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Western Dam Engineering Technical Note 13 Figure 4:Initial and Constant Loss methodology rainfall loss model (Source: DWR, 2008). Additional Watershed and Methodology Considerations Watershed Characteristics ·Wild fires can significantly alter the pertinent watershed characteristics that affect runoff. For this reason, burned areas within study watersheds should be assessed with particular scrutiny. ·For some watershed studies, it could be prudent to consider not only present watershed characteristics, but also potential future watershed characteristics. Potential development and “urbanization” of watersheds can significantly increase runoff. Green and Ampt Methodology ·Use of the Green and Ampt methodology is generally preferable for projects that warrant precision. ·Although perhaps more time consuming than other methodologies, it can be applied for any event frequency with a relatively high level of accuracy. Curve Number Methodology ·Application of the curve number methodology is generally not recommended for soil and land cover combinations that yield curve numbers less than about 40. ·The curve number methodology is used successfully and extensively by the engineering community due to its ease of use and sufficient accuracy. Readers are cautioned, however, because the curve number has been shown to be less accurate than some physically-based infiltration methodologies due to its empirical nature. This aspect is presented not to discourage its use, but rather to highlight its applicability to a particular project. If a high level of accuracy is required for a particular project, use of the curve number methodology can be inadequate. ·It is recommended, particularly for projects where a high level of accuracy is required, to independently verify curve number methodology results with another runoff estimation methodology. Transforming Excess Rainfall to Flow Hydrographs Runoff rates are converted to flow hydrographs using a translation methodology. Numerous translation methodologies exist; however, the unit hydrograph methodology is used extensively and is generally the most preferred. A unit hydrograph is defined as the time distribution of one inch of runoff from a storm event of a specified duration for a particular watershed, as presented in Figure 5. Unit hydrographs are reflective of the physiography, topography, land-use, and other unique characteristics of the individual watershed and assume that rainfall is uniformly distributed across the watershed. As such, different unit hydrographs are developed for the same watershed for different durations of rainfall excess. Ideally, a unit hydrograph would be developed based on gage and calibrated watershed data; however, engineers are frequently confronted with project watersheds that lack sufficient data to develop a unit hydrograph. As such, synthetic unit hydrographs are developed based on available watershed data at other locations that have hydrologic characteristics similar to those of the project watershed. Numerous synthetic unit hydrograph methodologies exist; however, for the purposes of application to the western U.S., the following methodologies are most pertinent: ·U.S. Department of the Interior, Bureau of Reclamation (Reclamation) synthetic unit hydrograph as presented in the Flood Hydrology Manual (Cudworth, 1989) ·Clark synthetic unit hydrograph ·U.S. Geological Survey (USGS) synthetic unit hydrograph specific to Montana as presented in