Analysis Techniques: Flow Duration Analysis
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What is it?
How is it calculated?
Step 1: Sort (rank) average daily discharges for period of record from the largest value to the smallest value, involving a total of n values.
Step 2: Assign each discharge value a rank (M), starting with 1 for the largest daily discharge value.
Step 3: Calculate exceedence probability (P) as follows:
What kind of graphs and charts can be generated?
What does this particular information tell you about your stream?
The shape of a flow-duration curve in its upper and lower regions is particularly significant in evaluating the stream and basin characteristics. The shape of the curve in the high-flow region indicates the type of flood regime the basin is likely to have, whereas, the shape of the low-flow region characterizes the ability of the basin to sustain low flows during dry seasons. A very steep curve (high flows for short periods) would be expected for rain-caused floods on small watersheds. Snowmelt floods, which last for several days, or regulation of floods with reservoir storage, will generally result in a much flatter curve near the upper limit. In the low-flow region, an intermittent stream would exhibit periods of no flow, whereas, a very flat curve indicates that moderate flows are sustained throughout the year due to natural or artificial streamflow regulation, or due to a large groundwater capacity which sustains the base flow to the stream.
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