APPLICATION OF AN INTEGRATED BASIN-SCALE HYDROLOGIC MODEL TO SIMULATE SURFACE-WATER AND GROUND-WATER INTERACTIONS
Z. Yu and F. W. Schwartz

Hydrologic models have become an indispensable tool for studying processesand water management in watersheds. A physically-based, distributed-parameter model, Basin-Scale Hydrologic Model (BSHM), has been developed to simulate the hydrologic response of large drainage basins. The model formulation is based on equations describing water movement both on the surface and in the subsurface. The model incorporates detailed information on climate, digital elevation, and soil moisture budget, as well as surface-water and ground-water systems. This model has been applied to the Big Darby Creek Watershed, Ohio in a 28-year simulation of rainfall-runoff processes. Unknown coefficients for controlling runoff, storativity, hydraulic conductivity, and streambed permeability are determined by a trial-and-error calibration. The performance of model calibration and predictive capability of the model was evaluated based on the correlation between simulated and observed daily stream discharges. Discrepancies between observed and simulated results exist because of limited precipitation data and simplifying assumptions related to soil, land use, and geology.

Journal of American Water Resources Association, v. 34, no. 2, p. 1-17, 1998.

APPLICATION OF VECTOR AND PARALLEL SUPERCOMPUTERS TO GROUND-WATER FLOW MODELING
Z. Yu

A new generation of high-performance computers offers geoscientists new and exciting tools for computationally-intensive scientific problems. This paper presents preliminary results on optimizing the performance for ground-water flow models on a Cray supercomputer. The traditional method of solving a tridiagonal matrix system, that of forward solution and backsubstitution (FB), is intrinsically data dependent, in that the equations at a given grid node depend on values from adjacent grid nodes. This feature effectively prohibits vectorizing and parallelizing the method for solution on vector and parallel processors. An alternative method of solution, that of reduction and backsubstitution (RB), has no data dependence and was implemented in Fortran for vector and parallel processors to solve a tridiagonal matrix system in a ground-water flow model. The vectorized code has an overall execution rate of 110 MFLOPs on a Cray Y-MP that is five to 16 times faster than the scalar code. Implementing the code in vector-parallel mode using eight undedicated processors on a Cray Y-MP results in an overall additional speedup of 2.8 times in wall clock time for a two-dimensional flow problem and 2.2 times for a three-dimensional flow problem. In comparison with the FB code, The RB code requires 26% less CPU time and 66% less wall clock time on the same flow problem. Demonstrative application of the model is included fortwo- and three-dimensional flow problems.

Computer & Geosciences, v. 23, no. 9, p. 917-927, 1997.

ON EVALUATING THE SPATIAL DISTRIBUTION OF WATER BALANCE IN A SMALL WATERSHED, PENNSYLVANIA
Zhongbo Yu, W. J. Gburek, F. W. Schwartz

A conceptual water balance model was modified from a point application to a distributed format for evaluating the spatial distribution of watershed water balance based on daily precipitation and temperature and other hydrologic parameters. The model was validated by comparing simulated temporal daily variation in soil moisture with field observed data and simulated results of another model that simulates the vertical soil moisture flow by numerically solving Richard's equation. The impacts of soil and land use on the hydrologic components of the water balance, such as evapotranspiration, soil moisture deficit, runoff, and subsurface drainage, were evaluated in this study. Given the same meteorological conditions and land use, soil moisture deficit, evapotranspiration, and surface runoff increase, and subsurface drainage decreases as the available water capacity of the soil increases. Among various land uses, alfalfa produced high soil moisture deficit and evapotranspiration and lower surface runoff and subsurface drainage, while soybeans produced an opposite trend. The simulated distribution of various hydrologic components shows the combined effect of soil and land use. The study demonstrated that the distributed water balance approach is efficient and superior to the use of single average values of hydrologic variables and the application at a single point in the traditional practice.

Hydrological Processes, vol. 14, 941-956, 2000.

ASSESSING THE RESPONSE OF SUBGRID HYDROLOGIC PROCESSES TO ATMOSPHERIC FORCING WITH A HYDROLOGIC MODEL SYSTEM
Zhongbo Yu

An integrated Hydrologic Model System (HMS), a physically-based distributed watershed model for large river basins was developed and used 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 focus of this study was to improve the presentation of rainfall-runoff partitioning by implementing the subgrid scale spatial variability in the precipitation and hydraulic conductivity. 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 data preparation, model calibration, and subgrid scale spatial variability are addressed in the hydrologic simulation. With the implementation of subgrid spatial variability, the simulated results were improved in terms of fit between the simulated and observed streamflows.

Journal of Global and Planetary Change, vol. 25, 1-17, 2000.

ON EVALUATING THE INTERACTION OF SURFACE WATER AND GROUND WATER IN WATERSHEDS
Zhongbo Yu, X. Shangguan, T. Shun, C. Duffy, and F. W. Schwartz

Understanding the interaction of surface water and ground water in watersheds is a crucial key to optimizing water resources, controlling pollution, and understanding hydrologic processes like sediment and contaminant transport. Hydrologic models have been a routine tool in studying the interaction of surface water and ground water. Basin-Scale Hydrologic Model (BSHM), a physically based distributed watershed model, was used in this study to simulate basin-scale hydrologic processes, such as evapotranspiration, surface runoff, and ground-water baseflow. The water budget among hydrologic components in the Upper West Branch watershed, Pennsylvania was analyzed through the long historical daily measurements of streamflow and hydrograph separation procedure. The results of water budget analysis from the observed data was compared with the simulated results with BSHM for the same watershed. The water budget was simulated using BSHM in the Big Darby Creek watershed, Ohio for the 1992-93 water year and in the Upper West Branch watershed, Pennsylvania for the 1980-81 water year. The simulated hydrologic components are compared well with the observed streamflow data at the watershed outlet. The results indicate that hydrologic models, such as BSHM, are very useful tools in studying hydrologic processes and aquifer-stream interaction in large watersheds.

Physical, Chemical, and Biological Aspects of Aquifer-Stream Relations, 45-51, 1998.