Irrigation Water Requirement Calculator
Calculate the total daily irrigation water requirement in cubic meters by multiplying crop area and water need per day, then dividing by system efficiency. Use it for irrigation scheduling, pump sizing, and water allocation planning.
Last updated: May 2026
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About this calculator
The formula is: dailyWater (m³) = (cropArea × waterRequirement) / irrigationEfficiency, where cropArea is hectares, waterRequirement is mm/day (1 mm over 1 hectare = 10 m³), and irrigationEfficiency is a decimal (0.50 to 0.95). The 1 mm × 1 ha = 10 m³ conversion is implicit in the units. Edge cases: zero efficiency causes division by zero; zero area or requirement produces zero output. Crop water requirement (also called evapotranspiration or ET) depends on crop, growth stage, and climate. Typical mid-season peak ET values: 5–9 mm/day for high-water crops (corn, alfalfa, rice) in warm climates; 4–6 mm/day for moderate-water crops (wheat, soybeans, vegetables); 2–4 mm/day for low-water or cool-climate conditions. ET values are calculated from weather station data (reference ET₀ × crop coefficient Kc) — most US states publish daily ET₀ through agricultural weather networks (CIMIS in California, AgWeatherNet in WA, NEWA in NY). Subtract effective rainfall (rain that infiltrates and reaches the root zone, typically 50–80% of total precipitation) from crop ET to get net irrigation requirement. Irrigation efficiency varies by method: surface flood 40–60%; furrow 50–70%; sprinkler 70–85%; micro-sprinkler 80–90%; surface drip 85–95%; subsurface drip 90–95%. Higher efficiency means less applied water for the same crop benefit, saving water and energy. Distribution uniformity (DU) is a related concept — how evenly water is delivered across the field; low DU forces over-application to ensure the driest spots get enough water. For seasonal water budgeting, multiply daily requirement by days in the growing season; for pump sizing, use peak daily requirement plus 20% headroom.
How to use
Example 1 — Mid-season corn in Iowa. 80-hectare corn field, peak ET 7 mm/day, center-pivot sprinkler efficiency 0.80, no recent rainfall. Enter cropArea 80, waterRequirement 7, irrigationEfficiency 0.80. Result: (80 × 7) / 0.80 = 560 / 0.80 = 700 m³/day. ✓ At a center-pivot delivery rate of typical 800 m³/hour (~3,500 gpm), the system needs to run ~52 minutes per day at peak demand, or roughly 6 hours every 7 days if running daily. For pump sizing, ensure 800 m³/hour minimum capacity with 20% headroom = ~950 m³/hour. Verify against well capacity and water rights allocation; many irrigation districts cap daily withdrawals. Example 2 — Drip-irrigated almond orchard. 40 hectares, mid-season ET 6 mm/day, drip efficiency 0.92. Enter cropArea 40, waterRequirement 6, irrigationEfficiency 0.92. Result: (40 × 6) / 0.92 = 240 / 0.92 ≈ 261 m³/day. ✓ Drip orchards typically run 8–14 hours/day at peak; this matches a system delivering ~30 m³/hour or ~130 gpm. For seasonal planning: at 6 mm/day × 200 days growing season × 40 ha = 4,800 m³ × efficiency adjustment = ~5,200 m³ total seasonal irrigation. Verify against well/canal allocation; California almond water budgets typically allow 30–40 acre-inches/acre/year (~9–12 ML/ha).
Frequently asked questions
How do I find crop water requirements for my location?
