How irrigation water need is calculated
The calculator subtracts entered effective rainfall from the selected crop’s preset peak ET, with a minimum net need of zero. It divides by the selected system’s efficiency to estimate gross water, then converts acre-inches to US gallons and pump runtime.
net inches/day = max(0, crop ET − effective rainfall); gross inches/day = net inches/day ÷ efficiency; gallons/day = gross inches/day × acres × 27,154; pump hours/day = gallons/day ÷ (flow in gpm × 60)Use the same time period and US units
ET and effective rainfall are in inches per day. Efficiency is a decimal fraction, acres are field area, and pump flow is US gallons per minute. The calculator uses approximately 27,154 US gallons per acre-inch. Weekly acre-inches multiply gross daily inches by acres and seven days.
Planning, not a live irrigation decision
ET and system-efficiency values are presets, not measurements of your farm. Verify local ET, rainfall, soil moisture, crop conditions, system performance and water-allocation limits before irrigating. This simplified estimate is not a full soil-water balance or an application recommendation.
University of Minnesota Extension: irrigation water-balance guidance explains field-specific water-balance scheduling. It does not certify this calculator or its crop and efficiency presets.