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Hydrological Routing Algorithm (HYDRA): Overview
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The Hydrological Routing Algorithm (HYDRA) is a computer model that simulates the regional scale flow of water (Coe et al., 2000). HYDRA simulates a set of physical hydrologic processes including interception, infiltration, interflow, base flow, overland routing and channel routing. The HYDRA model uses river transport directions based on digital elevation model (DEM) representations of the land surface. River discharge is calculated at each grid cell as the accumulated flow of water across the drainage basin surface. Lake and wetland area and volume are calculated as a function of the local precipitation minus evaporation, the stream flow into and out of the grid cells, and the potential for storage of water on the surface as derived from DEMs. HYDRA is being periodically modified to include effects of man-made structures (i.e., dams and reservoirs) and management practices (i.e., irrigation and reservoir operations), to account for the human controls over terrestrial hydrological systems.
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HYDRA uses inputs of long-term monthly precipitation from interpolated weather station data sets, and surface evapotranspiration, snowmelt, and soil water retention from the coupled NASA-CASA (Carnegie-Ames-Stanford Approach) land surface model (see below). Satellite remote sensing products are used to define the land cover variations statewide. The inherent uncertainty in the climate model inputs (combined) likely introduce a baseline of 5-15% error into HYDRA's estimates of yearly and seasonal river discharge rates. Uncertainties in precipitation inputs alone could be even higher in remote locations where weather station coverage is sparse.
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