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TimE~-Area Relation: The time-area relation is a graphical parameter that specifies the <br />accui~nulated area of the watershed that contributes runoff to the outlet of the watershed at any <br />time, Two methods can be used to develop a time-are relation: 1) by analysis of the <br />watershed to define incremental runoff producing areas that have equal incremental travel <br />time:~ to the outflow location, or 2) by use of synthetic time-area relations. The development of <br />a tirr~e-area relation by analysis of the watershed is a difficult task and well-defined and reliable <br />prou~dures for that task are not available. Unless the watershed has an extremely unusual <br />shape, or has several distinct areas of dramatically different land-use, this analysis should not <br />be undertaken. In general, synthetic time-area relations can be used in Colorado. <br />The ~~imensionless, synthetic time-area relations that can be used in Colorado are shown in <br />Figure 1 and the coordinate values of the curves are listed in Table 2. Curve A should be used <br />if thE~ watershed or subbasin physiographic type is urban or predominantly urban. Curve C <br />should be used if the watershed or subbasin is mostly undeveloped mountains and/or plains <br />possibly with some interspersed agricultural fields. Curve B should be used for all other <br />situations. <br />Curv~~ B is the default time-area relation in HEC-1 and will be used with the Clark unit <br />hydr~~graph if a time-area relation (UA record) is not supplied. Curves A and C are <br />dimensionless and those curves are input to HEC-1 by inserting the percent of total area values <br />from Table 2 in the UA record. <br />19 March 2007 11 <br />