Free crop water requirement calculator. Estimate daily evapotranspiration, seasonal water needs, and irrigation schedules using FAO Penman-Monteith method for any crop and climate.
High water demand during tasseling and silking stages
Kc: 0.3 → 1.2 → 0.6 | Root depth: 1m
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Efficient irrigation starts with knowing exactly how much water your crops need. Our calculator uses the FAO Penman-Monteith equation—the international standard for evapotranspiration estimation—to calculate daily and seasonal water requirements based on your crop type, growth stage, climate conditions, and soil type.
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
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.
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.
The FAO Penman-Monteith equation is the international standard for calculating reference evapotranspiration (ETo). It combines solar radiation, temperature, humidity, and wind speed to estimate water loss from a reference grass surface, which is then adjusted for specific crops.
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.