# Water Yield: Evapotranspiration

Hello,
I have a question about which evapotranspiration layer I should use in the Water Yield model.
I’m working on a study that projects future land use in the future. Since evapotranspiration (ET) is directly related to temperature, I’d prefer to create my own layers to account for the effects of climate change.
To do so, I’m using the equation provided by the InVEST manual:
REFERENCE ET: ET0=0.0013×0.408×RA×(Tav+17)×(TD−0.0123P)^0.76

The problem is that when I compare values yielded by this equation (e.g. 40mm per year - REFERENCE ET) to values from the literature (e.g. 1,300mm per year - WorldClim: POTENTIAL ET) they have different magnitude. The manual says the potential ET “…may be used for reference ET”. However, these two layers yield completely different results. Hardly comparable. I believe the potential ET layer yields a more realistic output, even though the input data for this model should be the reference ET.
The study area is in the northern portion of Georgia, USA. Which value of ET you believe is more realistic?
Please, see both water yield outputs below (All other parameters constant, except by ET).

Thank you!

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It looks like you’re using the Modified Hargreaves method for calculating Reference ET, which I often use, so a few things come to mind. This equation is used to calculate a value for each month of the year, then the monthly values must be added together to get the annual value used in the model. Is the 40mm you mention the total over all 12 months?

Also be sure to verify the units for temperature and precipitation that go into the ET equation. I’ve been caught off guard when it turned out that my precip units were in cm, or temperature values were (real values * 10).

~ Stacie

@swolny Thank you very much for your contribution! I’m sorry I didn’t answer earlier, but just now I noticed your comment! My bad!

This value (40mm) is per year already and I double-checked all units, they are correct.

Thanks!