Planning and management of irrigation water

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Planning and management of irrigation water. Pieter van Heerden With material from: Prof Sue Walker, Dr Dirk Raes, Mr Felix Reinders. Introduction. Background to irrigation water planning. Need to know about: Crops and their characteristics Soils and their characteristics - PowerPoint PPT Presentation


  • Planning and management of irrigation waterPieter van HeerdenWith material from:Prof Sue Walker, Dr Dirk Raes, Mr Felix Reinders

  • Introduction

  • Background to irrigation water planningNeed to know about:Crops and their characteristicsSoils and their characteristicsClimate / weather of the areaEngineering considerationsEconomic considerations

  • CropsGrowing characteristicsSuitability for climateSuitability for soil Planting dateGrowing periodHeightDensity / foliage coverRooting characteristicsSoil water extractionAdaptation to water stress situation

  • SoilsTextureStructureDepthChemistryInfiltration rate

  • Climate / weatherAverage, maximum and minimum temperaturesHumidityFrostWindSunshine / radiationRain, seasonHail, storms

  • EngineeringSuitable and affordable systemsSuitable for cropSuitable for soilLosses between pump and rooting area

  • Economic considerationsMarketsFundingProfitAvailability of input material

  • Irrigation scheduling

  • Irrigation schedulingGetting the right amount of water to a crop at the right time

    This means that you should:Know your crop and its characteristicsKnow your weather and its influence crop water requirementKnow your soils, their water holding capacities and their limitations and or problemsKnow your irrigation systems, their applications and limitationsEstimate expected irrigation requirements within the scope of source, system, crop, soil management limitationsDo real-time irrigation management and adapt your plan if necessary

  • Soil, Crop, Atmosphere Continuum

  • Simplified soil-crop-atmosphere continuumWater moves from soil to crop roots: low water content and high salinity changes osmotic potential: less water for crop.Water moves through roots to stem: movement restricted: high atmospheric demand cause temporary wilting.Water moves through stem to leaves: as above.Water vapour from leaves during transpiration; speed depends on:Weather situationSoil water contentCrop characteristics

  • Determination of crop water useDirect measurementNot easy or cheapUsed mainly for management and less so for planningRelative to a known quantityA-pan and other evaporation pan approachesCalculation approachPenman-Monteith and others

  • Penman-MonteithUse a reference crop to which which water use of other crops are relatedDefinitionA hypothetical crop with an assumed height of 0.12 m, having a surface resistance of 70 s m-1 and an albedo of 0.23, closely resembling the evaporation of an extensive surface of green grass on uniform height, actively growing and adequately watered.

  • Relating crop water requirement to reference

  • Relating crop water requirement to reference

  • Relating crop water requirement to reference

  • Relating crop water requirement (ETc) to reference evapotranspiration (ET0)

  • Relating crop water requirement (ETc) to reference evapotranspiration (ET0)ET0: reference evapo- transpirationETc: crop evapo- transpirationKc: crop coefficient = relation between crop water use and reference water use

  • CROP COEFFICIENT APPROACHsingle crop coefficient used for applications to irrigation planning, design, and management. used for calculating crop evapotranspiration under standard conditions, ETc. standard conditions refer to crops grown in large fields under excellent agronomic soil water conditions. crop evapotranspiration ETc different from reference evapotranspiration, ETo, ground cover, canopy properties & aerodynamic resistance of the crop are different from grass. effects of characteristics that distinguish field crops from grass are integrated into the crop coefficient, Kc.

