Why crop water requirement matters more than ever
India irrigates 71 million hectares (CWC 2024) — the world's largest irrigated area. Yet the water-use efficiency of Indian agriculture remains under 40% (NITI Aayog 2023), among the lowest in major agrarian economies. Per-capita water availability has dropped from 5,177 m³/year in 1951 to 1,486 m³/year in 2024, with 21% of districts now classified "over-exploited" under the CGWB groundwater assessment. The crop water requirement (CWR) calculation is not academic — it is the single most actionable number for an Indian farmer to size pumps, plan drip systems, schedule irrigation, or apply for the per-drop-more-crop subsidy under PMKSY-PDMC.
The FAO-56 equation: ETc = Kc × ETo
FAO Irrigation and Drainage Paper No. 56 (Allen, Pereira, Raes, Smith 1998) is the global gold standard for CWR computation. Reference evapotranspiration (ETo) is the water lost from a hypothetical 12-cm grass surface under standard meteorological conditions, calculated by the Penman-Monteith equation from temperature, humidity, wind, and net radiation. The crop coefficient (Kc) is a dimensionless multiplier specific to the crop and its growth stage:
- Kc-initial — bare soil to 10% canopy, low transpiration; typical 0.3-0.4
- Kc-development — 10-70% canopy, ramping; 0.7-0.9
- Kc-mid — full canopy, peak transpiration; 1.0-1.25
- Kc-late — senescence; 0.4-0.8
FAO-56 Table 12 provides the canonical Kc values; ICAR-IIWM Bhubaneswar has published India-calibrated refinements. This calculator embeds Kc tables for wheat, paddy, cotton, maize, sugarcane, onion, and tomato.
Worked example: Punjab wheat season
Sowing 5 November in Ludhiana, harvest 15 April → 140 days. Mean ETo (IMD AgroMet Ludhiana climate normal): 2.5 mm/day initial, 3.0 development, 4.5 mid, 5.5 late. Stage lengths 21/35/56/28 days. Kc-wheat: 0.3, 0.7, 1.15, 0.4.
- Initial: 0.3 × 2.5 × 21 = 15.8 mm
- Development: 0.7 × 3.0 × 35 = 73.5 mm
- Mid: 1.15 × 4.5 × 56 = 290 mm
- Late: 0.4 × 5.5 × 28 = 61.6 mm
- Seasonal ETc ≈ 441 mm
Effective rainfall during Punjab wheat season averages 80-110 mm — net irrigation requirement is 330-360 mm. Translated to litres: 360 × 10,000 = 3.6 million L per hectare per season. With a tube-well discharging 12 lps, that is 83 hours of pumping for one hectare — about 12-14 irrigations of 6-7 hours each at 70 mm depth per irrigation. The Irrigation Scheduling calculator translates this further into when to apply each irrigation.
Crop comparisons: why paddy is so thirsty
Paddy ETc in standing-water cultivation is 1,200-1,500 mm including the 250-300 mm puddling losses and continuous percolation. Sugarcane spanning 10-12 months claims 1,800-2,200 mm. Cotton (medium staple) ranges 600-800 mm over 180 days. Maize 400-500 mm in 100-110 days. Pulses (chickpea, lentil) are remarkably water-efficient at 220-300 mm. These differences explain the case for crop diversification away from paddy in over-exploited blocks — the Punjab Direct Seeded Rice (DSR) push targets 12-15% water saving by skipping puddling.
From ETc to PMKSY-PDMC subsidy
PMKSY-Per Drop More Crop (PDMC) provides 45-55% capital subsidy on drip and sprinkler systems (55% for small/marginal farmers; 45% others). The application requires a project report including CWR estimation per the FAO-56 method. Department of Agriculture officers verify the proposed drip flow rate against the calculated peak-ETc multiplied by area and check pump HP and tank capacity for adequacy. This calculator gives you the seasonal total; the Drip Sizing calculator turns it into peak daily demand for system design.
Climate and uncertainty: how to use CWR robustly
Climate change is increasing ETo across India — ICAR-NICRA data shows +0.04 mm/day per decade ETo trend in north-west India since 1980. Use a 5-10 year average ETo, not last year's value. Build a 10-15% buffer in pump and storage sizing. For rainfed regions, the practical CWR calculation must include rainfall reliability (e.g., 50% probability rainfall used by ICAR-CRIDA for design) rather than mean rainfall, because monsoon failures hit the bottom 30% of years hardest.
Reading the calculator output
The widget returns seasonal ETc in mm and total water in litres for your area. Stage breakdown is shown so you can see the peak-water windows (e.g. wheat mid-stage of 290 mm in our example — booting to dough — when irrigation deficit costs the most yield). Combine with Irrigation Scheduling for timing and Drip Sizing for delivery infrastructure.
Sources
Allen R.G., Pereira L.S., Raes D., Smith M. (1998) FAO Irrigation and Drainage Paper 56 — Crop Evapotranspiration; IMD AgroMet Division product portal 2024; ICAR-IIWM Bhubaneswar Crop Coefficients for Indian Conditions 2022; CGWB Dynamic Groundwater Resources of India 2023; CWC National Water Use Efficiency Status Report 2024.