Provide more physically realistic minimum value threshold in NoahMP soil moisture solver

[aubreyd]

At the end of SOILWATER, there is a check for negative soil moisture. The code enforces a fixed minimum depth of 0.01mm. Where did this value come from? This seems problematic given that it is the same minimum depth for a shallow (e.g., 10cm) layer and a deep (e.g., 1m) soil layer. Why not SMCDRY or another ratio-based value? It does appear the solution is sensitive to this value, so needs more testing.

wrf_hydro_nwm_public/trunk/NDHMS/Land_models/NoahMP/phys/module_sf_noahmplsm.F

Line 7110 in 5cfc4d2

WATMIN = 0.01 ! mm