Calculate crop evapotranspiration ETc by multiplying reference ET₀ by the crop coefficient Kc. Determine daily water use by growth stage.
Crop evapotranspiration (ETc) is the actual water consumption of a specific crop under optimal growing conditions. It is calculated by multiplying the reference evapotranspiration (ET₀) by a crop coefficient (Kc) that accounts for the plant's canopy size, stomatal behavior, and growth stage.
Kc changes as the crop develops: it starts low during seedling establishment, ramps up through vegetative growth, peaks during full canopy and grain fill, and declines as the crop matures and senesces. By tracking ETc day-by-day or week-by-week, irrigators can schedule applications precisely to match crop demand.
This calculator provides a quick ETc estimate for any combination of ET₀ and Kc. Use it to determine daily water use at each growth stage, compare water demand among crops, or verify the output of an automated weather-station-based irrigation scheduler. Whether you are a beginner or experienced professional, this free online tool provides instant, reliable results without manual computation. By automating the calculation, you save time and reduce the risk of costly errors in your planning and decision-making process.
ETc tells you exactly how much water your crop is using each day. Without this number, irrigation scheduling is guesswork. Over-irrigating wastes water, energy, and risks nutrient leaching. Under-irrigating causes yield-limiting stress. A reliable ETc estimate keeps you in the sweet spot. Having a precise figure at your fingertips empowers better planning and more confident decisions.
ETc = ET₀ × Kc Where: ET₀ = Reference evapotranspiration (in/day) Kc = Crop coefficient (dimensionless, typically 0.2–1.3) Cumulative ETc = ETc × Days
Result: ETc = 0.322 in/day; 7-day total = 2.25 in
Daily ETc = 0.28 × 1.15 = 0.322 in/day. Over 7 days the crop uses 0.322 × 7 = 2.25 inches of water. If no rain falls, this is the depth the irrigation system must replace.
The single Kc method combines plant transpiration and soil evaporation into one coefficient. The dual method separates them into Kcb (basal crop coefficient) and Ke (soil evaporation coefficient). The dual method is more accurate under frequent irrigation or rain but requires daily soil water balance tracking.
The initial stage runs from planting to about 10% canopy cover. The development stage spans from 10% to full canopy coverage. Mid-season covers the period of full canopy until senescence begins. Late season extends from senescence onset to harvest.
Some crops tolerate moderate water stress during specific growth stages without significant yield loss. By tracking ETc and intentionally under-irrigating during tolerant stages, farmers can conserve water while protecting yield during critical stages like flowering and grain fill.
Kc is the crop coefficient, a dimensionless multiplier that converts ET₀ to crop-specific ETc. FAO Irrigation and Drainage Paper 56 and many university extension bulletins publish Kc tables by crop and growth stage.
Yes. Kc typically starts around 0.3–0.5, rises to 1.0–1.2 at mid-season, and drops to 0.5–0.8 at maturity. The four-stage model (initial, development, mid, late) is standard practice.
Yes. Tall, dense crops like sugarcane or alfalfa can have Kc above 1.0, meaning their actual water use exceeds the grass-reference baseline.
Under water stress, actual ET is less than ETc. A stress coefficient (Ks, 0–1) is multiplied: Actual ET = ET₀ × Kc × Ks. This calculator assumes no stress (Ks = 1).
One inch of water over one acre equals 27,154 gallons. Multiply ETc (in) by acreage and by 27,154 for total volume.
Not exactly. Net irrigation requirement = ETc minus effective rainfall. Gross irrigation also accounts for application efficiency (typically 75–95%).