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<br />Temporal Distribution of Unit Runoff- The determination of the basin's lag time is only Ilalf <br />of the information required to develop a synthetic unit hydrograph. The remaining Ilalf <br />required to determine the unit hydrograph is determining how the runoff from the unit <br />effective rainfall is distributed over time. The temporal distribution was determined by <br />using a dimensionless form of an observed unit hydrograph for a Ilydrolically similar basin. <br />By using the dimensionaless format, the differences in basin size, lag times, and unit <br />duration are automatically taken into consideration. <br /> <br />The Bureau developed dimensionless form by converting unit hydrographs from recorcied <br />flood events by: <br /> <br />a. The time bases is expressed in terms of time t as a percentage of the lag time Lg plus <br />50 percent of the unit rainfall duration D: <br /> <br />100 t <br />------------ <br />L'I '0. 50 <br /> <br />1'1-4) <br /> <br />Increments of time t are equal to the unit duration of fhe umt hydrograpll derived from the <br />recorded flood event. <br /> <br />b. Dimensionaless discharge ordinates q are expressed in terms of the product of tile <br />recorded flood unit hydrograDh ordinate Q, in cubic teet per second, and lag time Lg plus <br />50 percent of the unit rainfall duration, Lg + O.5D, divided by the unit runoff volume for the <br />basin V', in 1-day cubic feet per second: <br /> <br />_ Q( Lg +0.50) <br />q - -.-.... .---.-- <br />VI <br /> <br />('1-5) <br /> <br />ComputinQ Synthetic Unit HydroQraph Ordinafes- Once the unit hydrograph lag time 113.S <br />been determined. it is a mlatively simple process to determine the synthetic unit <br />hydrograph ordinates. The following process is useel to determine the complete syntretic <br />unit hydrograph: <br /> <br />a. Determine the unit duration of the synthetic unit ilydrograpll. To provide adequate <br />definition, especially at the peak of the unit hyelrograph, the unit duration should <br />approximately be the lag time divided by 5.5. This unit dura.tion should be rounded to the <br />closest of 5,10,15, or 30 minutes, or 1,2,3. or 6 tlours. If the result is greater than 6 <br />hours, the basin should be divided into subbasins amj an individual unit ilydrograph should <br />be developed for each subbasin. This is generally the case for basins exceeding 500 <br /> <br />Colorado Flood <br />Hydrology Manual <br /> <br />7-52 <br /> <br />C'FW:T <br />