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22 <br />2.3 Flow Duration Curves <br />Another approach that has- been proposed for deriving instream <br />flow recommedations from historical stream flow records uses flow <br />duration analysis (Fig. 2-1). The concept of flow duration in <br />descriptive hydrology is somewhat of a misnomer. It refers to the <br />discharge level which is equaled or exceeded in a certain percentage <br />of all stream flow observations (i.e., a cumulative probability <br />density function of flow events), but incorporates no direct measure <br />of how long the flow persists. When discussing low flows, the 90 <br />percentile annual flow (calculated from the data set of mean daily <br />flows) would be the stream flow which would be expected to be equaled <br />or exceeded nine out of every ten days of the year. Given a data set <br />of historical flow records, the calculation of percentile flows is a <br />relatively simple procedure common to many applications of descriptive <br />hydrology, including dam and reservoir design. <br />The Northern Great Plains Resource Program (NGPRP) proposed a <br />method that utilizes stream flow records and flow duration analysis <br />(Anonymous 1974). With this procedure, which requires at least 20 <br />years of daily flow records, a flow recommendation is made for each <br />month of the year to protect all aquatic resources potentially <br />affected by stream flow regulation. Although this method was <br />presented as being applicable to both warm and coldwater rivers in the <br />Midwest, a caution was added that it will not necessarily protect <br />against degradation of aquatic habitats which might occur from high <br />temperatures or low dissolved oxygen during low flows in warm lowland <br />streams. <br />The NGPRP method includes a step in which flow records are <br />modified by using Student's t distribution to identify and eliminate <br />abnormal events (floods and droughts). After high and low <br />observations are removed from the record for each month, the instream <br />flow recommendation is the flow that is equaled or exceeded by 90% of <br />the observations remaining in the record. This same procedure is