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The Watershed Systems Group (WSG) within the Coastal and Hydraulics Laboratory of the US Army Engineer Research and Development Center (ERDC) supports the US Army and the US Army Corps of Engineers (USACE) in both military and civil operations through the development, modification and application of surface and sub-surface hydrologic models. The Department of Defense (DoD) is also charged with managing approximately 200,000 km2 of land within the United States on military installations and flood control and river improvement projects. The WSG provides the Army with predictions of stream flow and stage, inundated areas, saturated areas, soil moistures, groundwater levels, and contaminant fate and transport. Predictions are provided for anticipated changes in weather conditions, project alternatives and land-use changes. The WSG uses a variety of models that are supported by the DoD graphical user interfaces (GUI) Watershed Modeling System (WMS) (Nelson, 2001), Groundwater Modeling System (GMS) (Jones, 2001), and Surfacewater Modeling System (SMS) (Zundel, 2001). These GUIs are commonly referred to the XMS system. The XMS interfaces support a variety of model classes, from simple lumped-parameter runoff models, to 2-D overland, and 3-D unsaturated groundwater models.

For many problems the distributed modeling approach may offer substantial potential improvement in capability compared with traditional lumped-parameter hydrologic models such as the USACE surface hydrologic model HEC-1 (USACE, 1985). The US Army, with additional support from the US Environmental Protection Agency (EPA), funded the development of the physically-based, distributed parameter, Hortonian runoff model CASC2D (Ogden and Julien, 2002; Downer et al., 2002a). Past experience with CASC2D has been favorable when the model has been properly applied, i.e. when Hortonian flow is the dominant process (Doe and Saghafian, 1992; Doe et al. 1996; Ogden et al., 2000; Senarath et al., 2000; Downer et al., 2002a). CASC2D Version 1.18b is linked with WMS Version 5.1 (BYU, 1997a; 1997b), which greatly simplifies model setup, results analysis and visualization. The WSG and the US Army no longer support the development or application of the CASC2D model. CASC2D development continues at Colorado State Univerity.

While Army experience with CASC2D has generally been favorable, there are many instances where the assumptions inherent in the CASC2D model limit its applicability (Senarath et al., 2000; Downer et al., 2002a). Figure 1 illustrates hillslope hydrology with an emphasis on the different runoff and streamflow generating processes. When saturation excess runoff, groundwater discharge to stream, exfiltration, etc., contribute significantly to the stream flow, the application of Hortonian runoff models is ill advised and can lead to erroneous results (Loague and Freeze, 1985; Loague, 1990; Grayson et al., 1992; Smith et al., 1994; Loague and Kyriakidis, 1997; Downer et al., 2002a).

GSSHA User's Manual

1 Introduction
1.1 History
1.2 Purpose