  • Effect of weatherOn evapotranspiration in ET0 Effects of cropped surface as differ from reference surface are integrated into the crop coefficient, ETc = Kc.ET0Effects of soil water stress Ksmultiplying Kc.KsETc = Kc.Ks.ET0

  • ETc = Kc.ET0ETc is the crop evapotranspiration [mm d-1] Kc is the crop coefficient [dimensionless] ET0 is the reference evapotranspiration [mm d-1]most weather effects in ET0 estimate. ET0 = index of climatic demand, Kc varies specific crop characteristics only to a limited extent with climate. transfer Kc values between locations & climatesreason for global acceptance & usefulness KcKc factors developed in past studies

  • 4 main differences: ET0 vs ETcCrop heightinfluences the aerodynamic resistance term in the FAO Penman-Monteith equation turbulent transfer of vapour from crop into atmosphere.Albedo (reflectance) of the crop-soil surface affected by the fraction of ground covered by vegetation by soil surface wetness. influences net radiation of surface, Rn, primary source of energy for evaporation process.Canopy resistance of crop to vapour transfer affected by leaf area index (number of stomata), leaf age & condition, & degree of stomatal control. influences surface resistance.Evaporation from soil, especially exposed soil.

  • Kc predicts ETc under standard conditionsNo limitations are placed on crop growth; evapotranspiration due to water shortage; crop density; disease, weed, insect or osmotic potentials. ETc predicted by Kc can be adjusted if necessary to non-standard conditions, where any environmental condition or characteristic is known to have an impact on or to limit ETc. Factors for correcting ETc to an adjusted value are described in Allen et al. (1998) and section 10.6.

  • FACTORS DETERMINING Kc1. CROP TYPEDifferences in albedo, crop height, aerodynamic properties, & leaf and stomata properties, =>ET from full grown, well-watered crops differs from ET0. Crop characteristics influence on Kc:Taller: Kc upShorter: Kc downLeafier: Kc upStomatal control: Kc downCloser spacing: Kc up

    Crops that grow taller with high roughness of full grown crops => have Kc factors larger than 1. 15-20% greater for some tall crops such as maize, sorghum or sugar cane


  • FACTORS DETERMINING Kc2. CLIMATEMore arid climates & conditions of greater wind speed => Kc up. More humid climates and conditions of lower wind speed => Kc down. Impact of climate on Kc for full grown crops upper bounds = extremely arid and windy conditionslower bounds = very humid & calm weather conditions. Ranges of Kc as climate and weather conditions change small for short crops BUT large for tall crops.


  • FACTORS DETERMINING Kc3. SOIL EVAPORATIONET= E + TSoil evaporation & crop transpiration in field crops integrated in KcAfter rainfall or irrigation, effect of evaporation is dominant when crop is small & little shade on ground For low-cover conditions, Kc coefficient a/c by frequency of soil surface wetting. Where soil is wet for most of time from irrigation or rain, Kc 1. Where soil surface is dry, evaporation is restricted and Kc small as low as 0.1

  • FACTORS DETERMINING Kc4. CROP GROWTH STAGESAs crop develops, ground cover, crop height & leaf area change.Due to differences in ET in various growth stages Kc vary over growing period. Growing period divided - 4 distinct growth stages: initialcrop development mid-season late season


  • PROCEDURE TO GET CROP COEFFICIENT KcID crop growth stagesdetermine lengths of each stageselect Kc coefficientsadjust Kc values for frequency of wetting or climatic conditionsconstruct crop coefficient curvecalculate ETc = ET0 x Kc

  • DEFENITION OF GROWTH STAGESInitial stage: sowing to 10% foliage coverDevelopment stage: 10% to 80% foliage coverMid-season stage: 80% foliage cover to first signs of senescenceLate season: First signs of senescence to physiologically inactive


  • LENGTH OF GROWTH STAGESdevelopment stage a/c weather/climatespecific temperature & growth & climate, latitude, elevation, planting date & crop varietymid-season stage a/c genotype & phenology & flowering, seed development, ripening & senescencelate season a/c frost or harvested fresh = sharp cutoff

  • Kc ini (initial stage)table for planningreal values a/c time interval between irrigation eventsevaporation power of atmospheremagnitude of wetting even surface areause Kc adjusted a/c climate & wind graphs & crop height

  • Kc dev (development stage)From 10% cover to full cover by leaves & closed between rows; For grain crops use flowering timeLeaf Area Index LAI = 3If more shadow lower EsKc = 0,5 if 25-40 % coverKc = 0,7 if 4060 % cover

  • Kc mid (mid-season stage)a/c climate RHmin if humid Kcif dry Kcfrequency of wettingbut smaller effect after full canopy

