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Western Dam Engineering <br />Technical Note <br />11 <br />Figure 1:The rainfall-runoff process with losses. <br />The rate and volume of rainfall losses, and subsequently <br />runoff, are influenced by a number of factors including: <br />·Rate of rainfall (i.e. intensity) as well as rainfall <br />distribution and patterns <br />·Watershed pervious and impervious areas <br />·Soil infiltration rates <br />·Watershed properties like roughness, vegetative <br />cover, soil properties and slope <br />Numerous rainfall loss estimation methodologies are <br />available; however, with an interest in brevity and <br />specific application to the western U.S., three of these <br />methodologies will be presented in the following <br />sections. <br />Green and Ampt Infiltration Loss Methodology <br />The Green and Ampt methodology is an approach based <br />on the soil-water system where the inherent soil <br />properties can be physically measured. It is particularly <br />applicable for frequency event storms more frequent <br />than and including the 100-year storm event, but can also <br />be applied to less frequent storm events. <br />In general, the Green and Ampt methodology is based on <br />the following factors: <br />·Surface Retention (Initial) Loss <br />o Depression storage <br />o Interception by vegetation <br />·Infiltration Loss <br />o Hydraulic conductivity at natural saturation; <br />o Wetting front capillary suction; and <br />o Volumetric soil moisture deficit at the start of <br />rainfall <br />·Imperviousness of the watershed <br />A simplified definition sketch of the Green and Ampt <br />methodology is presented in Figure 2 and is <br />representative of the aforementioned factors. <br />The Green and Ampt methodology is fairly <br />comprehensive and accurate; however, as a <br />consequence, the process of estimating the <br />aforementioned parameters has been historically more <br />cumbersome than other methodologies. However, <br />modern GIS techniques can be applied to simplify the <br />effort required to estimate pertinent parameters. See <br />references [2]and [6]regarding application and use of