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<br /> <br />UNIT HYDROGRAPHS <br /> <br />1. BACKGROUND THEORY. <br /> <br />A unit hydrograph is the direct runoff that would be observed at the <br />outlet of the watershed as a result of 1 unit of precipitation excess occurring within <br />the unit time interval. The unit of excess is normally taken as equal to 1 inch. Since <br />the physical features of the basin (shape, size, slopes, soils, etc.) do not vary from <br />storm to storm, hydrographs from storms of like duration and pattern are assumed to <br />have the same shape, but with ordinates of flow in proportion to the runoff volumes. <br />Thus, if 2 units of excess occurred in 1 unit time interval, then it would be expected <br />that the resulting hydrograph would have the same shape as the hydrographs from 1 <br />unit of excess in the same time, except that all of the ordinates would be twice as <br />large. As another example, if 1 unit of excess occurred in each of 2 consecutive unit <br />time intervals, the resulting hydrograph would simply be the sum of the two unit <br />hydrographs, but with the second unit hydrograph beginning 1 time unit later. <br /> <br />While it is known that some assumptions of linearity involved in the unit <br />hydrograph technique do not accurately apply, extensive experience indicates thatthe <br />limitations of the technique are not a major drawback, considering the quality of <br />rainfall and snowmelt data that are usually available, provided that the procedures <br />involved are applied with appropriate knowledge and judgment. <br /> <br />It would be erroneous to assume that one unit hydrograph would suffice <br />for any basin. Although the physical characteristics of natural basins may remain <br />relatively constant, variations in storm characteristics will have a significant effect on <br />the shape of resulting hydrographs. The primary variations in unit hydrographs are <br />due to variations (a) in areal distribution of rainfall and snowmelt, (b) rainfall and <br />snowmelt duration, and (c) time intensity patterns. <br /> <br />In general, a hydrograph resulting from precipitation concentrated in the <br />lower part of a basin will nave a rapid rise, a sharp peak, and a rapid recession, while <br />precipitation concentrated in the upper part of the same basin will have a slower rise <br />and recession and a lower, broader peak. It can be seen that unit' hydrographs <br />developed from these different areal distributions of runoff would have distinctly <br />different shapes. On occassions, unit hydrographs have been developed for upstream, <br />uniform, or downstream concentrations of runoff. However, this is not wholly <br />satisfactory due to the subjectivity involved, and a better solution is to limit the <br />application of the unit hydrograph technique to basins small enough so that the <br />differences in spatial distribution of rainfall and snowmelt do not significantly affect <br />runoff. This limitation does not ordinarily apply to rainfall variations caused by <br />topographic effects, since the effects can be considered as relatively fixed <br /> <br />7-24 <br /> <br />- <br /> <br />