FAO IRRIGATION AND DRAINAGE PAPER 24

1 Atuirementsrevi sed1977 FOOD AND AGRICULTURE ORGANIZATIONOF THE UNITED NATIONSROMEFAO IRRIGATION AND DRAINAGE PAPER24 FAO IRRIGATION AND DRAINAGE PAPERclu Mims for [preck'dngi:e[r [aqukramernmbyj. drenbos(2L.,revised1977water management specialistland and water development divisionfaoandW. O. PrErfg" coreamN:anr1isTigadzqiisrdivevzKyoaNcinT je,davis, california, ,in consultationa. thcautthEieE: gsbar-dc<, dastane 7,D(AF: c. van den be., ashINDscl ( , field sFOOD AND AGRICULTURE ORGANIZATION OF THE UNITED NATIONSRome 1977 SUMMARYThis publication is intended to provide guidance in determiningcrop waterrequirements and their application in planning, design and operation presents suggested methods to derive crop water use of four well-known methods for determining such requirements is definedto obtain reference crop evapotranspiration (ETo))))]]]

2 , which denotes the level of evapo-transpiration for different climatic methods are the , the Radiation, the Penman and Pan Evaporation methods, each requiringa different set of climatic derive the evapotranspiration for a specificcrop, relationships between crop (ETcrop) and reference cropevapotranspiration (ETo) are given in Part I . 2 for different crops, stages of growth,length of growing season and prevailing climatic conditions. The effect of localconditions on crop water requirements is given in Part ;this includes localvariation in climate, advection, soil water availability and agronomic and irrigationmethods and procedures are presented together with detailed discussion on selection and calibration of the preSented methodologiestogether with the data sources is given in Appendix II.

3 A computer programme onapplying the different methods is given in Appendix 11 discusses the application of crop water requirements data in irriga-tionproject planning, design and II. 1 deals deriving the fieldwater balance, which in turn forms the basis for predicting and peakirrigation supplies for general planning purposes. Attention is given to irrigationefficiency and water requirements for cultural practices and leaching of methods are presented to arrive at field and scheme supply schedules withemphasis towards the field water balance and field IRRIGATION management.

4 Criteriaare given for operating the canal system using different methods of water delivery,and for subsequent design parameters of the are made inPart on refinement of field and project supply schedules once the project is presented guidelines are based on measured data and experience obtainedcovering a wide range of practical, technical, social econo-mic considerations will, however, affect the planning criteria and a critical attitude should still be taken when the identification AND PRELIMINARY and Peak Project Supply water IRRIGATION requirements: effective rainfall, groundwatercontribution, stored.

5 Soil requirements:leaching requirements, of calculation of seasonal and peak project supplyrequirementsPROJECT and Project Supply IRRIGATION schedules: depth of IRRIGATION application, IRRIGATION application interval, calculation of field IRRIGATION supply schedules: surface, sprinkler and operation of supply system: continuous, rotational,supply on calculation of project design and operationPROJECT IAppendix IIAppendix IIIA ppendix IVAppendix VAppendix VITABLE OF CONTENTSS ummaryTable of ContentsList of TablesList of FiguresConversion FactorsClimatological NometiclaturePART I-CALCULATION OF CROP WATER REQUIREMENTSCALCULATION OF REFERENCE CROP Evaporation MethodSELECTION OF CROP COEFFICIENTFACTORS AFFECTING CROP : variation with time, distance, size of irrigationdevelopment, advection, Water: level of.

