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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.
Boone et al. (Mon,) studied this question.