On continental-scale hydrologic simulations with a coupled hydrologic model

A new method of coupling coarse-grid regional or global climate models with a much finer-grid hydrologic model is described, that is designed for interactive climate-hydrologic simulations with explicit changes in individual rivers, lakes, wetlands and water tables. Six vertical land-surface solutions with prescribed near-surface soil moistures or standing water depths within each coarse meteorological cell are obtained to disaggregate the relevant quantities (infiltration, runoff) to the finer hydrologic grid based on current near-surface soil moisture in the hydrologic model. Feedbacks on the climate (evaporation, surface heat flux) can be aggregated on the climate grid in the same way. The method is applied for the simulation over the North American continent using (i) NCEP/NCAR reanalyzed meteorologic data and Higgins precipitation data for recent decades, (ii) a vertical column land-surface model on the same coarse grid, and (iii) a new hydrologic model of river, lake and groundwater flow on a 20 × 20 km grid. The predicted routing of major rivers and most lake extents are realistic, reflecting the hydrologic consistency of the 20-km topography. The modeled continental patterns of water-table depths, vadose-zone soil moisture and recharge rates are reasonable. The predicted seasonal discharges at the outlets of four major US river basins are in fair to good agreement with those observed, except for the Colorado where human influences drastically reduce the natural flow.