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Revised FAO Procedures for Calculating Evapotranspiration: Irrigation and Drainage Paper No. 56 with Testing in Idaho
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2001
Year
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Irrigation ManagementFao ProceduresEnvironmental MonitoringEngineeringWater ResourcesDroughtReference EvapotranspirationAgricultural Water ManagementSustainable AgricultureAgricultural EconomicsCrop Water RelationIrrigationFao IrrigationPublic HealthHydrologyDry Soil SurfaceDrainage Paper NoWater Balance
In 1998, the FAO released FAO‑56, a revision of the earlier Paper No. 24, to provide updated methods for calculating evapotranspiration and crop water requirements. FAO‑56 calculates reference evapotranspiration with the Penman‑Monteith equation, supplies basal crop coefficients and soil‑surface evaporation equations, and compares its basal Kc method to lysimeter data and Wright’s time‑based procedure for three crops.
In 1998, the Food and Agriculture Organization of the United Nations (FAO) published FAO Irrigation and Drainage Paper No. 56, a revision of the earlier and widely used Paper No. 24 for calculating evapotranspiration (ET) and crop water requirements. The revision uses a single method, the FAO Penman-Monteith equation, for calculating reference evapotranspiration (ETo). In addition to the "mean" crop coefficient (Kc) values of FAO-24, FAO-56 provides tables of "basal" crop coefficients that represent ET under conditions having a dry soil surface. Associated equations for predicting evaporation from bare soil associated with crop transpiration are based on a water balance of the soil surface layer. Comparisons of daily ET from three agricultural crops are made between lysimeter measured ET and the basal Kc method of FAO-56 and the time-based basal Kc procedure of Wright (1982). Standard errors of estimate and accuracies were similar between the two methods and averaged about 0.77 mm/day or 15%.