Laserfiche WebLink
<br />" <br /> <br />Once this equation was developed, median curve numbers (CN) were <br />derived graphically by plotting the daily rainfall and runoff volumes measured in the <br />field. These curve numbers were .related to the maximum retention (S) through the <br />following relationship: <br /> <br />CN = 1000/( S + 10) <br /> <br />(6) <br /> <br />Therefore, the curve number could be selected from atable based on soil <br />type and land use and used in the following equation based on combining equation (6) <br />and (5) to compute the runoff. <br /> <br />Q = (p - (200/CN) - 2) * * 2/ (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 <br />Agricultural Research Service. This method is based on watershed characteristics and <br />accumulated soil moisture. The Holtan method has the same general form as the HEC <br />exponential loss rate function except it does not consider precipitation intensity. An <br />advantage of the Holtan method is that parameters may be derived directly form the <br />soil water infiltration characteristics of the watershed. The Holtan infiltration function <br />is expressed 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 <br />SCS handbook as shown in the section discussing the initial and uniform method. <br /> <br />5. HEC EXPONENTIAL METHOD. <br /> <br />The HEC Exponential Loss Rate method was developed by the Corps' <br />Hydrologic Engineering Center. It is an empirical function in which the parameters are <br />not readily determined from measurable watershed characteristics. The parameters <br />are generally obtained by calibration to observed rainfall and runoff data. For ungaged <br />areas where the loss rate parameters must be related to the variable soil types and <br /> <br />7-5 <br />