Flow Variation

The characterization of streamflow has practical application for the design of flood-control structures, evaluation of channel stability, and in determining whether sufficient water is available at the appropriate time to meet the needs of both people and the ecosystem. Abundant data often are available for gauged sites - as much as a century of continuous monitoring - and methods exist that allow extrapolation to ungauged sites. This has led to a great deal of hydrologic analysis of the spatial and temporal variation in streamflow.

Ecologists stress that one must consider five elements of streamflow to ensure that the needs of the stream ecosystem are met. These include the magnitude of flow, its frequency of occurrence, duration and timing of the event, and the rate of rise and fall (Richter et al. 1996). Because these five components are determined by natural variation in climate, vegetation, geology, and terrain, it is argued that rivers of a region have a characteristic flow regime, much like a region has a characteristic climate.

2.3.1 The likelihood of extreme events

Often we wish to know how frequently a flow of a given magnitude is exceeded in an average year, or the likelihood of an extreme annual flood such as one that we might characterize as a 10- or 50-year event. Several methods are available for estimating the probability of extreme events, whether flood or drought. The magnitude of events is inversely related to their frequency (or probability) of occurrence, a relationship captured in a flow-duration curve. These typically are constructed using daily streamflows over many years, so they include both seasonal and interan-nual variability. A flow-duration curve plots the cumulative frequency of daily records that equal or exceed a given value of average daily discharge against flow magnitude (Figure 2.10). Low flows are equaled or exceeded on most days, whereas high flows are equaled or exceeded only a small percentage of the time.

Several useful metrics are easily obtained from a flow duration graph. The median flow OQ050) is that exceeded 50% of the time. Because of the influence of a few large floods on the mean value, in humid regions the mean is exceeded only on 20-30% of the days (Dingman 2002). The Q0 05 is a streamflow exceeded only 5% of the time (18 days in an average year), so is a reasonable value for high flows that occur infrequently. Similarly Qo 95 is a streamflow exceeded 95% of the time. This value indicates how much water is available most of the time, and also provides a threshold below which we can identify extremely low flows. The flow-duration method is an excellent indicator of water availability, but does not by itself provide information on the timing of high and low flows, which may be ecologically significant.

If one wishes to compare among streams, it is usual to normalize flow to drainage area by plotting flow divided by drainage area. Such

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