Reference Evapotranspiration


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ET (
evapotranspiration) is the water evaporated from the ground back to the atmosphere both as transpiration from the leaves of plants and also as direct evaporation from open water and soil.

Reference Evapotranspiration is defined as "the ET from an extensive surface of clipped grass (ETo) or alfalfa (ETr) that is well-watered, and fully shades the ground." (Kimberly R&E Center, Univ. of Idaho)


Help CoCoRaHS measure EToWe know how much water comes from the sky (precipitation) since many of us measure it.  Knowing how much water is leaving the soil and returning to the atmosphere is just as important.  It matters for agriculture, lawn care, weather prediction, hydrology and much more, so we should try to measure it.

Interested in becoming a ETo observer?

Some things to consider as you apply:

1. We are looking for "ideal" locations (open exposures with surrounding vegetation that is representative of your area) and highly motivated observers (you have reported on a consistent daily basis for CoCoRaHS over time).

2. You are up for the challenge of taking on a more complex daily observation.

3. You or a sponsoring organization are willing to make the substantial investment in purchasing this delicate instrument, the ETgage ($239.75 plus tax and shipping from ).

Still interested?
Please contact Noah at with a photo of the site where you will install the gauge and he will follow up with you.

 View Evapotranspiration Reports

 View Evapotranspiration for a 7 day period
The CoCoRaHS Guide for Measuring Reference Evapotranspiration (ETo) is supported by NOAA’s Office of Education, the National Science Foundation and Colorado State University.

    For Reference ET articles/equations/software --

    University of Idaho:

    Google: fao56

    To see FAO Irrigation and Drainage Paper 56 – Crop Evapotranspiration

    Google: modified atmometer and irrigation

    Go to to see description sheets for Model A and E for data comparisons for reference ET.