculations can be done manually.Some farmers have developed their ownspreadsheet methods while others use PC-basedscheduling software (e.g. Happy Irrigator). Theseenable you to keep up to date with your irrigationand are useful tools for planning future irrigations asweather forecasts evolve. Rather than doing thecalculations yourself you can use specialistbureau services which offer irrigation advice anduse more sophisticated water balance models toforecast irrigation needs. Examples include theADAS Irriguide model and the CUPGRA potatoscheduling model developed by CambridgeUniversity Farm (CUF).

Bureau services rely on you to regularly measureand send in data on rainfall and irrigation on your

farm and provide agronomic data such as plantingdates, emergence dates and ground cover. Basedon this information the bureau then provides youwith the irrigation requirement for individual fieldsfor the coming week.

Soil water measurement

There are several in-field techniques available formonitoring soil water directly.

Neutron probes, though popular, are subject tostrict legal control because of their radioactivesource and so they are only used by commercialscheduling services. Readings are taken on aregular basis at different soil depths as the probe islowered into an access tube installed in the crop’sroot zone. Because the measurements are taken inthe same place each time it is a good method formeasuring changes in soil water rather thanabsolute values. They work well at depth but arenot so accurate in the top 200 mm of the soil. Sothey are not so useful for shallow rooting cropssuch as salads.

Tensiometers are not widely used but recentdevelopments in wireless telemetry and thedevelopment of systems such as IrriWise™ havere-awakened interest in them. They measure soilsuction and so they require calibrating to convertsuction data into soil water deficits. They arerelatively inexpensive, easy to read, and well suitedto scheduling water sensitive crops grown onsandy soils. However, they are rather delicateinstruments and must be installed with care.

Capacitance probes avoid the legal problems ofneutron probes and are accurate close to the soil

surface. Permanent systems, such as theEnviroscanTM, can be linked to the farm computerfor regular soil water monitoring. However, the initialinvestment cost is high and so the system isgeared to servicing larger irrigated farms.

Portable capacitance systems such as theDivinerTM, uses access tubes, rather like neutronprobes. They are sold directly to end-users or‘rented’ from consultants who download andinterpret the data and provide scheduling advice – often remotely.

Managing water better04 05Managing water better

Belt and bracesWater balance models can deal with large irrigated areas but they areprone to cumulative errors and sensitive to the data input. But in-situ soilwater measurements are time consuming, subject to spatial variability andare difficult to use for forecasting irrigation need.

Thus there is no ‘best’ approach to scheduling because all methods havelimitations. It is best to use a combination of methods and always checkthe soil and crop in the field.

Feeling the soil Neutron probes

Tensiometers

Capacitance probes

Resistance blocks

Operator judgement Water balance calculation Soil water measurement

Manual balance sheet

Computer water balance model

“Each year we provide irrigation scheduling services for about60 growers representing 6000 ha, and have run this servicesince 1990.” Dr Mark Stalham (CUF)

Soil moisture or soil suction?

It is important to recognise the fundamentaldifference between soil water content and soilwater suction (or ‘potential’). The soil watercontent is the volume of water in a givenvolume of soil. In contrast, soil water suction isa measure of how tightly the water is held insoil, and therefore the suction plant roots needto exert in order to extract water.

At the start of each season you will needto think about your irrigation ‘strategy’ –what critical soil water deficits will youallow and how much water will youapply each time you irrigate. Will youbring your soil back to field capacityeach time or will you leave a deficit totake advantage of any rainfall? Yourstrategy will vary through the season asyou take account of crop sensitivity towater stress at different growth stages.

Summer rainfall can play havoc withirrigation schedules and so you will need to develop ‘tactics’ to help you decide how to respond tounexpected changes in weather.

Scheduling strategies and tactics

5343 - Managing Water Better 7/11/08 16:40 Page 5

Does scheduling impact on waterresources?

Yes – but good scheduling practices willminimise water losses

There is a tendency among many farmers to over-irrigate (see box). In simple terms, if youdeliberately over-irrigate by 10% then this means10% more water is abstracted. Also if you irrigate10% more than you think then you are using 10%more water than you need. This may sound likean obvious statement but unless you regularlyconduct a water audit on your irrigation system,it is quite easy to be over-irrigating withoutactually knowing it. A recent water audit on fourfarms growing potatoes in Norfolk showed that,on average, they tended to over-irrigate by some12% – even though the farmers involved thoughtthey were applying the right amount of water. Thisoccurred in spite of the use of sophisticatedcommercial scheduling services.

Rainfall is free irrigation but you can easily losethis advantage with poor scheduling practices.Irrigating to a fixed schedule, irrespective ofweather, and more commonly, failure to allowadequate storage capacity in the soil forunforeseen rainfall following irrigation will meanthat rainfall is not used to its full effect. Irrigating to a planned deficit can make best use of any rainfall event.

