Crop Water Requirement Calculator

Crop water requirement is expressed as crop evapotranspiration, ETc, the depth of water in millimetres per day that leaves a cropped field as soil evaporation plus plant transpiration. The two-step formula ETc equals ET0 multiplied by Kc separates the two things that drive that loss. ET0, reference evapotranspiration, carries all the weather: net radiation, air temperature, vapour pressure deficit and wind speed over a hypothetical 0.12 m clipped grass surface with a fixed surface resistance of 70 s/m. Kc, the crop coefficient, carries the plant and the calendar - canopy fraction, leaf area, stomatal behaviour and how wet the soil surface is kept. Kc rises from roughly 0.30 for wheat or maize at emergence to a mid-season peak near 1.15 to 1.25 at full canopy, then falls to 0.40 to 0.60 during senescence. Paddy rice is the exception, staying above 0.90 throughout because the field is ponded. Typical ET0 runs 1 to 3 mm/day in cool humid climates and 6 to 12 mm/day in hot arid ones, so a mid-season maize crop can consume 8 to 14 mm/day at peak. Subtracting effective rainfall gives the net irrigation depth; dividing that by application efficiency gives the gross depth actually pumped, since surface flood systems deliver only about 40 to 60 percent of applied water to the root zone, sprinklers 60 to 80 percent and drip 85 to 95 percent. One millimetre over one hectare is 10 cubic metres, which is how depth converts to pumped volume. The main caveats are that tabulated Kc values assume a well-watered crop with standard density and no salinity, pest or nutrient stress; deficit irrigation, mulching, partial canopies, frequent light wetting of bare soil and advection from dry surroundings all move real ETc away from the table value, sometimes by 20 percent or more.

The modern calculation descends from a single 1948 paper by Howard Penman at the Rothamsted Experimental Station in England, which combined the energy balance at an evaporating surface with an aerodynamic transport term to predict evaporation from open water, bare soil and grass. In 1965 John Monteith extended it by inserting a surface resistance representing stomatal control, producing the Penman-Monteith equation that treats a vegetated canopy as a single big leaf. Simpler empirical alternatives were widespread in the same era, notably the Blaney-Criddle temperature method developed for the western United States around 1950 and the Hargreaves-Samani radiation-temperature method, which is still used where only maximum and minimum temperature are recorded. The crop coefficient approach that pairs with it was formalised by the Food and Agriculture Organization. FAO Irrigation and Drainage Paper 24, published in 1977 by Jan Doorenbos and William Pruitt, introduced tabulated Kc values for the four growth stages and made the ET0 times Kc structure the international standard for irrigation design. In 1998 FAO Irrigation and Drainage Paper 56, by Richard Allen, Luis Pereira, Dirk Raes and Martin Smith, replaced the older ET0 definitions with a single standardised Penman-Monteith reference surface, revised the Kc tables for hundreds of crops and added the dual coefficient method that splits transpiration from soil evaporation. FAO distributed the CROPWAT software and the CLIMWAT climate database alongside it, and the FAO-56 formulation remains the reference method used by national irrigation advisory services today.