How it works
Weather sets a reference rate, ET0: the water a well-watered grass surface would lose on that day, computed from temperature, humidity, wind and sunshine with the FAO-56 Penman-Monteith equation.
A crop uses a fraction of that reference, set by its crop coefficient (Kc) and stage: ETc = Kc × ET0. Young crops use less, a full canopy at flowering uses the most.
Why it matters for Indian farmers
Evapotranspiration is the water a crop actually needs. On a hot, dry, windy day in April it can be three or four times what it is on a cool, cloudy day, which is why fixed irrigation calendars waste water in some weeks and leave the crop short in others.
How VarshaKrishi uses it
VarshaKrishi computes daily reference evapotranspiration for each field from the weather forecast and observations, and on the Decide plan runs a root-zone water balance with crop-specific coefficients to time irrigation.
Questions about Evapotranspiration
How much water does a crop lose to evapotranspiration?
Typically 2 to 8 mm a day in India, depending on the weather and the crop's stage. Over a season that adds up to the crop's total water need.
What is FAO-56?
FAO Irrigation and Drainage Paper 56 (Allen et al., 1998), the international standard method for computing crop water requirements. It defines the Penman-Monteith reference evapotranspiration and the crop coefficients.
Is evapotranspiration the same every day?
No. It follows the weather: heat, wind, sunshine and dry air increase it; cloud, humidity and cool weather reduce it.