Real-time data is usually available from state agricultural weather networks. California: CIMIS (cimis.water.ca.gov) provides daily reference ET₀ and crop coefficient lookups for 250+ weather stations. Washington: AgWeatherNet (weather.wsu.edu). New York and Northeast: NEWA (newa.cornell.edu). Texas: TexasET Network. Australia: BOM AgClimate. Most US states have similar services through state universities or USDA. Reference ET₀ × crop coefficient Kc = actual crop ET. Crop coefficients are published in FAO-56 Irrigation and Drainage Paper No. 56 (the global standard) and state-specific extension publications. Kc varies by growth stage: 0.3–0.4 early planting; 0.6–0.9 vegetative growth; 1.0–1.2 mid-season peak; 0.4–0.7 senescence. Apps like Open ET (openetdata.org), CropManage (cropmanage.ucanr.edu), and IrrigationMate provide automated daily ET-based recommendations integrating real-time weather, crop stage, and growth degree day tracking. For planning purposes, use seasonal-average ET; for in-season scheduling, use real-time data. Soil moisture sensors (Watermark, AquaSpy, Sentek) verify whether scheduling is on target.
What is irrigation efficiency and how do I measure it?
Irrigation efficiency = water meeting crop demand / water applied. The "lost" water goes to deep percolation below the root zone, surface runoff, evaporation from soil surface or droplet flight, wind drift, or distribution uniformity issues. Efficiency varies by system: surface flood 40–60% (highly dependent on field slope and soil); furrow 50–70%; sprinkler 70–85% (lower in hot/windy, higher with low-pressure nozzles); micro-sprinkler 80–90%; surface drip 85–95%; subsurface drip 90–95%. Measuring your own efficiency: 1) Catch-can test for sprinkler/drip — distribute small containers across the field, run the system, measure water collected in each, calculate uniformity (Coefficient of Uniformity, Distribution Uniformity). DU > 80% is good; > 90% is excellent. 2) Soil moisture monitoring — sensors before and after irrigation tell you how much water reached the root zone vs how much was applied. 3) Pump flow meter and runtime — total volume applied / total irrigation cycles compared to ET demand. Most irrigation districts and USDA NRCS offer free irrigation audits; identifying and correcting inefficiencies typically saves 10–25% water at low or no cost (pressure regulation, nozzle replacement, scheduling adjustments).
How do I schedule irrigation more efficiently?
Use ET-based scheduling rather than fixed calendar or visual stress cues. ET-based scheduling: each day, calculate crop ET (Kc × ET₀ from local weather station), subtract effective rainfall, accumulate the daily deficit; irrigate when accumulated deficit reaches your soil's allowable depletion (typically 50% of available water capacity in the root zone). Available water capacity (AWC) depends on soil texture: sandy soils 80–120 mm/m of soil depth; loam soils 150–200 mm/m; clay soils 180–250 mm/m. For a 1-meter root zone in loam, total AWC = ~175 mm; depleting 50% = ~88 mm — irrigate when you've accumulated 88 mm of net crop demand. Soil moisture sensors verify and refine: Watermark gypsum-block sensors are cheap ($30 each) and widely used; capacitance and TDR sensors (Sentek, Acclima) more expensive but more accurate. Modern apps integrate weather, soil, and crop stage (Climate FieldView, John Deere Operations Center, CropManage) for automated scheduling. Schedule to avoid runoff: shorter cycles with rest periods ("cycle and soak") allow water to infiltrate slowly without ponding. Irrigate early morning (before 10 AM) or evening to reduce evaporation losses 10–30% vs midday.
What are the most common irrigation scheduling mistakes?
The biggest is fixed-calendar scheduling regardless of weather; a 1-inch rainstorm can eliminate 5+ days of irrigation need but is often ignored. The second is over-watering "to be safe"; excess water leaches nutrients, increases disease, wastes energy, and can salinize soil over years. The third is using outdated Kc values that don't 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 on a 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 are under-watered — 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 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 60–90 cm; over-irrigation past that depth wastes water and leaches nutrients. The ninth is allowing systems to leak; small leaks compound to 10–30% water waste over a season. The tenth is treating tail-end of the season same as peak; late-season ET drops significantly as crops senesce — many growers over-irrigate in the last 3–4 weeks before harvest, wasting water with no yield benefit and potentially delaying maturity.
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. 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. For aquaculture and pond systems, water exchange and chemistry drive requirements, not ET-based calculations. And for variable-rate irrigation (VRI) where different parts of the same pivot apply different rates, use VRI-specific software (Valley AgSense, Lindsay FieldNET) rather than the whole-field average formula.