Improving the precision of simulated hydrologic fluxes in land surface models

The numerical method for estimating soil moisture transport coefficients in land surface models (LSMs) is evaluated through comparison to a weighted-a verage method based on successive steady states [Warrick, 1991] and a propo sed new scheme [Dong and Wang, 1997]. The focus is on situations with large gradients of soil moisture. The reference cases used to evaluate the perfo rmance of the various methods are eased on numerical integrations with fine spatial resolution and small time steps. A series of tests were performed to detect sources of discrepancies and to assess the impact of various nume rical methods on the simulation of evaporation, transpiration, and drainage . Off-line LSM simulations driven by observed atmospheric forcings were ana lyzed to understand the propagation of errors between the various hydrologi c processes. We suggest a simple modification to LSM that can improve the p recision of simulated hydrologic fluxes without increasing vertical resolut ion.