How it works
FAO-56 tabulates Kc for each crop at three points of the season: initial, mid-season and late season. A crop's water use is ETc = Kc × ET0.
For tomato, for example, Kc rises from about 0.6 after transplanting to about 1.15 at full canopy and falls towards harvest. Orchard crops such as citrus have a Kc that depends on canopy cover and season.
Why it matters for Indian farmers
Kc is why the same weather calls for different irrigation on different crops and at different stages. Using a crop's own curve avoids both over-watering young crops and under-watering at the critical flowering and fruit-set stages.
How VarshaKrishi uses it
On the Decide plan, VarshaKrishi uses FAO-56 crop coefficients, adjusted for drip irrigation and canopy cover, inside the field's water balance. Each crop's coefficients are reviewed by an agronomist before Decide opens for that crop.
Questions about Crop coefficient (Kc)
Where do crop coefficients come from?
From field research summarised in FAO-56 (Tables 11, 12 and 17), adjusted for local climate and irrigation method by agronomists.
Does drip irrigation change Kc?
Yes. Drip wets only part of the soil surface, so less water evaporates from the soil and the effective Kc is lower than for flood irrigation.
Where can I find a Kc table?
FAO Irrigation and Drainage Paper 56 lists crop coefficients for most crops in Tables 12 and 17. Local values can differ, so use them as a starting point.