Watershed and Receiving Water Model Linkage With HSPF: Issues and Example Applications

Capabilities of watershed models have continued to evolve over the past decade, with advances in computing power and scientific knowledge to greatly improve the ability to represent complex watersheds and water systems with increasing detail, both spatially and temporally. In many cases, watershed models must be linked to lake, estuary, or other detailed receiving water models in order to adequately represent the intricacies of the physical system under study. Watershed models are often called upon to provide "boundary conditions," both hydrologic/hydraulic and nonpoint source loading fluxes, to the receiving water models. Model linkage procedures must consider spatial and temporal characteristics of the systems being linked, correspondence and transference of the state variables between the models, and file format specifics for proper communications between the computer codes. All of these issues must be adequately investigated and analyzed to ensure proper and appropriate representation of the complex water system. The United States Environmental Protection Agency (US EPA) Hydrologic Simulation Program – FORTRAN (HSPF) has been applied to hundreds of watersheds within the United States and abroad over the past 2 decades. In recent years, a number of these studies have required linkage with more detailed hydrodynamic and water-quality receiving water models in order to adequately represent complex watersheds and water systems. This paper describes a few recent model linkage efforts to demonstrate the temporal, spatial, and state variable issues that arise when such complex water modeling systems must be linked.