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19 <br />Although the original guidelines include a recommendation that the <br />proposed flow requirements be supported by field data, such as <br />photographs of the proposed development site at critical flows, the <br />method has often been applied with no field work. <br />1 <br />Data required to apply a fixed percentage method such as the <br />Montana Method are easily obtained from the published stream gaging <br />records of the U.S. Geological Survey (USGS). Good techniques are <br />available for extrapolating the MAF statistic upstream or downstream <br />from existing gaging stations or estimating MAF on ungaged watersheds <br />(e.g., Chow 1964, Linsley and Franzini 1972). However, because of the <br />skewed nature of stream flow events (floods are relatively rare in <br />occurrence but very signficant in terms of their effect on mean flow <br />values), a sound argument can be made for the fact that the median <br />flow statistic is a more appropriate measure of central tendencies in <br />hydrologic data than the mean flow. The median statistic has been <br />used in other approaches to instream flow assessment (Sect. 2.2). <br />2.2 Constant Yield <br />The U.S. Fish and Wildlife Service (USFWS) in Region 5 recently <br />issued guidelines (the New England Flow Recommendation Policy or <br />NEFRP) that establish a process for formulating minimum flow <br />recommendations by using a combination of the median flow and a <br />constant yield statistic to represent watershed hydrology (U.S. Fish <br />and Wildlife Service 1981, Knapp 1980). For unregulated streams with <br />a drainage area greater than 130 km2 (50 sq miles) and good historical <br />flow records (>25 years and t1d accuracy of gage), the median monthly <br />flow (MMF) serves as the datum for evaluation of instream flow needs <br />in the NEFRP. For streams that do not meet these criteria, a constant <br />yield factor, runoff per watershed area, was calculated for the entire <br />New England region and is applied to a specific site to estimate <br />actual flow conditions (Table 2-1). The instream flow recommendation <br />based on this policy is called the Aquatic Base Flow (ABF) and is