Water yield Z parameter

Dear Nat Cap Team,

I am working with the Invest 3.8 water yield model. The model output looks good but I noticed when experimenting with the Z parameter (shifting from 8.4 to 27.2) the output values remained identical. Any idea what might explain that?

Thank you in advance for your help and for creating this super resource.


InVEST-Hydropower-Water-Yield-log-2020-02-27–14_18_04.txt (7.3 KB)

Hi @dgw_awf, welcome back to the forums!

The clearest possibility for this is if the climate equation from the User’s Guide. It looks like the \omega(x) parameter is calculated by a linear function that includes Z, but then the User’s Guide also says this:

Z is an empirical constant, sometimes referred to as “seasonality factor”, which captures the local precipitation pattern and additional hydrogeological characteristics. It is positively correlated with N, the number of rain events per year. The 1.25 term is the minimum value of ω(x), which can be seen as a value for bare soil (when root depth is 0), as explained by Donohue et al. (2012). Following the literature (Yang et al., 2008; Donohue et al. 2012), values of ω(x) are capped to a value of 5.

My guess is that the calculated \omega(x) is consistently greater than 5 given the Z, PAWC and Annual Precipitation inputs provided.

Hope this helps!

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Many thanks for the detailed and constructive reply James and apologies for the slow reply here. Indeed lower values of Z produce different results.