Inclusion of a Third Soil Layer in a Land Surface Scheme Using the Force–Restore Method

Abstract

The inclusion of a third soil layer in the Interactions between Soil, Biosphere, and Atmosphere (ISBA) model is presented in this paper. The soil water content between the base of the root zone and the deep soil layer is described using a generalized form of the force?restore method. The new force?restore coefficient is calibrated using a detailed high-resolution soil water transfer model and then is related to the soil textural properties using simple regression relationships. It is shown that the use of a calibrated coefficient gives better results, in general, than a direct solution method when using similar model geometry with the same number of layers. In the initial two-layer version of ISBA, it was not possible to distinguish the root zone and subroot zone soil water reservoirs. With the three-layer version, the deep soil layer may provide water to the system through capillary rises only, and the available water content (for transpiration) is clearly defined. Three test cases are examined in which atmospheric forcing, a good description of the soil properties and vegetation cover, and measured soil moisture profile data are present for an annual cycle. Use of the three-layer version of ISBA gives general improvement in modeling results, and values for key parameters that relate evapotranspiration to soil moisture are more consistent with those inferred from observations, compared with the two-layer version.