Tuesday, 11 February 2003: 3:30 PM
Evaluation of Three Topography-based Runoff Schemes in a Land Surface Model
Three different schemes of topography-based runoff production (VISA-TOP1, VISA-TOP2, and VISA-TOP3) are described for a land surface model developed for use with a general circulation model. The schemes’ sensitivities to some key parameters are assessed for two catchments using data sets developed for the Project for Intercomparison of Land-surface Parameterization Schemes Phase 2e. VISA-TOP1 differs from VISA-TOP2 only in how to treat over-saturated soil water from the soil layers. In VISA-TOP1, the over-saturated soil water is thrown out of the soil column and hence it no longer plays a role in the ensuing soil water budgets. In VISA-TOP2, this over-saturated soil water is recharged back to the unsaturated soil layers above the water table and hence it continues to involve in the water budgets. Unlike VISA-TOP1 and VISA-TOP2, VISA-TOP3 relaxes its dependence on the topographic parameters. The over-saturated soil water is treated the same in both VISA-TOP2 and VISA-TOP3. All three models reproduce daily and seasonal cycles of streamflow provided that different values of the saturated hydraulic conductivity decay factor are used. The decay factor controls the timing and partitioning of subsurface runoff. In both VISA-TOP1 and VISA-TOP2, an anisotropic parameter explaining different hydraulic conductivities in the vertical and horizontal directions is critical for using the topographic index in the land surface model. In the VISA-TOP2 scheme, the topography-controlled subsurface runoff is dominant because the over-saturated water is recharged to upper unsaturated soil layers to raise the water table. The water budgets in all these schemes show dramatically different responses to the decay factor, indicating that the calibrated parameters and the model formulations should not be separated.