# Water yield parameter

When I run the model with different z parameter,I get the same result.Can you help me explain this ?
And how can I get different water yield output?
Thank you very much.InVEST-Hydropower-Water-Yield-log-2021-09-02–09_45_53.txt (30.8 KB)
InVEST-Hydropower-Water-Yield-log-2021-09-02–10_22_24.txt (30.7 KB)

Hey @fiona -

I’m not entirely sure why you’re getting the same result with such different values of Z. But if I look at the model equations that include Z, the User Guide 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

So we see that Z is used for calculating ω(x), and ω(x) is limited to values between 1.25 and 5. So my guess is that the values that you are using, combined with the AWC and P rasters (I’d reference the ω(x) equation in the User Guide, but it doesn’t seem to be numbered, I’ll have to look into why) is causing ω(x) to have the same value for both of your values of Z.

Unfortunately, the Annual Water Yield model doesn’t provide as many intermediate files as some of our other models, so it’s harder to troubleshoot the equations step by step. But do look at the equation for ω(x), and evaluate your Z, AWC and P inputs, and see if that explains the results.

~ Stacie

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do you achieved get right results?