Irrigation & Water
Irrigation Water Need Calculator
Calculate daily and weekly irrigation water needs based on crop evapotranspiration and system efficiency.
Input
Fill in the fields below, then click Calculate.
Evapotranspiration rate from weather station or estimate
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How to Use This Calculator
Use this irrigation calculator to determine crop water needs for your fields. Enter your field size in acres and the daily evapotranspiration rate from your local weather station or crop ET table. Select your irrigation system type to account for application efficiency. Subtract any recent rainfall to see net irrigation needs.
Why This Matters
Under-irrigating reduces crop yield and quality, while over-irrigating wastes water, increases pumping costs, and can cause nutrient leaching and root diseases. Proper irrigation scheduling matched to actual crop water demand saves 20-40% on water costs.
Methodology
Net irrigation need = (ET × days) - effective rainfall. Gross need = Net / system efficiency. One acre-inch equals 27,154 gallons. Drip systems are 90-95% efficient, sprinklers 75-85%, and flood/furrow 50-70%.
Common Mistakes to Avoid
- Using a fixed irrigation schedule instead of adjusting for actual ET and rainfall.
- Not accounting for system efficiency — sprinklers lose 15-25% to evaporation and wind.
- Ignoring soil type — sandy soils need more frequent, lighter irrigations than clay soils.
- Watering during the heat of the day when evaporation losses are highest.
Tips & Best Practices
- Check local weather station data for daily ET rates.
- Peak ET rates in summer can reach 0.3-0.4 inches/day for many crops.
- Schedule irrigation based on soil moisture monitoring, not just calendar.
- Deficit irrigation can save water without significant yield loss for some crops.