On the Use of the Force–Restore SVAT Model Formulation for Stratified Soils

Abstract

An approach to simulate soil moisture content with the force?restore soil?vegetation?atmosphere transfer (SVAT) model in the case of stratified soils is proposed. Typical soil profiles possess marked variation in soil hydraulic properties from the soil surface to the base of the root zone. The force?restore method is extensively used for land surface modeling in atmospheric models but without any specific consideration for dealing with stratified soils. Drainage from and recharge to the near-surface soil layer is classically estimated on the basis of the volumetric soil moisture differences between the near surface and lower root zone, with an adjustment for gravitational effects. However, moisture differences do not relate uniquely to differences in hydraulic potentials when the soil properties are vertically inhomogeneous. As a consequence, the classical force?restore formulation does not correctly represent vertical fluxes, and it results in biased time series of predicted near-surface soil moisture. This bias will result in potentially incorrect land surface flux calculations and will also frustrate any efforts to assimilate periodic near-surface soil moisture data into the model. The authors propose a simple model modification that employs knowledge of vertical differences in soil texture and demonstrate its improved performance vis-à-vis the classical formulation. The focus of this study is on the special case of hydrostatic conditions for four theoretical soil profiles and on dynamic model performance in the context of an experimental field dataset.