WSU Tree Fruit Research & Extension Center

TFREC Computer & Web Resources

WSU-TFREC Orchard Calculator

This calculator is an implementation of the Penman equation in JavaScript. For real time data and estimated evapotranspiration in Washington state visit the WSU-AgWeatherNet site.

Estimating water use by plants: Penman calculator for water use by grass

1. Enter the daily (24 hour) AVERAGE solar radiation watts/meter-sq
OR time of year:
  sky conditions:

2. air temperature ° F

3. relative humidity %

OR dew point ° F

4. wind speed mph

OR wind condition:

Daily potential water use by grass in.