Growing season total. 75% efficiency assumed.
Cost Estimation
$5 to $15 typical in 2026
If purchasing water
Select a crop and irrigation method to calculate seasonal water requirements in gallons per acre per day, total volume, peak demand, and pumping costs.
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What This Calculator Does
This irrigation water requirement calculator estimates the gallons per acre per day, total seasonal water volume, and peak demand for irrigated crops based on crop type, growing season length, irrigation method efficiency, and rainfall. It covers major irrigated crops including corn (22 inches seasonal), soybeans (18 inches), cotton (24 inches), alfalfa (36 inches), and rice (48 inches). The tool adjusts gross water application for irrigation system efficiency ranging from 60% for furrow/flood to 95% for subsurface drip, and calculates pumping costs per acre-inch using 2026 energy cost estimates.
The Formula
Crop water need is the total seasonal evapotranspiration requirement in inches. Effective rainfall is typically 75% of actual rainfall (accounting for runoff and deep percolation). Net irrigation is the deficit that must be supplied by the irrigation system. Gross irrigation divides by the system efficiency (expressed as a decimal) to account for water lost to evaporation, wind drift, and deep percolation during application. One acre-inch of water equals 27,154 gallons. Peak demand is calculated from the highest weekly water use rate (typically during tasseling for corn or pod fill for soybeans) and determines the required pumping capacity in gallons per minute (GPM).
Step-by-Step Example
Select crop and method
Corn selected (22 inches seasonal need, 2.0 inches/week peak). Center pivot irrigation (85% efficiency).
Enter acres and rainfall
160 irrigated acres. Expected 8 inches of growing season rainfall (6 inches effective at 75%).
Calculate water requirement
Net irrigation: 16 inches. Gross irrigation: 18.8 inches. Total: 80.8 million gallons (248 acre-feet) for the season.
Review peak demand and costs
Peak: 729 GPM. Pumping cost at $8/acre-inch: $24,094 for the season.
Real-World Use Cases
Irrigation System Sizing
Determine the required pump capacity (GPM) based on peak demand to ensure the system can keep up with crop water needs during critical growth stages.
Water Rights and Allocation Planning
Calculate total seasonal water volume in acre-feet to compare against water rights allocations and groundwater pumping permits, especially in restricted regions.
Irrigation Cost Budgeting
Estimate annual pumping costs as an input to crop production budgets, comparing the cost of irrigation against the expected yield increase from irrigated versus dryland production.
Common Mistakes to Avoid
Using total rainfall instead of effective rainfall. Only 60% to 80% of rainfall is available to the crop. Intense storms lose water to runoff, and light rains may evaporate before reaching the root zone.
Not matching irrigation capacity to peak demand. A system that delivers adequate water on average may fall short during the 2 to 3 weeks of peak demand (tasseling, pod fill) when crop water use doubles.
Ignoring system efficiency differences. Flood irrigation delivers only 50% to 65% of pumped water to the crop. Switching to center pivot (85%) or drip (92%) can reduce water use by 25% to 40%.
Assuming uniform soil water holding capacity. Sandy soils hold 0.5 to 1.0 inches per foot of depth versus 1.5 to 2.0 for loam. Sandy soils require more frequent, lighter irrigations.
Not accounting for energy cost increases. Pumping costs depend on fuel/electricity prices, pump efficiency, and pumping depth. Deeper wells and higher energy costs significantly increase the per-acre-inch cost.
Frequently Asked Questions
Accuracy and Disclaimer
This calculator provides irrigation water estimates based on typical crop water use data and standard efficiency values. Actual water requirements depend on specific climate conditions, soil type, crop variety, management practices, and local evapotranspiration rates. Consult your local extension irrigation specialist or use weather-based ET models (such as NDAWN or CoAgMet) for field-specific scheduling.
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