Irrigation Water Requirement Calculator
Calculate daily irrigation water requirement in gallons based on crop water demand (Kc × reference ET), rainfall, system efficiency, and field area. Use it for irrigation scheduling that meets crop needs without overwatering or wasting energy.
Last updated: May 2026
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About this calculator
The formula applies the FAO-56 reference evapotranspiration framework: cropET = Kc × ET₀, where Kc (crop coefficient) is the crop-and-stage-specific multiplier on reference ET (the ET of a standard short grass under full water). The formula computes net crop demand (cropET − rainfall), divides by irrigation efficiency (because not all applied water reaches the root zone), multiplies by field area, and converts to gallons (27,154 gallons = 1 acre-inch). Typical Kc values: bare soil or early planting 0.3–0.4; vegetative growth 0.7–1.0; mid-season peak (most crops) 1.0–1.20; late season/senescence 0.4–0.7. Crops with high water use: alfalfa 0.95–1.20; corn 1.10–1.20 mid-season; rice 1.05–1.20 (plus standing water); cotton 1.10–1.15. ET₀ varies by climate and season: arid summer (Arizona, Central Valley CA) 0.3–0.4 inches/day peak; humid summer (Iowa, Indiana) 0.15–0.25 inches/day peak; spring/fall in most US climates 0.10–0.18 inches/day. Rainfall to subtract is effective rainfall — rain that infiltrates and reaches the root zone — typically 50–80% of measured precipitation depending on intensity, soil, and storage capacity. Irrigation efficiency: surface flood 40–60%; sprinkler 70–85%; drip 85–95%. Edge cases: rainfall exceeding cropET produces negative output (no irrigation needed); zero efficiency causes division by zero. The result is the daily gross irrigation water that the system must deliver to meet net crop demand. For weekly or seasonal scheduling, multiply daily by days between irrigation events. Modern irrigation management uses real-time ET₀ from weather networks (CIMIS in CA, ET-based scheduling apps in most states); seasonal averages work for planning but real-time data optimizes weekly decisions.
How to use
Example 1 — Mid-season corn in Iowa. 80-acre field, peak corn Kc 1.15, summer ET₀ 0.22 inches/day, rainfall over the past day 0.05 inches/day effective, center-pivot sprinkler efficiency 0.80. Enter cropCoefficient 1.15, referenceET 0.22, rainfall 0.05, irrigationEfficiency 0.80, fieldArea 80. Net crop demand = 1.15 × 0.22 − 0.05 = 0.203 inches/day. Gross requirement = 0.203 / 0.80 = 0.254 inches/day. Daily gallons = 0.254 × 80 × 27,154 = ~551,200 gallons/day. ✓ A typical center-pivot delivers 800–1,000 gpm; 551,200 gpd requires ~9–11 hours of run time per day at peak demand. Example 2 — Drip-irrigated almond orchard. 40 acres, mid-season Kc 1.05, hot Central Valley summer ET₀ 0.32 inches/day, no rainfall, drip efficiency 0.92. Enter cropCoefficient 1.05, referenceET 0.32, rainfall 0, irrigationEfficiency 0.92, fieldArea 40. Net demand = 1.05 × 0.32 = 0.336 inches/day. Gross requirement = 0.336 / 0.92 = 0.365 inches/day. Daily gallons = 0.365 × 40 × 27,154 = ~396,500 gallons/day. ✓ Drip orchards typically run 8–14 hours/day at peak; this matches a system delivering ~500 gpm.
Frequently asked questions
What is reference evapotranspiration (ET₀)?
ET₀ is the rate of water loss from a standardized hypothetical short reference crop (clipped grass, well-watered, full ground cover) under the prevailing weather. It is a climatic demand measurement — how thirsty the atmosphere is — and is independent of the actual crop in the field. ET₀ is calculated from weather station data (temperature, humidity, wind speed, solar radiation) using the FAO-56 Penman-Monteith equation, the international standard. Most US states publish ET₀ daily through agricultural weather networks (CIMIS in California, NEWA in the Northeast, AgWeatherNet in Washington, etc.); apps like CropManage and Wateright pull this data automatically. Daily ET₀ ranges from 0.05 inches in cool overcast winter conditions to 0.40 inches in hot windy desert summer. Multiplying ET₀ by a crop coefficient (Kc) gives the actual crop water requirement; ET₀ alone tells you climatic demand but not what the crop is using.
