Evapotranspiration Water Need
Calculator
Results
- Crop evapotranspiration ETc (mm/day)
- 6.324999
- Irrigation need (mm/day)
- 5.124999
- Irrigation need over the period (mm)
- 35.874999
- Irrigation need (m³/ha)
- 358.749999
Farming results
| Crop evapotranspiration ETc (mm/day) | 6.324999 |
| Irrigation need (mm/day) | 5.124999 |
| Irrigation need over the period (mm) | 35.874999 |
| Irrigation need (m³/ha) | 358.749999 |
formula-map diagram
- Crop evapotranspiration ETc (mm/day)
- 6.324999
- Irrigation need (mm/day)
- 5.124999
- Irrigation need over the period (mm)
- 35.874999
- Irrigation need (m³/ha)
- 358.749999
Formula map
Formula
ETc = Kc × ETo; irrigation need = ETc − effective rainfall= 6.325
Note
Simplified agronomic model: results use the rates, grades and coefficients you enter and ignore soil type, weather, variety and local regulations. Confirm with a soil test, the product label and an agronomist before applying anything.
More in Farming and crops
See all →Frequently asked questions
What is evapotranspiration and why does it drive irrigation need?+
Evapotranspiration (ET) is the combined water loss from soil evaporation and crop transpiration, and it represents the water the crop is actually using each day. Irrigation scheduling based on ET replaces exactly what the crop has consumed rather than watering on a fixed calendar.
What is the difference between reference ET and crop ET?+
Reference ET (ET0) is calculated from weather data for a standard grass surface; crop ET (ETc) adjusts that reference figure using a crop coefficient (Kc) that reflects your specific crop's growth stage and canopy development. Always apply the crop coefficient — using ET0 directly will misstate actual crop water use.
Why does the crop coefficient change during the season?+
A young crop with little leaf area transpires much less than the same crop at full canopy, so the crop coefficient rises from planting through peak growth and then typically falls again as the crop matures and senesces. Using a single fixed coefficient all season overestimates early-season needs and can underestimate peak-season needs.
Should I subtract rainfall from the ET-based water need?+
Yes — only 'effective rainfall,' the portion that actually infiltrates and is available to roots (excluding runoff and deep percolation), should be subtracted from ET to determine the remaining irrigation requirement. Total rainfall alone overstates how much water the crop actually received.
Does soil type affect the ET-based irrigation need?+
Soil type doesn't change ET itself, but it changes how much of that need can be met from stored soil moisture between irrigations. Sandy soils hold less water and need more frequent, smaller irrigations, while clay soils allow longer intervals with larger applications for the same total ET.