Irrigation Water Use Calculator

Irrigation scheduling starts from the soil water balance: what a crop transpires plus what the soil surface evaporates must be supplied by rainfall, stored soil moisture, or applied water. Crop water need is normally expressed as crop evapotranspiration in millimetres per day, obtained by multiplying a reference evapotranspiration for a short grass surface by a crop coefficient that changes with growth stage. Subtracting effective rainfall gives the net irrigation requirement, the depth of water the crop actually needs from the irrigator. That net figure is then divided by system efficiency because conveyance losses, deep percolation below the root zone, surface runoff, wind drift, and evaporation from droplets all mean more water must leave the pump than reaches the roots. Since one millimetre spread over one hectare is exactly 10 cubic metres, converting a daily depth to a seasonal volume is a matter of multiplying by growing days and area. Realistic efficiency values are well characterised: surface and furrow systems generally achieve 40 to 65 percent, centre pivot and other sprinklers 70 to 85 percent, surface drip 85 to 95 percent, and subsurface drip can exceed 95 percent. Seasonal crop demand ranges from roughly 450 mm for wheat to 1,200 mm or more for paddy rice. Two caveats matter. Effective rainfall is not gross rainfall; intense storms run off and typically only 70 to 90 percent of measured precipitation is usable, so entering raw gauge readings will understate the irrigation needed. And treating daily demand as a constant hides the fact that requirement peaks sharply during flowering and grain fill, so a season-average figure is adequate for sizing storage and pumps but not for day-to-day scheduling.

Quantitative irrigation requirement began with attempts to predict evaporation from physical principles. In 1948 Howard Penman at Rothamsted Experimental Station in England combined an energy balance with an aerodynamic transfer term to estimate evaporation from open water and grass. John Monteith extended the approach in 1965 by adding surface and canopy resistance terms, producing what is now known as the Penman-Monteith equation and shifting the treatment from evaporation to plant-mediated transpiration. Separately, the concept of irrigation efficiency as the ratio of water beneficially used to water delivered was formalised in American irrigation engineering in the 1930s, notably in the work of Orson Israelsen at Utah State Agricultural College. The operational framework most of the world now uses came from the FAO. Irrigation and Drainage Paper 24, published in 1977 by Jan Doorenbos and W. O. Pruitt, introduced the crop coefficient method that let practitioners convert a reference evapotranspiration into demand for a specific crop and growth stage. In 1998 FAO Irrigation and Drainage Paper 56, by Richard Allen, Luis Pereira, Dirk Raes and Martin Smith, standardised the FAO-56 Penman-Monteith equation as the sole reference method and published crop coefficient tables that remain the international benchmark. Later refinements shifted the emphasis from field-level efficiency to basin-scale consumptive use, recognising that water lost from one field is often recovered downstream.