Irrigation & Water
Rainfall Deficit Calculator
Calculate irrigation needs by comparing crop water demand to actual rainfall.
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How to Use This Calculator
Enter the weekly crop evapotranspiration (ET) from your local extension office or weather station data. Enter the average weekly rainfall for the same period. Set the growing season length in weeks. The calculator shows how much supplemental irrigation you need.
Why This Matters
Rainfall rarely matches crop water demand perfectly. Knowing your seasonal deficit helps you plan water storage, well capacity, and irrigation system sizing before the growing season starts, preventing mid-season water shortages that devastate yields.
Methodology
Weekly deficit = Weekly ET - Weekly rainfall (minimum 0). Season total deficit = Weekly deficit × Number of weeks. Gallons per acre = Total deficit (inches) × 27,154. Not all rainfall is effective — only 50-75% of heavy rain infiltrates the root zone.
Common Mistakes to Avoid
- Assuming all rainfall is effective — heavy storms produce runoff, not root zone moisture.
- Using annual average rainfall instead of weekly data during the actual growing season.
- Not adjusting ET for crop growth stage — peak water demand occurs at reproductive stages.
- Ignoring that ET varies significantly by region, crop, and time of year.
Tips & Best Practices
- Not all rainfall is effective — heavy rains may run off without infiltrating.
- Use soil moisture sensors for precise irrigation scheduling.