Is the default value of fates_hydro_vg_alpha_node too small?

[jennykowalcz]

In my FATES-Hydro simulations over the Amazon Basin, I get unrealistic values of stem conductivity fraction when using the van Genuchten hydraulic transfer function for plants (i.e. fates_hydro_htftype_node = 2 for all organs) with the default value of fates_hydro_vg_alpha_node = 0.0005 MPa^-1. This is shown in the center plot below. The left figure shows the distribution of stem conductivity fraction with fates_hydro_htftype_node = 1 (i.e. TFS hydraulic transfer function, the default), and the right figure shows the distribution with fates_hydro_htftype_node = 2 and fates_hydro_htftype_node values several orders of magnitude larger than the default (specifically, 0.11855 MPa^-1 for the early successional tropical tree PFT and 0.088026 MPa^-1 for the late successional). These values are from Ding et al. (2023) for conifer forests in California, so may not be appropriate for tropical trees, but the default vg_alpha value is perhaps not appropriate either.

[jennykowalcz]

[jennykowalcz]

I'm working on fitting better values for the van Genuchten HTF and will submit a PR to update the parameter file