Calculating ET

ET stands for evapotranspiration and is a measurement of the loss of moisture in the landscape through evaporation from the soil and transpiration from plant leaf surfaces. It is provided in inches per day. ET is determined by the following four factors:

  • Solar radiation
    Length of daylight hours and intensity of sun. Longer daylight hours, such as during the summer season, with little or no cloud cover increases water loss.
  • Temperature
    Higher temperatures result in higher water loss.
  • Wind
    Stronger winds result in higher water loss.
  • Humidity
    Lower humidity results in higher water loss.
Also, please keep in mind, precipitation replaces water lost, and reduces the need for supplemental irrigation. Other factors, such as slope, soil conditions and exposure must be considered when determining how much water to apply. 

Determining how much water to apply

For a typical residential lawn, add together all the ET values since you last watered - check out the historical data from our weather stations. Multiply the total by 75 percent. This is the amount, in inches, that should be applied to nourish plants. 



This ET value is standardized for an average cool season turf grass, such as Kentucky Blue Grass or a fescue blend. Trees, shrubs, alternative turf types, and especially xeric plants may require significantly less water. Learn more with our Run Time Calculator.