Inclusion of a third soil layer in a land surface scheme using the force-restore method

The inclusion of a third soil layer in the Interactions between Soil, Biosp here, and Atmosphere (ISBA) model is presented in this paper. The soil wate r content between the base of the root zone and the deep soil layer is desc ribed using a generalized form of the force-restore method. The new force-r estore coefficient is calibrated using a detailed high-resolution soil wate r transfer model and then is related to the soil textural properties using simple regression relationships. It is shown that the use of a calibrated c oefficient 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 distinguis h the root zone and subroot zone soil water reservoirs. With the three-laye r version, the deep soil layer may provide water to the system through capi llary rises only, and the available water content (for transpiration) is cl early defined. Three test cases are examined in which atmospheric forcing, a good description of the soil properties and vegetation cover, and measure d soil moisture profile data are present for an annual cycle. Use df the th ree-layer version of ISBA gives general improvement in modeling results, an d values for key parameters that relate evapotranspiration to soil moisture are more consistent with those inferred from observations, compared with t he two-layer version.