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<br />Chapter II <br /> <br />Development of a Physically Based Distributed <br />Parameter Rainfall Runoff Model <br /> <br />To capture the spatial distribution of precipitation in the Upper Gunnison River Basin and <br />communicate this distribution of precipitation to the model (i.e., assign precipitation correction <br />factors), GIS technology was again used. Spatial data sets of October through April isohyets and <br />May though September isohyets were developed. These two data sets were digitized from maps <br />developed by the National Weather Service for the State of Colorado, based on precipitation data <br />for the period 1931 to 1960. <br /> <br />The isohyetal spatial data set for the Upper Gunnison River Basin for the October through April <br />time period is displayed in figure 8. The extreme spatial variability of precipitation distribution <br />for this time period can be visualized from this figure. Figure 9 displays May through September <br />isohyets for the Upper Gunnison River Basin. <br /> <br />Mean elevation data is another input parameter which must be developed for all HRU's in a <br />PRMS model. Mean HRU elevation is used in the adjustment of observed temperature data to <br />higher elevation HRU's, in the computation of evapotranspiration, and in energy balance <br />calculations in the model. A GIS data set of elevation was used to develop this model parameter. <br />Figure 10 shows elevation zones in the Taylor Park Basin. Like most of the Upper Gunnison <br />River Basin, the Taylor Park Basin contains significant relief. <br /> <br />The mean slope of all HRU's in the model must also be determined. These HRU slopes are used <br />in model solar radiation calculations. A GIS data set of slope, similar to the aspect spatial data set <br />described in the previous section, was used. <br /> <br />A second spatial processing program was written, using AML with ARCIINFO GIS software, to <br />capture attribute data from the isohyet, elevation, and slope GIS data sets and then transfer this <br />information to the HRU GIS data seL Average summer period precipitation, average winter period <br />precipitation, mean slope, and mean elevation of all the polygons in the HRU coverage are <br />determined in this program. Subsequently, attributes for polygons sharing the same HRU identifier <br />are averaged on an area weighted basis to determine the average elevation, average summer and <br />winter precipitation, and slope for all 132 HRU's. These data were then used to assign <br /> <br />14 <br />