WSP09829 - Laserfiche WebLink

Home Browse Search

f\Q?22C\ ?~!':,< DESCRIPTION OF MODEL AND SEQUENCE OF STREAMFLOW SIMULATION ANALYSIS Jj ~f~j. The PRMS model used in this .study is a modeling system developed in modules to enable flexibility in a variety of uses (Leaves ley and others, 1983). PRMS was used in this study because it is a distributed-parameter model that accounts for the spatial and ,temporal variation in hydrologic characteristics within the drainage basins. Although the modeled drainage basins are small, elevation differences can be substantial, and changes in precipitation can be dramatic. The variety of vegetation types within the drainage basins indicates the variability in precipitation. PRMS also has snow accumulation and snowmelt algorithms, and for drainage basins in this study area the primary input to the water balance is snow. :':;~4: ~~l~ Characteristics of a drainage basin are distributed in this modeling system..by dividing the drainage basin into hydrologic response units. (HRU' s). In theory, these HRU's represent homogeneous areas in the drainage basin that have a uniform and characteristic response to hydrologic input. Dividing the drainage basin into HRU's enables variation in such factors as different infiltration rates resulting from changes in soils, different precipitation input resulting from changes in elevation, or different evapotranspiration rates resulting from changes in vegetation. In practice, HRU's can be. difficult to describe because sufficient information about the drainage basin is not always available. The designation of HRU's is based on features that can be observed from aerial photographs and from topographic and soils maps and from general observations of vegetation, elevation, slope, and aspect. i..:\'~' rn ,x.-, c::'~ :'$ :ftf "~;~.:" . ;k~~ ,.:::..:< . ~~~ I . .:~.); ~:}~ . '~";' ';~1~!( '..:"; :~:._;~~., PRMS, as used in this study, requires daily precipitation and daily'max- imum and minimum air temperature as input data. A schematic diagram of the watershed system used in PRMS is shown in figure 2. The model computes a daily water balance using values of net precipitation, adjusted maximum and minimum air temperature based on the elevation of the HRU, interception, solar radiation, potential and actual evapotranspiration, soil-moisture content, subsurface and ground-water reservoir contents, and water equivalent in the snowpack. Daily mean streamflow for the drainage basin is computed from an area-weighted average of these water-balance computations. i~-'i~:;;' .~ Daily solar shortwave radiation that is needed to compute the energy balance of the snowpack is estimated from air-temperature data using the method developed for a part of the Rocky Mountains and described by Leaf and Brink (1973). The daily solar shortwave radiation is adjusted for the particular slope and aspect of each HRU and for the time of year at the specific latitude of each HRU (Frank and Lee, 1966). Daily potential evapotranspiration is computed for each HRU using the Jensen-Raise technique (Jensen and Haise, 1963). Actual evapotranspiration then is estimated for each HRU from the potential evapotranspiration and the available soil moisture. ".';'; ;:~~~::~ Within each HRU, an accounting of soil moisture is maintained in PRMS. Water is added from rainfall and snowmelt and water is lost through evapo- transpiration and seepage to .subsurface and ground-water reservoirs. The maximum available water-holding capacity of the soil profile is the. difference ,..... ;.\.::' "'-'. ~.}h: ' :.;.;~", ' '. ." .~ :.'. (;:;;;:;:~ i ~"'~:\:;;-i (i.'i) ,,- :;;~'/'> 9 " .;-[