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SEC'TION 2: UNIT HYDR~GRAPHS <br />U~ut Hvdro4rap~ Selection <br />A unit hydrograph is defined as the time distribution of one inch of direct runoff from a storm of <br />a spe~cified duration for a particular watershed. Unit hydrographs reflect the physiography, <br />topo~araphy, land-use, and other unique characteristics of the individual watershed. Different <br />unit liydrographs are produced for the same watershed for different durations of rainfall excess. <br />For e~xample, a unit hydrograph for a particular watershed can be developed for a rainfall <br />exce:~s duration of 5 minutes, or 15 minutes, or 1 hour, or 6 hours, etc. Any duration can be <br />selected for unit hydrograph development as long as the upper limit for duration is not <br />exceeded for the specific unit hydrograph (discussed in the Instructions section). <br />Only a few watersheds in Colorado have an adequate data base (rainfall and runoff records) <br />from which to develop a unit hydrograph for the watershed. Therefore, indirect methods are <br />freq~iently used to develop unit hydrographs. Such unit hydrographs are called synthetic unit <br />hydr~~graphs. Several procedures are available to develop synthetic unit hydrographs, and <br />virtu~~lly all of those procedures are empirical. The selection of a synthetic unit hydrograph <br />procE:dure should be made such that the data base for the empirical development is <br />reprE~sentative of the study watershed. <br />The ~~nit hydrograph itself is a lumped parameter in that it represents the composite effects of <br />all of the watershed and storm characteristics that dictate the rate of runoff from the <br />watershed. Although there are numerous watershed and storm characteristics that determine <br />the shape of a unit hydrograph, only a limited number of those characteristics can be quantified <br />and iased to calculate a unit hydrograph. One or more unit hydrograph parameters (depending <br />on tFie selection of synthetic unit hydrograph procedure) are needed to calculate a unit <br />hydn~graph. <br />19 March 2007 <br />