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