ON THE COUPLING OF CLIMATE AND HYDROLOGY MODELS AT A CONTINENTAL SCALE

A new method of coupling coarse-grid regional or global climate models with a much finer-grid hydrologic model is designed for interactive climate-hydrologic simulations with explicit changes in individual rivers, lakes, wetlands and water tables on scales down to a few km. The method involves running a small number (6) of vertical land-surface solutions with prescribed near-surface soil moistures or standing water depths within each coarse meteorological cell, and disaggregating the relevant quantities (infiltration, runoff) to the finer 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 tested over the North American continent using (i) NCEP/NCAR reanalyzed meteorologic data for recent decades at ~1.9 degree resolution, (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 20x20 km grid. The predicted routing of major rivers and most lake extents are realistic, reflecting the hydrologic consistency of the 20 km topography. The model continental patterns of water-table depths, vadose-zone soil moisture and recharge rates are reasonable. The simulated seasonal discharges at the outlets of four major U.S. river basins are in fair to good agreement with those observed, except for the Colorado where human influences drastically reduce the natural flow. Further simulations at scales of 5 and 10 km are conducted to evaluate the scaling effect on the hydrologic process.