A flood hydrograph, also called a storm hydrograph, represents a stream or river's response to a rainfall event. The hydrograph shows this response in terms of the flow rate. On a normally-shaped hydrograph, the graph line rises quickly to a peak discharge rate, then gradually falls back down to the normal flow rate of the stream. The x axis of a hydrograph shows the time in days or hours, and the y axis shows the flow rate, usually in cubic feet per second. Many hydrographs include a hyetograph, a graph of the rainfall intensity over time.
- Skill level:
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Estimate the total runoff from the storm by performing a rough calculation of the area under the hydrograph. Approximate the hydrograph by a series of rectangles; calculate the area of each rectangle and sum the areas.
Pinpoint the highest spot on the hydrograph and read the corresponding number on the y axis to determine the peak discharge rate.
Move down from the peak discharge point and read the corresponding number on the x axis to find the time between the beginning of the storm and the peak discharge, called time to peak.
Subtract half the time of the duration of the rainfall---determined using a hyetograph---from the time to peak to determine the lag time. Lag time is sometimes defined as the time from the centre of mass of the rainfall to the centre of mass of the runoff, according to the authors of "Design Hydrology and Sedimentology for Small Catchments." Skip this step if your hydrograph does not include a hyetograph.
Tips and warnings
- Ensure that the units of time are consistent when calculating the area under the hydrograph.
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