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<br />I <br /> <br />unit hydrograph technique, and translates the streamflow through valley <br /> <br />storage and reservoir storage by simple routing procedures. <br /> <br /> <br />Modelling a complex basin consists of defining its topologic structure <br /> <br /> <br />(subbasin boundaries and areas, stream channels and the logical relation- <br /> <br /> <br />ships between the subbasins and stream channels), and defining the parameters <br /> <br />that describe the rainfall-runoff response of the subbasins and channels <br /> <br />of the river basin. Parameters are needed to determine: (1) basin average <br /> <br />total precipitation and its time distribution for each subbasin, (2) precipi- <br />tation loss rates for selected storm events, (3) unit hydrographs for each <br />subbasin, (4) base flow controls for each subbasin, and (5) routing criteria <br />for each channel reach. The detailed computational algorithms can be found <br /> <br /> <br />in reference 2. Figure 1 is a simplified flow chart of HEC-l. <br /> <br /> <br />Topology. The topology of the basin is described in the program <br /> <br /> <br />essentially by the way the sequences of operations are specified. Hydrographs <br />are computed, routed and combined with other hydrographs in accordance with <br /> <br />the data sequence provided. In this manner any complex basin consisting of <br /> <br />a very large number of subbasins and routing reaches can be accurately described. <br /> <br />Precipitation, The precipitation applied to a subarea for runoff compu- <br /> <br />tations may be determined by three methods. The computed precipitation will <br />then be treated as rainfall or snowfall depending upon temperature criteria. <br />a. Subbasin total precipitation is computed from non recording <br />station precipitation according to weights provided for each station. The <br />subbasin total precipitation can also be computed considering longer term <br /> <br />weighting values such as normal annual amounts that may reflect consistent <br /> <br />6 <br />