Hero Image

ETo Irrigation

What is evapotranspiration (ET)?

Evapotranspiration or ET is a measure of all the water lost into the atmosphere from the combined processes of evaporation from the soil and plant surfaces and transpiration from plants. It is an indicator of the amount of water that must be replaced in order for plants to stay alive and grow. ET in the U.S. is measured in inches. It varies by plant type, and to some degree by soil type, and is most affected by the following factors:

  • Temperature
  • Solar radiation (length and strength)
  • Wind speed
  • Relative Humidity

What is reference evapotranspiration (ETo)?

Reference evapotranspiration (ETo) is derived by measuring weather conditions and estimating the ET of a reference plant. In California this is a standardized planted surface of well-maintained cool season turf. 

Where do we get ETo data?

ETo data is available online from over 100 weather stations throughout the state of California from the California Irrigation Management Information System (CIMIS). Minute-by-minute weather data is collected and used to calculate hourly, daily, weekly, or monthly ETo. We use ETo data from the two sites located near each of our trial fields (#6 Davis and #71 Irvine).  

How is ETo data used for irrigation?

In the trials, ETo is used to modify the irrigation frequency (READ MORE). By using real-time, local ETo data from CIMIS, we are ensuring that the irrigation treatments we apply in our trials are based on the most accurate estimate of actual plant water needs expressed as some percentage of the need of a cool-season grass lawn.

In landscapes, most "smart" irrigation controllers use either weather data collected on site, historical weather data from weather stations, or wireless links to current, real-time weather data.

Since the results from the trials are expressed as a percentage of ETo (e.g. ETo × 0.2 for Low; ETo × 0.5 for Moderate; ETo × 0.8 for High), they should be translatable to almost any location with available ETo data.  Drier, hotter areas will have higher ETo readings and therefore need greater amounts of water, while cooler areas with the same percentage of ETo will have lower water needs, even though the percentage is constant.