SIMULATING THE RIVER-BASIN RESPONSE TO ATMOSPHERIC FORCING
I. FUNDAMENTAL CONCEPTS OF THE HYDROLOGIC MODEL SYSTEM

This first paper of two paper series describes a methodology for the development of an integrated Hydrologic Model System (HMS), a physically-based distributed watershed model for large river basins. HMS was designed to study hydrologic processes and systems responding to various climatic forcings. The modeling system operates with a time step of minutes to days to facilitate coupling with a mesoscale meteorological model. The major emphasis with HMS is on the interactions among climate, land surface, surface water, and ground water. HMS utilizes spatially-detailed information on climate, soil type, land use, digital elevation, and hydrologic parameters. The practical application of HMS is demonstrated in the hydrologic simulation of a major sub-basin of the Susquehanna River Basin in Pennsylvania. Questions concerning flow routing and rainfall-runoff generating schemes are addressed in the hydrologic simulation. Based on the fit between the simulated and observed streamflows, the simulated results compare well with observed data.

Journal of Hydrology, under revision, 1998.

SIMULATING THE RIVER-BASIN RESPONSE TO ATMOSPHERIC FORCING
II. LINKING A MESOSCALE METEOROLOGICAL MODEL AND A HYDROLOGIC MODEL SYSTEM

The purpose of this paper is to use the Hydrologic Model System (HMS) developedby Yu et al. (this issue) to simulate the hydrologic response to three single-storm events passing over the Upper West Branch of the Susquehanna River Basin. Observed and simulated precipitation data for those storms are used to drive HMS and output from HMS is compared to the measured hydrographic trace at the sub-basin outlet. The high-resolution precipitation fields are provided by observations and by the Penn State-NCAR Mesoscale Meteorological Model (MM5) with three nested domains. To support the linkage of HMS and MM5, special attention is given to data resampling and reprojection. The MM5 simulation successfully captures the storm patterns over the study area, although some temporal and spatial discrepancies exist between observed and simulated precipitation fields. The Curve Number and Green-Ampt methods of rainfall-runoff partitioning are used in HMS and evaluated for streamflow simulation. The results from the Storm 1 hydrologic simulation are realistic when using the Curve Number partitioning, but they show considerable underestimation when using the Green-Ampt method because of the low intensity of the rainfall event. In contrast, Storm 2 has much higher rainfall intensities and produces good results with both partitioning methods. The results from the hydrologic simulations for Storm 3 are also realistic. In sum, the methodology given here provides a useful framework for appraising the effects of weather and climate on basin hydrology.

Journal of Hydrology, under revision, 1998.

8/26/98