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<br />, - '. <br /> <br />002268 <br /> <br />Chapter II <br /> <br />Development of a Physically Based Distributed <br />Parameter Rainfall Runoff Model <br /> <br />precipitation correction factors,' mean HRU elevations, and mean HRU slopes for the model. The <br />total area of each HRU is also calculated in this program. Table 3 summarizes these data for the <br />Taylor Park Basin. These data for all 132 HRU's in the model can be found in attachment C. <br /> <br />The soil zone in PRMS is a conceptualized reservoir consisting of an upper and lower zone. (See <br />figure 3.) Parameters used in the model include soil type, maximum water storage capacity of <br />each zone, and coefficients which govern the rate of transfer of water between the soil and <br />subsurface reservoirs. Soil parameters for the Gunnison Model were obtained from data collected <br />in the field and by the inspection of Natural Resource Conservation Service (NRCS) State <br />Geographic Database. The water storage capacities used in the Gunnison Model range from <br />0.76 centimeter to 3.8 centimeters (0.3 inch to 1.5 inches) in the upper zone for various HRU's. <br />The lower zone ranges from a capacity of 2.54 centimeters to 12.7 centimeters (I inch to 5 <br />inches). <br /> <br />Evapotranspiration in the Gunnison Model is handled using a Illodified Jensen-Haise technique <br />(Jensen et aI., 1963, 1969). Potential evapotranspiration is computed using air temperature, solar <br />radiation, and two coefficients that can be estimated using regional air temperature, elevation, <br />vapor pressure, and vegetation data. Values for the Jensen-Haise coefficients used in the Gunnison <br />Model were developed by the USGS. <br /> <br />Parameters for interception and solar radiation transmission coefficients were determined as a <br />function of land cover and aspect. HRU's in forested areas with north aspects were assigned the <br />highest values for interception parameters and the lowest solar radiation transmission coefficients <br />because these areas are the most densely foliated. Conversely, HRU's in tundra areas with south <br />aspects had the smallest interception values but the largest solar radiation transmission coefficients. <br /> <br />Daily solar radiation data were not available as input for the Gunnison Model. Because of this, <br />dai1y solar radiation is estimated using maximum daily temperature, according to the method <br />described by Leaf and Brink (1973). <br /> <br />~ This correction factor is determined using the ratio of HRU avemge precipitation to average climate station <br />precipitation. <br /> <br />15 <br />