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<br />001943 <br /> <br />CHAPTER 2 <br />NATURAL FLOW AND HISTORIC DEPLETION ANALYSIS <br /> <br />USGS Stream Gages in the San Juan River Basin <br /> <br />Natural flows for this study were computed by adjusting stream flows at USGS gaging stations <br />throughout the Basin for man-made influences. This includes adjustments for depletion of water <br />through consumptive use, diversion of water to other basins or around a gage, import of water from <br />other basins, and evaporation and change in storage from man made reservoirs, The general equation <br />used is: <br /> <br />Natural Flow = Historic (Gaged) Flow <br />+ Irrigation Depletions <br />+ Sprinkler Evaporation) <br />+ Major Reservoir Evaporation <br />+ Change in Major Reservoir Storage <br />+ Livestock Depletions <br />+ Stock Pond Depletions <br />+ Domestic Depletions <br />+ Fish and Wildlife Depletions <br />+ Power Plant Depletions <br />+ Mining Depletions <br />+ Minor Reservoir Depletions <br />+ Exports <br />- Imports <br /> <br />About ninety stream gaging stations were identified in the San Juan River Basin of which twenty-five <br />were used in the natural flow development (Table 2-1). These gages define the subbasins used to <br />compute natural flows (Figure 2-1). Figure 2-2. is a schematic showing depletion nodes and major <br />imports and exports. Selection of each of the gaging stations is based upon data availability and <br />project requirements. <br /> <br />Depletions of water are estimated in terms of Units. For Colorado, a Unit consists of a County-HUe <br />combination. County boundaries were incorporated to dissagregate annual crop data which are <br />reported by county for Colorado. A Hue (Hydrologic Unit Code) is a geographic area representing <br />part or all of a surface drainage basin or distinct hydrologic feature as delineated by the Office of <br />Water Data Coordination on State Hydrologic Unit Maps, A HUC may consist of a number of <br />subbasins. Figure 2-3 is the San Juan River Basin HUe map and Figure 2-4 is the San Juan River <br />Basin Unit map, In New Mexico, the New Mexico Interstate Stream Commission (NMISC) defined 8 <br />units in the basin based upon: <br /> <br />. Physiographic boundaries of river drainage (with few exceptions allowed) <br />. Location of stream gages and integrity of flow information <br />. Geographic extent of the regional water plans <br /> <br />DRAFT - SJRIP Hydrology Model Documentation <br />June 18, 1999 <br /> <br />4 <br />