A weekly water budget is easier to understand when crop use, useful rainfall and irrigation losses are kept separate. A rainfall total cannot simply be treated as irrigation delivered to the roots, and a crop coefficient is not a weather reading.
The Crop Water Budget Calculator starts with your entered reference evapotranspiration, or ETo, and crop coefficient, Kc. It uses their product for crop ET, subtracts entered effective rainfall for the whole period and divides the remaining requirement by application efficiency.
A seven day worksheet
Use ETo of 5 mm per day, Kc of 0.9 and seven days. Estimated crop ET is 4.5 mm per day, or 31.5 mm across the period. With 10 mm of effective rainfall, net irrigation depth is 21.5 mm.
At 85% application efficiency, gross applied depth is about 25.294 mm. Across 20 m², that corresponds to approximately 505.882 litres for the period, or an average 72.269 litres per day.
| Water balance stage | Period depth |
|---|---|
| Entered ETo × Kc × days | 31.5 mm |
| Entered effective rainfall | 10 mm |
| Net irrigation requirement | 21.5 mm |
| Gross depth at 85% efficiency | 25.294 mm |
Effective rain is already a net input
This field expects the rain considered useful to the crop during the period. It is not automatically the full amount recorded in a gauge. The calculator does not estimate runoff, infiltration or soil storage before accepting that entry.
If entered effective rainfall exceeds the calculated crop ET, net irrigation is clipped to zero. The model does not carry surplus rainfall into a future week or maintain a daily soil water balance.
The crop coefficient method follows the relationship explained in FAO’s chapter on crop evapotranspiration. Your ETo, crop stage and coefficient still need an appropriate local basis.
Average daily litres are not a timer setting
The daily result spreads the period estimate evenly for comparison. It does not say that the same volume must be applied every day. Soil water storage, crop stage and actual weather can change the useful timing.
For an actual irrigation session, enter its chosen net litre target into Drip Irrigation Runtime Calculator with the zone's measured flow and efficiency. Make sure you transfer the net volume, not a gross volume that has already been divided by efficiency, or you will apply the loss allowance twice.