Can we use PET to replace ETo in Water-yield model?

Hi,

I am a bit confusing about which data of evatranspiration that I can use in water-yield model. As shown in the User Guide of InVEST 3.2.2, it seems to me that I could use PET to replace ETo (page 126). 

I got PET raster from http://www.cgiar-csi.org/, can I use this raster directly? Or is there a way to calculate ETo raster from this PET raster? 

Thank you very much,
Duc


I quoted the method of calculating ETo as shown in page 126:
"The reference evapotranspiration could be also calculated monthly and annually using the Hamon equation
(Hamon 1961, Wolock and McCabe 1999):

Comments

  • PerrinePerrine Moderator, NatCap Staff
    Hi Duc, 

    You are right, the CGIAR dataset can be used for the ETo input in InVEST water yield. 

    The confusion between "reference" evapotranspiration and "potential" evapotranspiration is common in the literature. We prefer the terminology used by Allen et al. (1998): "reference" evapotranspiration is only a function of climate variables and represents the evapotranspiration for a well-watered lawn (~ equivalent to forests). "Potential" evapotranspiration comprises information on the vegetation characteristics (e.g. it is lower than ETo if the plant has a low leaf area index).

    The CGIAR dataset represents "reference" evapotranspiration, since it is only a function of climate variables, as described in the Methods.
    Cheers

    Perrine

    Ref:
    Allen, R.G., Pereira, L.S., Raes, D., Smith, M., 1998. Crop evapotranspiration - Guidelines for computing crop water requirements, FAO Irrigation and drainage paper 56. Rome, Italy.
  • Thank you very much Perrine!

    I have another question. I want to convert a map of wyield (mn) estimated by the model to a map of wyield (m3). Can you show me the conversion formula?

    Thank you again,
    Duc
  • PerrinePerrine Moderator, NatCap Staff
    Hi, 

    The map of wyield is in mm. To convert mm to m3, you need to divide by 1000 (to obtain meters), and then by the area of the pixel (in m2).

    If you use the mean wyield value in the attribute table (also in mm), you follow the same procedure: divide by 1000, then multiply by the area of the subwatershed.

    Best,
  • Hi Perrine,

    If my map's cell size is 30x30 = 900 (m2), then to convert wyield in mm to m3, what I need to do is dividing the values by 1000 then multiple by 900. Am I correct?

    Best regards,
    Duc
  • PerrinePerrine Moderator, NatCap Staff
    Correct. This gives you the volume of water "produced" by the pixel (i.e. precipitation falling on the pixel and not evapotranspired by its vegetation).
    Best,

    Perrine
  • Thanks a lot, Perrine!
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