Watershed Scale Physically Based Water Flow, Sediment and Nutrient Dynamic Modeling System - Landscape Ecology for Sustainable Environment and Culture - page 141

Fig. 8.1 Topographical representation of overland flow and channel routing schemes within a

watershed

reformulation and enhancement of the CASC2D (Fig. 8.1 ). The model incorpo-
rates 2D overland flow, 1D stream flow, 1D unsaturated flow and 2D groundwater
flow components. Within GSSHA, sediment erosion and transport processes take
place both on the land and within the channel. The GSSHA model employs mass
conservation solutions of partial differential equations and closely links the
hydrologic components to assure an overall mass balance. GSSHA had already
been tested and applied for hydrologic response and sediment transport in several
watersheds across the United States and achieved satisfactory results (CHL 2012 ).
A brief introduction is given as follows however details of the GSSHA model can
be found in Downer and Ogden ( 2004 ). A review of hydrologic and sediment
erosion and transport process descriptions is informative to illustrate the physics
behind individual process representations and specific to those needed to drive full
nitrogen and phosphorus cycling at the watershed scale.

8.2.1 Hydrologic Simulation

The modeling of hydrologic processes begins with rainfall being added to the
watershed, some of which is intercepted by the canopy cover, evapotranspirated or
infiltrated. Hydrologic processes that can be simulated and the methods used to
approximate the processes with the GSSHA model are listed in Table 8.1 .
GSSHA uses two-step, finite-volume schemes to route water for both 2D
overland flow and 1D channel flow where flows are computed based on heads and
volumes are updated based on the computed flows. Several modifications were
made to both the GSSHA channel routing and the overland flow routing schemes