I am working with the annual water yield model, specifically with the construction of the parameter Z. The definition given in the user guide is:
" 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 w(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 w(x) are capped to a value of 5."
How can I interpret this? Should I deliberately assume w(x) as 5? Or should I calculate it separately? If it is the latter, what would be the correct way to do it?
I look forward to your answers and thank you very much.
Z and w are some of the harder parameters to determine. The Appendix to the Annual Water Yield model gives pointers to a few studies that provide values for w that you can look at. In particular, Xu et al. 2013, Fig. 3 shows a global map with their estimates for w for basins around the world. Unfortunately, the graphic is rather low-resolution and coarse, and not made for zooming in to particular smaller areas. Still, I have used that image to at least get an idea of the closest value for my general study area.
The Appendix also mentions a perhaps simpler way of calculating Z as "0.2*N, where N is the number of rain events per year. ", which of course depends on whether you have rain event data.
Does anyone else have more specific advice for determining w (omega)?