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)?
I am stuck while calculating the z parameter and i am clueless about how to calculate it because i am not getting the exact number of rainy days/events per year for my study area i.e cauvery river basin (India) and as a result i am not able to apply the given formula given in invest model user guide i.e. N = P/5 and 0.2*N and also for determinig N is N = P/5 applied universally or is it flexible according to the study area…please reach me asap it would be great help for me.
Aside from the advice given previously in this thread, I don’t have additional guidance for determining Z. It is, unfortunately, one of the harder parameters. If you don’t have number of rainy days, then try the User Guide equation involving w (omega), which you’ll need to determine from one of the listed sources.
I have one more quiery that in the uploaded raster file which i have dowloaded from ISRIC for root restricting layer depth…is it correct or can i use this raster as root restricting layer depth for my study area and second i am not able to determine the root depth of the land use land cover classes to metion in biophysical table…so how can i calcuate the root depth please provide me guidance in this regard. and in biophysical table i have taken KC value from the literature of tropical areas river basin since my study area also falls in the tropical region…will that be fine?
