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<br />002253 <br /> <br />Chapter II <br /> <br />Development of a Physically Based Distributed <br />Parameter Rainfall Runoff Model <br /> <br />respect to subbasin, land cover, and aspect. In this way, 132 HRU's for the Upper Gunnison <br />River Basin were defined. It should be noted that areas that define an HRU in the Gunnison <br /> <br />Model are not necessarily contiguous. While the areas that make up one HRU are physically <br />fragmented, the model treats each HRU as one unit. <br /> <br />Figure 7 shows the HRU data layer for the Taylor Park Basin. There are numerous polygons in <br />this figure. However, there are only nine unique combinations of subbasin (all being Taylor Park <br />in this case), land cover, and aspect, defining nine HRU's for the Taylor Park Basin. The <br />noncontiguity of HRU's is also depicted in figure 7. <br /> <br />MODEL PARAMETERS <br /> <br />Partitioning a basin into HRlJ's represents a signifIcant accomplishment in PRMS model <br />development. However, it is only the first step. Model parameters for all HRU's must then be <br />developed. Subsequently, time series input data must be prepared, and finally, model calibration <br />and validation can take place. <br /> <br />In PRMS, there are over 80 parameters for which values must be determined prior to running and <br />calibrating a model. Parameters which describe physiography, soils, vegetation density, ground- <br />water characteristics, subsurface flow characteristics, evapotranspiration, solar radiation, etc., must <br />all be established. Because of the volume of model parameters in PRMS, a detailed discussion of <br />all model parameters will not be possible in this document. However, some of the more <br />significant model parameters will be discussed. <br /> <br />In PRMS, precipitation is distributed to HRU's from precipitation data obtained at climate stations <br />(point data). Because of the limited number of climate stations, model HRU's obtain precipitation <br />data from climate stations commonly located not only at a significant distance from the HRU but <br />also in areas where precipitation can be quite different due to physiographic effects. Precipitation <br />correction factors (a PRMS model parameter) account for these differences by adjusting observed <br />precipitation data (point data) before applying this precipitation to a particular HRU. <br /> <br />13 <br />