ASSESSMENT OF SOIL MOISTURE SCHEMES IN SiBUC LAND SURFACE MODEL

Abstract

 Soil moisture (SM) is a key component in land surface model (LSM). An accurate prediction of SM is therefore important since it affects other variables when estimating water and energy budgets. This study aimed to evaluate the impacts of SM schemes in a simple biosphere model including urban canopy (SiBUC) on water balance and streamflow estimations. SiBUC calculates SM of a three-layers soil model using Richards’ Equation (RE) with an explicit-midpoint method. The default soil parameterizations are based on the Clapp and Hornberger equation, and the van Genuchten (vG) function, popularly used in hydrological models, was newly added here. In SiBUC, the thickness of the first soil layer was 2 cm, whereas the second was several meters thick. Because of the RE’s nonlinear behavior, this coarse partition may not accurately estimate SM because the changes of a hydraulic gradient in near-surface soil may be large, requiring a finer discretization. A reference model called HYDRUS was used to assess SM by SiBUC. For sandy loam and loam soils, the SM by the default schemes in SiBUC was close to that predicted by HYDRUS but overpredicted for clay loam. Applying a finer partition in the near-surface soil and an implicit-midpoint method seem to improve the estimated SM and reduce the overestimated discharge in the Ping River Basin in Thailand. However, because of the relatively large time steps applied in SiBUC, the estimated runoff using vG was less accurate, possibly causing an overestimate of streamflow. Overall, the potential problem of SM using the RE was explored here, which may help the LSMs users to alleviate the numerical errors resulting from the inadequate discretization of the soil layers.