Digital terrain modeling is one of the strong areas where GIS has been widely utilized in hydrology. Digital elevation model (DEM) data are employed to derive watershed characteristics such as slope, aspect, curvature, drainage network structure (e.g., Fairfield and Leymarie, 1991), hydrologic response units (HRUs), and also to delineate watershed boundaries (Band, 1986; Jensson and Domingue, 1988; Schultz, 1994). Lozar (1992) delineated drainage paths and watersheds of the entire Earth based on the 5-arc-minute DEM of Earth's land surface. Remotely sensed information can be integrated with DEM for a variety of hydrologic applications. For example, Dubayah (1992) employed a DEM, Landsat TM data, and a radiative transfer algorithm to model spatial variability of net solar radiation at fine spatial resolution. Also, remotely sensed products are employed in conjunction with a DEM to produce realistic perspective views of a watershed that aid visualization and understanding of spatial and temporal variability of hydrological parameters (Gugan and Dowman, 1988).
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