However, not all the excess water is necessarilylost. Most of it returns to the water resource asdrainage and may be available for someone elseto use – one person’s loss is another’s gain. Thebad news is that it may not be available at the righttime for other users and the quality of the watermay be affected as well. It largely depends on thehydrology of the catchment.

Does scheduling impact on water quality?

Yes – but timely irrigation can reduce nitrateconcentrations in groundwater.

Irrigation, particularly over-irrigation, can increasedrainage and hence nitrate leaching intogroundwater – but only when soils are wettedbeyond field capacity. Leaching risks are high inthe spring when nitrogen is first applied to thecrop. This is particularly true for potatoes whensoils are kept close to field capacity for scabcontrol. Any unexpected rainfall is likely to resultin drainage and leaching.

Summer irrigation can increase the likelihood ofdrainage and leaching but good scheduling, suchas maintaining a controlled soil water deficit, willreduce losses to a minimum.

Autumn and winter bring another period of riskwhen there may be surplus nitrate in the soilfollowing harvest. Even without irrigation, in mostwinters on light soils, enough water drains throughthe soil to leach most of the available nitrate. Butirrigation will produce wetter autumn soils and sodrainage and leaching may begin earlier than onnon-irrigated soils. However, because summerirrigation improves the growing environment andhence nitrogen uptake, there may be less residualnitrate in the soil at harvest and so less to leach intothe groundwater.

Does scheduling pay?

Good irrigation scheduling is not only time consuming it can also be expensive, particularly when you are buying incommercial services. So does it pay? What are the costs and the benefits?

Costs

A significant proportion of growers still do notschedule scientifically – why? Some believe thatby over-irrigating they do not need to schedule.But the marginal cost of applying water is risingand over-irrigating can prove costly (Figure 3).Others have irrigation systems that are under-capacity and cannot keep up with irrigationdemand. Scheduling is of little value in thesecircumstances but reduced crop yield and qualitywill reduce farm profits.

Benefits

Irrigation scheduling can pay for itself. It can be a‘win-win’ situation:

• If the schedule suggests one more irrigationthen the additional yield and quality benefits willoutweigh the scheduling cost.

• If the schedule suggests you do not need thatlast irrigation the saving will outweigh thescheduling cost.

Managing water better06 07Managing water better

Estimated costs to apply 25 mm on 1 ha, using a raingun hosereel anddiesel pump (2008 prices)

Jan-

05

May

-05

Sep

-05

Jan-

06

May

-06

Sep

-06

Jan-

07

May

-07

Sep

-07

Jan-

08

May

-08

Sep

-08

80

70

60

50

40

304

2

0

20

8

6

20

10

0

Gas

oil(

p/lit

re)

Gas oil Electricity

Ele

ctric

ityp/

kwh

Typical scheduling costs

There is no such thing as an ‘un-scheduled’irrigation – no-one irrigates completely at random.However, there is ‘good’ and ‘bad’ scheduling.

Some farmers deliberately over-irrigate toensure their crops are well watered.

Over-estimating ET by 10% – can result in a 20% increase in the volume of water applied.

Over-estimating a soil’s field capacity. A10% over estimate in field capacity (e.g. a volume water content 23.4%instead of 21.3%) would mean 30% over watering.

A poor irrigation plan that does not makebest use of rainfall (free irrigation).

Incorrect operating pressures affectingwater distribution and resulting in-field soilwater measurements.

Operating irrigation systems to suitoperator convenience rather than optimumsoil water management.

Inaccurate irrigation equipment calibration.Lack of infield checks on actual water applied.

Main causes of over-watering

Bureau water balanceconsultancy service

Purchase schedulingsoftware for use by farmer

Neutron probe service

Capacitance probe service

Automatic weather station

ETgage™

£200 – £250 per site per season£20 – £25 per ha per year

£750 – £1000 depending on number of fields requiring scheduling

£1500 – £3000

£300

£300 – £500 per site (based on 2 or 3 access tubes) per season

£1750 – £2500 for a basic system including sensors,logger and staff training

Scheduling and the environmentThe environment is no longer a ‘bolt-on,’ it is now central to all aspects ofirrigation planning and management. Most environmental concerns aboutirrigation scheduling stem from a tendency of farmers to over-irrigate. Itimpacts on both water resources and water quality. But good schedulingcan limit these problems and thus protect the environment.

Figure 3 – The rising energy costs for irrigation.

Water

Cost £

Labour

Energy

Total

0 10 20 30 40 50 60

5343 - Managing Water Better 7/11/08 16:40 Page 7