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Western Dam Engineering Technical Note 12 the Green and Ampt methodology. Reference [6]also provides a spreadsheet solution to reduce the time required for parameter development. Figure 2:Green and Ampt methodology rainfall loss model (Source: DWR, 2008). NRCS Curve Number Methodology The curve number methodology represents a more simplified approach to estimating runoff as compared to the Green and Ampt methodology. The curve number methodology estimates the runoff using antecedent moisture conditions and empirical curves estimated from a series of field studies, which were conducted using numerous soil and vegetative cover combinations. As shown in Figure 3, high curve numbers indicate high potential for runoff with a maximum value of 100 representing a total conversion of rainfall to runoff. Conversely, progressively lower curve numbers represent more pervious soil conditions and subsequently lower potential for runoff. The curve number is most easily applied to watersheds with relatively homogenous soil and vegetative cover properties; however, the methodology can also be applied to non-homogenous watersheds by estimating a composite curve number based on area weighted averages. The curve number is among the most widely used methodologies due to its relative ease of application and extensive parameter database. See references [2],[4]and [5]for more information regarding the use and application of the curve number methodology, selection of curve numbers, and worked example problems. Figure 3:NRCS Curve Number methodology (Source: Viessman and Lewis, 2003). Initial and Constant Loss Methodology Another simplified methodology that is commonly used for estimating runoff is the initial and constant loss methodology. This methodology is similar to the Green and Ampt methodology and assumes that rainfall losses can be simulated as a two-step procedure where: ·Step 1: Rainfall is initially lost to a combination of infiltration and surface retention until the initial loss depth is exceeded. ·Step 2: Upon initial loss depth exceedance, a constant loss rate is applied to the rainfall that occurs during the remainder of the event duration. Based on this methodology, if the rainfall intensity exceeds the constant loss rate, the rate difference is representative of runoff, as presented in Figure 4. Conversely, if the constant loss rate exceeds the rainfall intensity, no runoff is produced. Step 1 above is often set to zero when developing an inflow design flood hydrograph as a conservative approach. The initial and constant loss methodology is particularly applicable to the modeling of very infrequent storm events characterized by significant precipitation. It can also be applied to more frequent storm events (no more frequent than the 100-yr storm event), particularly where watersheds are characterized by moderate to high infiltration rates.