impact of modified richards equation on global soil moisture simulation in the community land model (clm3.5)

A fundamental deficiency has been found in the numerical solution of the soil moisture-based Richards equation using the mass-conservative scheme in the Community Land Model (CLM) in the first part of our efforts (Zeng and Decker 2009). This study implements the revised form of the Richards equation from that study (which doesn’t change the property of the differential equation but does remove the deficiency of the numerical solution) along with a new bottom boundary condition into the current version of CLM (CLM3.5) for global offline modeling evaluations. CLM3.5 represents a significant improvement over its earlier version (CLM3.0), but it also introduces a new deficiency in the vertical distribution of the soil moisture variability. Mean soil moisture in CLM3.5 is also too wet. It is found that the new treatments (primarily a numerically correct solution of Richards equation with a new bottom boundary condition) with minimal tuning are able to maintain the improvements of the CLM3.5 over CLM3.0 and, at the same time, remove the new deficiencies of CLM3.5 based on in situ and satellite data analysis. Because the deficiency in the numerical solution of the soil moisture-based Richards equation is also expected in other land models, implementation details are provided to facilitate similar tests using other land models in the future.