Irrigation Water Requirement: How to Calculate Daily Water Needs for Your Crop
Water is the largest variable input on most irrigated farms, and getting the amount wrong costs in both directions: too little and the crop stresses and yield drops; too much and you leach nutrients, run the pump longer than necessary and pay for water the roots never use. The irrigation water requirement is the volume you must actually deliver each day, after allowing for losses in the system. The Irrigation Water Requirement Calculator computes it in gallons per day from the crop's water use, rainfall, system efficiency and field area. This guide explains each input, works an example and shows how the figure drives scheduling and pump sizing.
How crop water demand is estimated
The calculator follows the FAO-56 approach used by extension services and irrigation schedulers worldwide. Crop water use (ETc) is reference evapotranspiration multiplied by a crop coefficient:
ETc = Kc × ET₀
- ET₀ (reference evapotranspiration) is the water a well-watered short grass surface would lose to the atmosphere under the day's weather, computed with the FAO Penman-Monteith equation from temperature, humidity, wind and solar radiation. Agricultural weather networks publish it daily; the calculator takes it in inches per day.
- Kc (crop coefficient) scales ET₀ to the actual crop and its growth stage. FAO-56 tabulates Kc by crop and stage; values are low at emergence, peak at mid-season (above 1.0 for many field crops) and fall as the crop matures. The calculator offers four presets (0.6 wheat/barley, 1.2 corn/soybeans, 1.05 cotton, 0.95 tomatoes); treat them as rough mid-season figures and look up the stage-specific value for your crop when precision matters.
The formula
Daily gallons = max(0, (Kc × ET₀ − effective rainfall) ÷ efficiency) × acres × 27,154
- Effective rainfall is the part of rain that infiltrates and reaches the root zone, in inches per day; subtracting it gives net irrigation need.
- Irrigation efficiency is the fraction of applied water that reaches the root zone. The calculator's presets are 45% for flood irrigation, 75% for sprinklers and 90% for drip. Dividing by efficiency inflates the net need to the gross amount you must pump.
- 27,154 is the number of gallons in an acre-inch (43,560 ft² × 1 in ÷ 12 × 7.4805 gal/ft³), so the inches-per-day depth times acres becomes gallons per day.
1. Crop water use: 1.2 × 0.22 = 0.264 in/day
2. Net need after rain: 0.264 − 0.05 = 0.214 in/day
3. Gross need: 0.214 ÷ 0.75 = 0.2853 in/day
4. Volume: 0.2853 × 80 × 27,154 ≈ 619,800 gallons per day
The crop itself uses only 0.214 in/day net, but you must apply 0.285 in/day because a quarter of the sprinkler water is lost. Switching to drip at 90% efficiency drops the gross need to 0.238 in/day and about 516,500 gallons, a saving of over 100,000 gallons a day for the same crop.
Using the requirement
Pump and run time. Divide the daily volume by the pump's flow. At 900 gallons per minute, 619,800 gallons takes 619,800 ÷ 900 ≈ 689 minutes, about 11.5 hours a day at peak demand. If that exceeds the hours available, the pump or the system capacity is too small for peak season.
Scheduling. Crops need not be watered daily; the soil's water-holding capacity lets you apply several days' requirement in one longer set.
Season budgeting. Sum the daily requirements across the season to size an allocation, reservoir or well draw, and to check it against any regulatory limit.
Common mistakes
A fixed Kc all season. Demand peaks at mid-season and in hot, dry, windy weather. Update Kc and ET₀ as the crop and forecast change.
Using measured rainfall instead of effective rainfall. Heavy rain on dry or sloping ground runs off; only the infiltrated fraction offsets irrigation.
Overstating efficiency. Worn nozzles, leaks and poor uniformity pull real efficiency below the preset. Use a conservative figure.
Mixing units. ET₀ and rainfall must both be in inches per day and area in acres; a millimetre ET₀ figure must be divided by 25.4 first.
Key takeaways
- Formula: gallons/day = (Kc × ET₀ − rain) ÷ efficiency × acres × 27,154, with ET₀ and rain in inches per day.
- Example: 80 acres of mid-season corn (Kc 1.2) at ET₀ 0.22 in/day with 0.05 in/day effective rain and 75% sprinkler efficiency needs about 619,800 gallons a day.
- Efficiency is the big lever: drip at 90% cuts that to about 516,500 gallons.
- Keep inputs current as growth stage, weather and rainfall change.