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<br />la = 0.2 . S <br /> <br />(11-4) <br /> <br />When equation (11-4) is substitut~!d into equation (11-3), the resuit is the SCS one <br />parameter infiitration equation: <br /> <br />Q'o (P~/a) 2 <br />(P+0.8S) <br /> <br />(11-5) <br /> <br />Once this equation was developed, median curve numbers (CN) were derived graphically <br />oy plotting the daily rainfall and runoff volumes measured in the field. These curve <br />numbers were related to the maximum retention (S) through the following relationship: <br /> <br />1000 <br />CNo, <br />( S+10) <br /> <br />(11-6) <br /> <br />Therefore, the curve number could be selected from a table based on soillype and land <br />use and used in the following equation based on combining equation (6) and (5) to <br />compute the runoff. <br /> <br />Qo <br /> <br />(pj2g~ )2t <br />(,0+( 8~) -8) <br /> <br />(11-7) <br /> <br />4. HOLTAN'S METHOD. <br /> <br />The Hoitan infiitration method was developed by H. Holtan of the Agricuitural <br />Research Service. This method is based on watershed characteristics and accumulated <br />soil moisture. The Holtan method tlas the same general form as the HEC expollentialloss <br />rate function except it does not consider pmcipitation intensily. An advantage of the Hoitan <br />method is that parameters may be derived directly form the soil water infiitration <br />characteristics of the watershed. The Hoitan infiitration function is expressed <br />mathematically as: <br /> <br />L=aSe+c <br /> <br />(11-8) <br /> <br />Colorado Flood <br />Hydrology Manual <br /> <br />DRAFT <br /> <br />7.231 <br />