THE INTERPLAY BETWEEN TRANSPIRATION AND RUNOFF FORMULATIONS IN LAND-SURFACE SCHEMES USED WITH ATMOSPHERIC MODELS

The Project for Intercomparison of Land-surface Parameterization Schem es (PILPS) has shown that different land surface models (LSMs) driven by the same meteorological forcing can produce markedly different surf ace energy and water budgets, even when certain critical aspects of th e LSMs (vegetation cover, albedo, turbulent drag coefficient, and snow cover) are carefully controlled. To help explain these differences, th e authors devised a monthly water balance model that successfully repr oduces the annual and seasonal water balances of the different PILPS s chemes. Analysis of this model leads to the identification of two quan tities that characterize an LSM's formulation of soil water balance dy namics: 1) the efficiency of the soil's evaporation sink integrated ov er the active soil moisture range, and 2) the fraction of this range o ver which runoff is generated. Regardless of the LSM's complexity, the combination of these two derived parameters with rates of interceptio n loss, potential evaporation, and precipitation provides a reasonable estimate for the LSM's simulated annual water balance. The two derive d parameters shed light on how evaporation and runoff formulations int eract in an LSM, and the analysis as a whole underscores the need for compatibility in these formulations.