  • Kc end (late season)fresh harvest irrigate to end / last dayEsoil highmature dry harvest dryout to endEsoil lowarid, high wind high Kc endhumid, lower wind lower Kc end


  • CONSTRUCT Kc CURVEDivide growing period into 4 periodsdetermine lengths of each periodIdentify Kc ini; Kc mid; Kc end Adjust Kc a/c climatea/c wetting frequencyConstruct curveconnect straight linesKc ini horizontalKc mid horizontaljoin Kc ini & Kc mid diagonal linesKc mid to Kc end diagonal lines

  • CONSTRUCT Kc CURVEForage crops have Kc end after each cut or grazingFruit trees need wind & RH adjustment- deciduous Kc end = time of leaf dropDetermine Kc from graph Can use any of following:-daily, 10 days, monthly values

  • Estimating irrigation requirements

  • Soil water balanceI + P - RO - DP - ET S = 0

    Where:I=irrigationP=precipitationRO=runoffDP=drainageET = evapotranspirationS = change in soil water content.I & P & RO can be measuredD is difficult to measure, usually 0S is difference between begin & end soil water contentET can be calculated as difference:- ET = I + P - RO - DP S

  • Soil water balance

  • Water available in the soil profileTAW = total available water [mm m-1 soil depth]Field capacity wilting point [mm m-1 soil depth]RAW =readily available water [mm (rooting depth)-1]f.rd.(Field capacity wilting point)where:f = allowed depletionrd = rooting depthAllowed depletion:Rule-of-thumb = 50%Actual: varies from crop to crop varies according to crop growth stagevaries according to atmospheric demandRooting depth expressed in m

  • Typical soil water capacities

  • How much? Amount?must use rainfalluse soil water balanceI + P - RO - DP - ET S = 0Where I=irrigation, P=rain, RO=runoff, DP=drainage, ET = evapotranspiration, S = change in soil water content.use weather data to calculate ETuse sum of daily values of ET since last I use FAO Penman-Monteith equations to calculate ETneed temperature (dry bulb & wet bulb), wind, radiation dataCan use crop factor to reduce potential evaporation for crop condition (age, stress)availability of water from dam or river NB

  • Calculating daily irrigation water requirementET0 = 6.5 mmCrop growth stage: MidseasonCrop Kc = 1.15ETc = 6.5 * 1.15 = 7.5 mm

  • Calculating readily available waterSoil = LoamTAW = 130 mm m-1ET0 = 6.5 mmCrop growth stage: MidseasonCrop Kc = 1.15Crop rooting depth = 1.1 mAllowed extraction = 50%RAW = 130 * 1.1 * 0.5 = 71.5 mm

  • Calculating time to next irrigation Soil RAW = 71.5 mmKc = 7.5 mmDays before next irrigation = 71.5 / 7.5 = 9.5 days

    We therefore plan to irrigate in 9 days time and then add 9 * 7.5 = 67.5 mm water

  • Planning irrigation water requirementOn a day by day basis during expected crop growth:Do water balance calculationRainfall included as per historic dataETc as demonstratedWater extraction as demonstratedPlan irrigation amount when soil water content reaches end of RAW levelTotal irrigation amounts for months and for seasonTotal for season is the estimated irrigation requirement

  • Managing irrigation water

  • Irrigation water managementMeasure, measure, measure ..Use irrigation requirement estimate as basis Measure irrigation water applicationMeasure rainfallMeasure soil water contentKeep track of water supply at sourceAdapt irrigation management plan if necessary

  • Indicators as when to irrigatesoil water measurementsuse probes, neutron moisture meters, tensiometers, gypsum blocksplant water status measurements on weekly basisleaf water potentialleaf temperature - infra-redreduction in transpirationIf not enough water available, change irrigation strategy to:Irrigate only during critical stages - e.g. flowering, tilleringApply deficit irrigation somewhat less than optimum crop requirement, increasing water productivity but reducing yieldEconomic analysis: full irrigation area at less than maximum yield vs. smaller area at maximum yield.

  • Water use planning:using SAPWAT3 SAPWAT3 available from the Water Research

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