MODELING VADOSE ZONE LIQUID WATER FLUXES - INFILTRATION, RUNOFF, DRAINAGE, INTERFLOW

Because of the large water storage capacity of soil relative to the at mosphere, changes in soil moisture storage can significantly affect th e regional atmospheric budgets of water and energy on monthly, seasona l and longer time scales. Therefore proper modeling of soil liquid wat er processes is essential to a correct representation of the climate s ystem. This study focuses on the class of summary models of liquid wat er fluxes in the vadose, or unsaturated zone of the soil, which are ap plicable to global or regional climate modeling studies, Fourteen such models are represented in this intercomparison study, Observational d ata from the HAPEX experiment provide validation, Because only limited observational data were available to constrain these models during th eir development and validation, the models have evolved very diverse t reatments of the relevant processes: the basic Darcian (soil internal) and Hortonian (surface liquid flow) processes, as well as the boundar y conditions of baseflow drainage and lateral interflow. The annual to tal local runoff is systematically underestimated by all but one of th e participant models. This is one of the few significant biases betwee n the consensus of participant models and the observations. The modele d runoff, averaged over the 14 models, differs from the budget estimat e from observations by about 40%. During the period of runoff generati on (late winter and early spring) the average model fails to deplete t he soil water store as rapidly as is observed, a result consistent wit h the underprediction of runoff. One cannot rule out insufficient char acterization of the field site soils as a primary cause of these discr epancies, Results suggest that model sources of the discrepancy are ab out equally likely to be related to the prediction of bare soil evapor ation (discussed elsewhere in this issue) as they are to the parameter ization of runoff and drainage processes.