EVALUATING GCM LAND-SURFACE HYDROLOGY PARAMETERIZATIONS BY COMPUTING RIVER DISCHARGES USING A RUNOFF ROUTING MODEL - APPLICATION TO THE MISSISSIPPI BASIN

To relate general circulation model (GCM) hydrologic output to readily available river hydrographic data, a runoff routing scheme that route s gridded runoffs through regional- or continental-scale river drainag e basins is developed. By following the basin overland flow paths, the routing model generates river discharge hydrographs that can be compa red to observed river discharges, thus allowing an analysis of the GCM representation of monthly, seasonal, and annual water balances over l arge regions. The runoff routing model consists of two linear reservoi rs, a surface reservoir and a groundwater reservoir, which store and t ransport water. The water transport mechanisms operating within these two reservoirs are differentiated by their time scales; the groundwate r reservoir transports water much more slowly than the surface reservo ir. The groundwater reservoir feeds the corresponding surface store, a nd the surface stores are connected via the river network. The routing model is implemented over the GEWEX (Global Energy and Water Cycle Ex periment) Continental-Scale International Project Mississippi River ba sin on a rectangular grid of 2-degrees x 2.5-degrees. Two land surface hydrology parameterizations provide the gridded runoff data required to run the runoff routing scheme: the variable infiltration capacity m odel, and the soil moisture component of the simple biosphere model. T hese parameterizations are driven with 4-degrees x 5-degrees gridded c limatological potential evapotranspiration and 1979 First GARP (Global Atmospheric Research Program) Global Experiment precipitation. These investigations have quantified the importance of physically realistic soil moisture holding capacities, evaporation parameters, and runoff m echanisms in land surface hydrology formulations.