Irrigation Water Requirement Calculator
Calculates how many gallons of water your irrigation system must deliver daily, accounting for crop type, evapotranspiration, rainfall, and system efficiency. Ideal for scheduling irrigation runs during the growing season.
About this calculator
Crop water demand is driven by evapotranspiration (ET) — the combined loss of water through soil evaporation and plant transpiration. The FAO Penman-Monteith method estimates crop-specific ET by multiplying reference ET (ET₀) by a crop coefficient (Kc): crop ET = Kc × ET₀. Rainfall offsets some of this demand, so the net irrigation requirement per acre-inch is: max(0, Kc × ET₀ − rainfall). Converting acre-inches to gallons uses the factor 27,154 gallons per acre-inch. Finally, dividing by irrigation system efficiency accounts for distribution losses. The full formula is: Water (gal/day) = fieldArea × 27,154 × max(0, cropCoefficient × evapotranspiration − rainfall) / irrigationEfficiency. This gives the total gallons your pump must deliver each day to sustain crop growth without deficit stress.
How to use
Assume a 10-acre tomato field (Kc = 1.15), reference ET of 0.25 inches/day, expected rainfall of 0.05 inches/day, and drip irrigation at 90% efficiency (0.90). Net crop water need = max(0, 1.15 × 0.25 − 0.05) = max(0, 0.2875 − 0.05) = 0.2375 inches/day. Then: 10 × 27,154 × 0.2375 / 0.90 = 10 × 27,154 × 0.2375 / 0.90 ≈ 271,540 × 0.2375 / 0.90 ≈ 64,490 / 0.90 ≈ 71,656 gallons/day. Your drip system must deliver about 71,656 gallons per day to meet the tomato crop's water demand under those conditions.
Frequently asked questions
What is the crop coefficient (Kc) and how does it affect irrigation water requirements?
The crop coefficient (Kc) is a dimensionless factor that adjusts reference evapotranspiration (ET₀) to reflect the water use of a specific crop at a specific growth stage. Values typically range from 0.3 for sparse early-season crops to over 1.2 for full-canopy vegetables at peak growth. Multiplying ET₀ by Kc gives the actual crop water demand. Using the wrong Kc — such as applying a mid-season value during establishment — can lead to significant over- or under-irrigation. FAO Irrigation Paper No. 56 provides standard Kc tables for most common crops.
How does irrigation system efficiency affect the total water I need to pump?
Irrigation efficiency represents the fraction of pumped water that actually reaches the plant root zone. Flood irrigation may be only 60–70% efficient due to runoff and deep percolation, while well-maintained drip systems can exceed 90%. Dividing your net crop water need by efficiency scales up the pump requirement to compensate for losses. For example, at 70% efficiency you must pump about 43% more water than the crop strictly needs. Improving efficiency through better system maintenance, scheduling, or upgrading to drip or micro-sprinklers can dramatically reduce water and energy costs.
When should I use the irrigation water requirement calculator during the growing season?
You should recalculate irrigation requirements weekly, or whenever weather patterns shift significantly. Reference ET changes throughout the season as temperature, humidity, wind speed, and solar radiation fluctuate. Crop coefficients also change with growth stage — a young transplant has very different water needs than a mature fruiting plant. Incorporating current rainfall forecasts lets you avoid unnecessary irrigation on days when precipitation is expected. Many irrigators integrate this calculation into a daily scheduling routine alongside soil moisture sensor readings for the most precise water management.