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z <br />`~ RUNOFF CALCULATIO"1 <br />The volume of runoff is estirated by the method described in Section <br />4, Hydrology, (and Supplement A), 'lational Engineering Handbook, U. S. <br />Department of Agriculture, Soil Conservation Service, 1908. This method <br />is also described and utilized in Hydrology and Sedimentology of Surface <br />lined Lands, by C. T. Haan and B. J. Barfield. <br />The basic equations used to compute the runoff volume are <br />(P Ia)2 <br />Q = P-Ia-S <br />and <br />S = 1000 _ 10 , <br />Ctl <br />where <br />Q = runoff depth (inches) <br />`• P = precipitation (inches) <br />~' <br />Ia = initial abstraction (inches) <br />S = potential maximum retention (inches) <br />Ctl = SCS curve number. <br />The procedure is to select a curve number appropriate for the soils, <br />antecedent moisture, land use, and general hydrologic conditions of the <br />watershed. This is accomplished from tables provided in the above <br />references. The second step is to compute the potential maximum retention <br />(S). The initial abstraction (fa) is estimated as a fraction of S; the <br />relation Ia = 0.25 being the coruaon assumption. Tlie depth of runoff is <br />then calculated. <br />Reliable estimates of runoff by this procedure depend centrally upon <br />an appropriate selection of CN. The soils on the study area were assigned <br />• the SCS classification A, described as "soils having high infiltration <br />rates even if thoroughly wetted and consisting chiefly of deep well to <br />