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<br /> <br />- <br /> <br />account for some of the factors, C. O. Clark developed a technique which uses the <br />concept of the instantaneous unit hydrograph (IUH). This is conceptually the <br />hydrograph that would result from 1 unit of excess occurring over the basin in a <br />specified areal pattern and zero time. The IUH can then be used to compute a unit <br />hydrograph for any unit duration equal to or greater than the time interval used in the <br />computations. <br /> <br />The Clark method used two parameters and a time-area relation to define <br />the instantaneous unit hydrograph. The first parameter, time of concentration (t 01 is <br />the travel time of a water particle from the most upstream point in the basin to the <br />outflow location. An estimate of this lag time is the time from the end of effective <br />rainfall plus snowmelt over the basin to the inflection point on the recession limb of <br />the surface runoff hydrograph. The time of concentration is used in developing the <br />time-area relatiol). <br /> <br />The second parameter is the attenuation constant, R, which has the <br />dimension of time. This parameter is used to account for the effect that storage in the <br />river channel has on the hydrograph. This parameter can be estimated by dividing the <br />flow at the point of inflection of the surface runoff hydrograph by the rate of change <br />of discharge (slope) at the same time. Another technique for estimating R is to <br />compute the volume remaining under the recession limb of the surface runoff <br />hydrograph following the point of inflection and divide by the flow at the point of <br />inflection. In either case, R should be an average value determined by using several <br />hydrographs. <br /> <br />The other necessary item to compute an lUH is the time-area relation. <br />When t 0 has been determined, the basin is divided into incremental runoff-producing <br />areas that have equal incremental travel times to the outflow location. The distance <br />from the most upstream point in the basin is measured along the principal watercourse <br />to the outflow location. Dividing this distance by t 0 gives an estimate of the rate of <br />travel. Isochrones representing equal travel time to the outflow location are laid out <br />using the rate of travel to establish the location of the lines. The areas between the <br />isochrones are then measured and tabulated in upstream sequence versus the <br />corresponding incremental travel time for each incremental area. <br /> <br />The increment of time used to subdivide the basin need only be small <br />enough to adequately define the areal distribution of runoff while the time period <br />selected as the computation interval must be equal to or less than the unit duration <br />of excess. Since the former is frequently larger than the latter, a plot percent of time <br />of concentration versus accumulative area is useful in determining time-area <br />relationships. Such a curve facilitates rapid development of unit hydrographs for <br />various computation intervals and unit durations of excess. This is especially helpful <br />when making flood predictions and for basins where to is not firmly established at the <br />outset, since unit hydrographs may be easily modified to reflect subsequent changes <br /> <br />7-31 <br /> <br />