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001592 32.8--4 WATER INFORMATION MANAGEMENT SYSTEM HANDBOOK To explain: the shape of a flow-duration curve is largely determined by the hydrologic and geologic characteristics of the watershed it represents. If the ordinate (y) axis is normalized by an indexing discharge (bankfull), the flow-duration curve may be used to describe the behavior of ungaged watersheds which have similar hydrologic and geologic characteristics. This concept is employed in the procedure for the purpose of constructing the channel maintenance flow hydrograph for the ungaged point(s) of quantification. Common Step 2 - Identify QB' QA, QLl, and QL2 ratios and their durations on the normalized flow-duration curve and calculate rise-recession ratios and durations. 1. Bankfull Discharge (QB): The ratio value of 1.0 is the point on the y-axis of the flow-duration curve which represents bankfull discharge. Extend a horizontal line from this point to its intersection with the flow-duration curve. Extend a vertical line from this intersect down to the x-axis and read the value for the percent of time bankfull discharge is equaled or exceeded. This time increment, converted to days, becomes the duration of bankfull discharge. 2. Mean Annual Dischar~e (QA): Locate along the flow-duration curve the point WhlCh represents t e ratio value of OA/QB (QA is identified from representative station data). 3. Baseflow Discharge (QLl): Locate on the flow-duration curve the point which represents the ratlo value of QLl/Oa lOLl is identified from representative station data and/or measurements). 4. Lowflow Discharge (QL2): Locate on the flow-duration curve the point which represents the lowest .ratio value of flow (QL2/0B) (QL2 is identified from representative station data). Note: OL2 represents the flow with the highest exceedance value. 5. Rise-Recession Flows: For ease of construction, to approximately mimic the natural hydrograph, and to reasonably construct the rise and recession segments of the channel maintenance flow hydrograph, this common step incorporates the following assumptions: a. Most sediment transport will occur during the period of flow which begins with the occurrence of mean annual discharge and continues through bankfull discharge. b. Sediment transport wi 11 be provid,'d for in the channel maintenance flow hydrograph if the constructed hydrograph includes hankfull di';ch,irge and one-half of the calculated rise-recession flow with most of this flow heing di s tri buted to the ri sing segment of the cons tructed hydrograph.