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<br />When equation (4) is substituted into equation (3), the result is the 8C8 one <br />parameter infiltration equation: <br /> <br />Q = ( P - 0.2 ' S) .. 2 I ( P + 0.8 ' 8 ) <br /> <br />(5) <br /> <br />Once this equation was developed, median curve numbers (CN) were derived <br />graphically by plotting the daily rainfall and runoff volumes measured in the field. These <br />curve numbers were related to the maximum retention (8) through the following <br />relationship: <br /> <br />CN = 1000 I (8 + 10) <br /> <br />(6) <br /> <br />Therefore, the curve number could be selected from atable based on soil type and <br />land use and used in the following equation based on combining equation (6) and (5) to <br />compute the runoff. <br /> <br />Q = (P - (2oo/CN) - 2)"2 I (P + (800/CN) - 8) <br /> <br />(7) <br /> <br />4. HOLTAN'S METHOD. <br /> <br />The Holtan infiltration method was developed by H. Holtan of the Agricultural <br />Research Service. This method is based on watershed characteristics and accumulated <br />soil moisture. The Hollan method has the same general form as the HEC exponential loss <br />rate function except it does not consider precipitation intensity. An advantage of the Holtan <br />method is that parameters may be derived direcUy form the son water infiltration <br />characteristics of the watershed. The Holtan Infiltration function is expressed <br />mathematically as: <br /> <br />L=a'S.'e+c <br /> <br />(8) <br /> <br />where, <br /> <br />L = Loss rate in Inches per hour <br />a = Infiltration capacity <br />S = Available Storage in inches <br />e = Exponent of storage <br />c = constant rate of infiltration <br /> <br />Estimates of 'c' can be based on the hydrologic soil group given in the 8CS <br />handbook as shown in the section discussing the initial and uniform method. <br /> <br />Cowrado Fwod <br />Hydrowgy Manual <br /> <br />DRAFT <br /> <br />7.17 <br />