6 Soil water, groundwater, salinity,water and crop of IRRIGATION :surface, sprinkler, drip or trickle, sub-surface Practices: fertilizers, plant population, tillage, mulching,windbreaks, anti-transpirantsPART II-APPLICATION OF CROP WATER REQUIREMENT DATA IN IDENTI-FICATION , DESIGN AND OPERATION OF IRRIGATION PROIECTS67 Refinement of Field Supply Schedules: adaptive research, datacollection, project monitoringApplication of Field IRRIGATION DataPersons and Institutes ConsultedBackground and Development of Methods to Predict Reference CropEvapotranspirationComputer Programme for Estimation of Reference Crop EvapotranspirationGlossaryReferencesCons tants for the Radiation Equation Rs = (a + b njol)Ra338153035555559626568686971727681 8383838690961001021021041071081201331371 44 LIST OF daily percentage (p) of annual daytime hours for different latitudes6 Extra terrestrial radiation (Ra)

7 Expressed in equivalent evaporation in mm/day12 Mean daily duration of maximum possible sunshine hours (N) for differentmonths and latitudes13 Values of weighting factor (W) for the effect of radiation on ETo at differenttemperatures and altitudes13 Saturation vapour pressure (ea) in mbar as function of mean air temperature(T) in oC21 Vapour pressure (ed) in mbar from dry and wet bulb temperature data in C(aspirated psychrometer)21 Vapour pressure (ed) in mbar from dry and wet bulb temperature data in C(non-ventilated psychrometer)22U2 Values of wind function f(u) = (1 + Tdd) for wind run at 2 m height in km/day23 Values of weighting factor (1-W) for the effect of wind humidity on ETo atdifferent temperatures and altitudes24 Values of factor (W) for the effect of radiation on ETo at differenttemperatures and altitudes24 Extra terrestrial radiation (Ra) expressed in equivalent evaporation in mm/day25 Mean daily duration of maximum possible sunshine hours (N) for differentmonths and latitudes26 Conversion factor for extra-terrestrial radiation (Ra) to net solar radiation(Rns)

8 For a given reflection oc of and different ratios of actual to maximumsunshine hours (1- + n/N)27 Effect of temperature f(T) on longwave radiation (Rnl)27 Effect of vapour pressure f(ed) on longwave radiation (Rn1)27 Effect of the ratio actual and maximum bright sunshine hours f(n/N) onlongwave radiation (Rnl)27 Adjustment factor (c) in presented Penman equation28 Ratios between evaporation from sunken pans mentioned and from Coloradosunken pan for different climatic conditions and pan environments33 Pan coefficient (Kp) for Class A pan for different groundcover and levels ofmean relative humidity and 24 hour vrind34 Pan coefficient (Kp) for Colorado sunken pan for different groundcover andlevels of mean relative humidity and a hour wind34 Approximate range of seasonal ETcrop in mm36 Crop coefficient (kc) for field and vegetable crops for different stages ofcrop growth and prevailing Climatic conditions40 Length of growing season and crop development stages of selected fieldcrops.

9 Some indications42kc values for alfalfa, clover, grass -legtimes and pasture45 Pagekc values for bananas46kc values for citrus (grown in predominantly dry areas with light tomoderate wind)47kc values for full grown deciduous fruit and nut trees49kc values for grapes (clean cultivated, infrequent IRRIGATION , soil surfacedry most of the time)50kc'values for rice51kc values for sugarcane52kc values for aquatic weeds and coefficients for open water54 Tolerance levels of crops to high groundwater tables waterlogging60 Critical periods for soil water stress for different crops63 Project planning stages and IRRIGATION supply data67 Average monthly effective rainfall as related to average monthly ETcropand mean monthly rainfall75 Effect of IRRIGATION water quality on soil salinity, permeability and toxicity77 Crop salt tolerance levels for different crops78 Conveyance (Ec), field canal (Eb), distribution (Ed)

10 And field applicationefficiency (Ea)80 Relation between soil water tension in bars (atmospheres) and available soil-water in mm/m soil depth86 Generalized data on rooting depth of full grown crops, fraction of availablesoil water (p) and readily available soil water ( ) for different soil types(in mm/m soil depth) when ETcrop is 5- 6 mm/day88 Average intake rates of water in mm/hr for different soils and .correspondingstream size 1/sec/ha91 Size of basins and stream size for different soils93 Length of furrows and stream size for different soil type, land slopeanddepth of water application93 Size of borders and stream size for different soil t3rpe and land slope(deeprooted crops)9344.