Free crop water requirement calculator. Find daily ETc, seasonal needs and an irrigation schedule from your crop, growth stage, climate and soil.
Kc: 0.3 1.2 0.6 · Root depth: 1 m
Enter a measured ETo, or leave it blank to estimate from your climate. For a rigorous FAO-56 Penman-Monteith ETo, use the Evapotranspiration calculator.
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Efficient irrigation starts with knowing how much water your crops need. This calculator works out crop water use as ETc = ETo × Kc — a reference evapotranspiration (ETo), estimated from your climate or entered directly, multiplied by a crop coefficient (Kc) that changes with growth stage — then turns it into seasonal totals and an irrigation schedule for your crop, soil, and method.
Crop water requirement (ETc) is the amount of water needed by a crop to compensate for evapotranspiration losses. It's calculated by multiplying reference evapotranspiration (ETo) by a crop coefficient (Kc) that varies with growth stage. This scientific approach ensures you irrigate based on actual plant needs, not guesswork.
Crop Water Requirement Formula
Determine when and how much to irrigate based on daily evapotranspiration rates.
Size pumps, pipes, and reservoirs based on peak and seasonal water demands.
Plan water allocations for the growing season and negotiate water rights.
Strategically reduce water during stress-tolerant stages to conserve resources.
Apply exactly what crops need—no over-irrigation that wastes water and causes nutrient leaching.
Prevent water stress during critical growth stages when under-irrigation reduces yields significantly.
Lower pumping costs, reduce fertilizer waste, and minimize waterlogging and disease pressure.
Use water resources responsibly while meeting crop needs throughout the growing season.
The FAO Penman-Monteith equation is the international standard for reference evapotranspiration (ETo), combining solar radiation, temperature, humidity, and wind speed. This calculator uses a simpler climate-based ETo estimate (or a value you enter) multiplied by the crop coefficient; for a full Penman-Monteith ETo from weather data, use our Evapotranspiration calculator.
Kc values adjust reference ET for specific crops at different growth stages. Initial stage Kc (0.3-0.5) is low because small plants use less water. Mid-season Kc (0.9-1.2) peaks during full canopy. Late-season Kc drops as crops mature and senesce.
Sandy soils hold less water (50-100 mm/m) and drain quickly, requiring frequent light irrigations. Clay soils hold more (150-200 mm/m) but have slower infiltration. Knowing your soil's water holding capacity helps determine irrigation frequency and amount.
Irrigation efficiency varies: drip (85-95%), sprinkler (70-85%), furrow (50-70%), flood (40-60%). Lower efficiency means more water must be applied to deliver the same amount to plant roots. The calculator adjusts gross irrigation needs accordingly.