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<br />PART IV . UNIT HYDROGRAPHS <br /> <br />1. BACKGROUND THEORY. <br /> <br />A un~ hydrograph is the direct runoff that would be observed at the outlet of the <br />watershed as a resuit of 1 unil of precipitation excess occurring within the unit time <br />interval. The unit of excess is normally taken as equal to 1 inch. Since the physical <br />features of the basin (shape, size, slopes, soils, etc.) do not vary from storm to storm, <br />hydrographs from storms of like duration and pattern are assumed to have the same <br />shape, but wilh ordinates of flow in proportion to the runoff volumes. Thus, ~ 2 unils of <br />excess occurred in 1 unit time interval, then it would be expected that the resuiting <br />hydrograph would have the same shape as the hydrographs from 1 unil of excess in the <br />same time, except that all of the ordinate!; would be twice as large, As another example, <br />~ 1 unit of excess occurred in each of :2 consecutive unil time intervals, the resuiting <br />hydrograph would simply be the sum of the two unit hydrographs, but with the second unit <br />hydrograph beginning 1 time unillater. <br /> <br />While it is known that some assumptions of linearity involved in the unit hydrograph <br />technique do not accurately apply, extensive experience indicates that the Iimilations of the <br />technique are not a major drawback, considering the quality of rainfall and snowmeit data <br />that are usually available, provided that the procedures involved are applied with <br />appropriate knowledge and judgment. <br /> <br />It would be erroneous to assume that one unil hydrograph would suffice for any <br />basin. Aithough the physical characteristics of natural basins may remain relatively <br />constant, variations in storm characteristics will have a significant effect on the shape of <br />resuiting hydrographs. The primary variations in unit hydrographs are due to variations (a) <br />in areal distribution of rainfall and snowmelt, (b) rainfall and snowmeit duration, and (c) <br />time intensily patterns. <br /> <br />In general, a hydrograph resulting from precipitation concentrated in the lower part <br />of a basin will have a rapid rise, a sharp peak, and a rapid recession, while precipitation <br />concentrated in the upper part of the same basin will have a slower rise and recession and <br />a lower, broader peak. It can be seen that un~ hydrographs developed from these <br />different areal distributions of runoff would have distinctly different shapes. On occasions, <br />unit hydrographs have been developnd for upstream, un~orm, or downstream <br />concentrations of runoff. However, this is not wholly satisfactory due to the subjectivity <br />involved, and a better solution is to limit the application of the unit hydrograph technique <br />to basins small enough so that the differences in spatial distribution of rainfall and <br />snowmeit do not significantly affect runoff. This Iimilation does not ordinarily apply to <br />rainfall variations caused by topographic ~!ffects, since the effects can be considered as <br />relatively fixed characteristics of the basin. It is variations from the normal areal pattern <br />of precipilation that cause the differences in unit hydro graphs. <br /> <br />Colorado Flood <br />Hydrology Manual <br /> <br />7-3S <br /> <br />a=w=r <br />