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<br />la~O.2'S <br /> <br />(11-4) <br /> <br />When equation (11-4) is substituted into equation (11-3). the result is ':he SCS one <br />parameter infiltration equation: <br /> <br />(1"1a ) 2 <br />Q (,0.0 Bsf <br /> <br />(11-5) <br /> <br />Once this equation WaS developed, median cu,ve numbers (CN) were derived graphicaily <br />by plotting the daily rainfall and runoff volumes measured in the field. n18se curve <br />numbers were relat'3d to the maximum retention (S) through tile following relationship: <br /> <br />eN <br /> <br />1000 <br />(s,To) <br /> <br />(11-6) <br /> <br />Therefore, the curve number could be selected from a table based on soil type and land <br />use and used in tile following equation bas'ld en combining equation (6) and (5) to <br />compute the runoff. <br /> <br />.200 <br />[p- '-eN' <br />Q ..------- <br />, 800 <br />IP'I......... . <br />I GN,i <br /> <br />" <br />-- 2 I" <br /> <br />8i <br /> <br />(11-7) <br /> <br />4. HOLTAN'S METHOD. <br /> <br />The Holtan infiltration methocl WaS developed by H. Holtan of the Agricultural <br />Research Service. This method is based on watershed cllaracteristics and accumulated <br />soil moisture. The Holtan method has the same general form as the HEC exponential loss <br />rate function except it does not consider precipitation intensity. An advanta(le of the Holtan <br />method is that parameters may be derived directly form the soil water infiltration <br />characteristics of the watershed. The Holtan infiltration 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 />7.23 <br />