What is a crop coefficient (Kc) and where do I get values?
Kc is a dimensionless multiplier on ET₀ that scales reference grass ET to the actual crop's ET at its current growth stage. Kc varies with crop, growth stage, canopy cover, and water management. The FAO-56 Irrigation and Drainage Paper No. 56 (Allen et al., 1998) is the global reference; most US extension services publish state-specific Kc tables for major crops. Generic patterns: Kc starts low at planting (0.3–0.4 for bare soil), rises with canopy development (0.6–0.9 mid-vegetative), peaks at full canopy and reproductive stage (1.0–1.20 for most field crops), then declines as the crop matures (0.6–0.8 grain fill to senescence). For tree crops, Kc rises from spring leaf-out to summer (peak ~1.0–1.15) and falls in autumn. Real-time crop water use apps (Open ET, IrrigationMate) provide Kc-based estimates that account for stage automatically; manual Kc lookup is fine for planning but real-time data improves scheduling.
What is irrigation efficiency and how is it measured?
Irrigation efficiency is the fraction of applied water that meets crop water demand. The rest is lost to deep percolation below the root zone, runoff, evaporation from soil surface or droplet flight, wind drift, or uneven distribution across the field. Typical efficiencies vary widely by method: surface flood 40–60%, furrow 50–70%, sprinkler 70–85%, micro-sprinkler 80–90%, surface drip 85–95%, and subsurface drip 90–95%. Distribution uniformity (DU) is a related measure of how evenly water is applied across the field. A system with 70% DU means the driest quarter of the field gets 70% as much water as the average area, requiring more total water to fully irrigate the dry zone. Improving efficiency through pressure regulators, properly sized nozzles, and regular system audits reduces water and energy costs simultaneously. Shorter run times during high-evaporation midday conditions further trim losses with no equipment investment.
What are the most common irrigation scheduling mistakes?
The biggest is irrigating on a fixed calendar regardless of weather; a rainstorm that delivers 1 inch may eliminate irrigation need for 5+ days but is often ignored. The second is over-watering "to be safe"; excess water moves nutrients below the root zone, increases disease pressure, wastes energy, and can salinize soil over years. The third is using outdated Kc values that do not match modern crop varieties — newer hybrids often have higher peak water use than 1980s references. The fourth is failing to adjust for system efficiency — running drip irrigation on a fixed schedule designed for sprinkler over-irrigates significantly. The fifth is ignoring distribution uniformity; uneven application means parts of the field are always over-watered while others under-water — fix the system rather than over-irrigate to compensate. The sixth is irrigating during the hottest part of the day, increasing evaporation losses 10–30%; early morning or night irrigation is far more efficient. The seventh is failing to monitor soil moisture; sensors confirm whether scheduling is on target, eliminating guesswork. The eighth is forgetting that root zone depth limits stored water — most annual crops effective root zone is 24–36 inches; over-irrigation past that depth wastes water.
When should I not use this calculator?
Skip it for rice paddies and other crops irrigated to flood depth rather than to soil moisture replacement; flooded systems use a different framework (water depth maintenance plus seepage losses). It is the wrong tool for highly variable terrain where field-wide averages mask large within-field differences; use zone-based scheduling tied to soil moisture sensors. Do not use it for greenhouse and container irrigation where root volume is tiny and demand is driven by container size and substrate, not field ET. For sub-surface drip on permanent crops with established root systems, real-time soil moisture and sap flow monitoring outperform any ET-based estimate. For deficit irrigation strategies (intentionally under-irrigating to control vegetative growth in wine grapes, for example), the standard formula does not capture intentional stress targets. And for rainfed dryland production, the goal is conservation rather than supplementation; this calculator does not apply. For frost protection irrigation (sprinklers running below freezing to protect blossoms), water requirement is determined by frost duration and intensity, not crop ET.