Question about calculating Reference Evapotranspiration

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Dear Community,

I had some problem when calculating Reference Evapotranspiration by using the ‘Modified Hargreaves’ Method formula :𝐸𝑇0=0.0013×0.408×𝑅𝐴×(𝑇𝑎𝑣+17)×(𝑇𝐷−0.0123𝑃)^0.76.
My result of ET0 raster showed that some value of pixels were negative. Concretely，in some pixels, 𝑇𝐷−0.0123𝑃<0，resulting in 𝐸𝑇0<0. I am confused about it because ET0 refers the amount of water that vaporizes from land into the air over a given period of time.

Thanks all in advance!

Best,

Jiangxuan

2

Hi @oohjiangxuan -

What are the temperature values where the ET0 values are negative? It’s certainly possible for this equation to create negative numbers if the temperature is very low.

~ Stacie

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