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<br />Chapter II <br /> <br />Development of a Physically Based Distributed <br />Parameter Rainfall Runoff Model <br /> <br />input parameters to PRMS are largely descriptive physical data for a given watershed. This <br />descriptive data includes infonnation on physiography, vegetation, soils, and distribution of <br />precipitation within a watershed. <br /> <br />PRMS simulations can be run in a daily time step mode or in a storm mode time scale (using <br />intervals shorter than I day). All Gunnison Model simulations were performed at a daily time <br />interval. When run in daily mode, PRMS simulates hydrologic components as daily average or <br />total values. Streamflow is computed as a mean daily flow. <br /> <br />The development of distributed model parameters in PRMS is accomplished by partitioning a <br />watershed into units by using characteristics such as physiography (slope, aspect, and elevation), <br />vegetation, soils, and precipitation distribution. Each of these units is assumed to be homogeneous <br />with respect to its hydrologic response. These units are the "building blocks" of PRMS and are <br />termed "hydrologic response units" (HRU's). As PRMS simulations are performed, a water and <br />energy balance is computed at each time step for each HRU. The sum of all the responses of all <br />HRU's, weighted on a unit-area basis, produces the daily watershed response. <br /> <br />Within PRMS are a number of physical-process components which simulate the daily accretion, <br />depletion, storage, and movement of water within model HRU's. The rates and volumes of these <br />processes are a function of HRU physical, hydrologic, and climatic characteristics. These <br />physical-process components form the conceptualized watershed system utilized by PRMS. This <br />conceptualized watershed is schematically depicted in figure 3. <br /> <br />Input data to PRMS for daily time step simulations are daily minimum and maximum <br />temperatures, daily solar radiation data, and daily precipitation at multiple climate data collection <br />sites within the watershed. Where solar radiation data is not available, maximum daily <br />temperatures may be used to estimate solar radiation. <br /> <br />6 <br />