Introduction to the Integrated Hydrologic Model (ihm)

The public domain Integrated Hydrologic Model (IHM) combines the EPA surface water model HSPF and the USGS groundwater model MODFLOW with near simultaneous integration and fundamentally new interpretations of vadose zone process dynamics. The two model components are concurrently run, linked through an interface that provides a surfacewater to groundwater transition that maintains strict water balance and smoothness in physical processes. The computational elements of the surface-water components are combinations of hydrologically distinct landforms made up of specific land use and soil types. Traditionally, HSPF has been a lumped-parameter model where parameters were averaged over a basin or smaller pervious area. The approach used in the IHM is to disaggregate basins into discrete landform elements that have similar hydrologic properties. These landforms can include impervious areas, irrigated areas, areas with different soils or significantly different depth-towater table, and areas with different types of land (vegetative) cover or different land uses. Using discrete landforms within basins allows significant distributed parameter analysis. To facilitate this level of discretization for regional domains the model has been uniquely written to maintain efficient computational structure. IHM integration code interprets and manages fluxes and storages across the component model interfaces, accounting for disparate discretization of land segments, hydrography (HSPF) and ground water (MODFLOW).