USGS Home Contact USGS Search USGS

USGS Water Science Center, Minnesota

Explanation of Duration Hydrographs

A hydrograph is a plot of streamflow against time or date. Duration hydrographs (plots of selected percentiles of x-day flows versus day-of-year) help resource managers to quantify the variability of flow at a gaging station. For example, a daily duration graph is a plot of flow percentiles (based on historic average daily flows for each day of the year) against the days of a calendar year. A daily duration hydrograph can be used to statistically quantify how much water typically is passing the gaging station and, possibly, to compare the hydrology of one stream to the hydrology of another stream.

Most importantly, a trace of daily flows for the current year and the preceding year are also plotted on the duration hydrograph for this web site. The severity and trends of dry or wet periods at a gaging station can be determined by comparison of the current daily flow trace on the hydrograph with the lines depicting the flow ranges of selected historic percentiles. For example, if recent streamflows at a station are in the minimum flow to 10-percentile area of a duration hydrograph, and decreasing in magnitude, a dry period is being experienced, and conditions are getting drier.

Daily value, running 7-day average, and base-flow hydrographs for regulated and unregulated conditions are presented on the web site, and are described below. A discussion of how to analyze the hydrographs follows the description of the three types of hydrographs.

An unregulated stream is one in which the effects of transfer, withdrawal, or storage of water by human activities are minimal. Conversely, a regulated stream will reflect significant effects on the hydrology of the basin as a result of transfers, withdrawals, and storage. Though duration hydrograph computation is identical, different types of duration hydrographs for regulated sites may provide more meaningful interpretation.

1. Daily value duration hydrographs depict percentiles for daily flows, which are the average flow, in cubic feet per second, over a day's time. Daily value duration hydrographs are appropriately used to depict the daily variability of flow for unregulated conditions and regulated conditions which do not have contrasting week-day/weekend flow patterns.
2. 7-day running average duration hydrographs are duration graphs of running averages of 7 days of flow, plotted at the middle of the 7-day period. These hydrographs should primarily be used for smoothing hydrographs of any streams with highly variable flow or for regulated streams that have distinctly different week-day/weekend flow patterns. The percentiles of 7-day averages are less affected by the week-day/weekend patterns.
3. Daily value base-flow duration hydrographs depict the hydrology of the approximated ground water contribution for unregulated streams, and are useful for evaluating the severity of droughts. There are two general sources of flow to a stream: overland runoff and flow originating from ground water, commonly referred to as "base flow". During droughts, most of the streamflow will be derived from water temporarily stored in the ground after infiltration from a previous rainfall event. Similar to the 7-day running average duration graphs, the base-flow duration hydrographs are smoother than the daily value hydrographs. Base flows were approximated by the sliding-interval HYSEP method ("HYSEP: A Computer Program for Streamflow Hydrograph Separation and Analysis," 1996, WRIR 96-4040, by R. A. Sloto and M. Y. Crouse). Base-flow duration hydrographs are not applicable for regulated streams.

Daily values, 7-day running averages, and base flows were first computed for each day of the year. The 7-day running averages are plotted against the mid-points of the 7-day periods. These flows were put into groups by month of the year. Then, the minimum, maximum, and 10-, 25-, 75-, and 90-percentiles of these monthly groups were computed and plotted on the duration graphs at the midpoint of the months. The percentile lines were interpolated between these data points.

1. Both the current year and the previous year are excluded from percentile computations so that it will be known how far the current data may exceed the minimum and maximum values of the historic period.
2. The historical minimum (0-percentile) and maximum (100-percentile) are plotted at the midpoint of the month, even though they may have occurred on another day of the month. Therefore, the plotted minimum and maximum lines on the plots depict the trend of the minimum or maximum, and not the actual date of occurrence.

1. Current flows less than the minimum flow or higher than the maximum flow lines may represent dry or wet conditions that exceed the historic record at a station.
2. Current flows in the minimum to 10-percentile range depict very dry conditions, and current flows in the 90-percentile to maximum range depict very wet conditions.
3. Current flows in the 25- to 75-percentile range, or even the 10- to 90-percentile range can be considered to depict "normal" conditions.
4. The hydrographs are useful not only for quantifying the severity of wet or dry periods, but the trends to drier or wetter conditions.
5. The "troughs" between rainfall-runoff events are an index of base flow, which is a better measure of drought severity than total flows that include runoff. A study of the daily flow duration hydrographs will typically show that during dry periods, when daily average flows are below the 10-percentile flow, a rainfall event will put current flows into the normal-flow zone, after which they may quickly return to below the 10-percentile line when direct runoff from the rainfall event is gone. Therefore, to analyze a drought, it is a good idea to analyze the base-flow duration plots or constrain your focus on the daily duration hydrographs to the current flow troughs/recessions rather than the current flow peaks that might include surface runoff.
6. Although it is almost impossible to predict streamflows without some prediction of rainfall, water-management agencies should anticipate dry periods in drought months when flows are in lower percentiles during the wet-season months. This is because water is not being detained as ground water storage during the wet-season months for release to the stream during the dry-season months. Though the likelihood of drought is probably greater following a "dry" wet season, it is completely possible for a severe drought to occur after a "wet" wet season. Similarly it also is possible for a "wet" dry season to occur after a "dry" wet season.
7. The three types of duration graphs were designed to present different aspects of the hydrology of a stream:
   1. The daily value hydrographs show the variable status of daily value flows and primarily are used with unregulated streams.
   2. The 7-day running average hydrographs accomplish a smoothing effect on the current and percentile flows, and may be used with unregulated streams and regulated streams with weekend/weekday flow patterns.
   3. The base-flow hydrographs are useful for evaluating the status of stream flow originating from ground water, which is the primary source of flow during droughts. These hydrographs are also smoother than both the daily value and 7-day running average flow duration hydrographs and are primarily used with unregulated streams.
   4. The cumulative-flow duration hydrographs are useful for extreme smoothing of flows and percentiles, and provide insight to the total production of water from a river basin.

Accessibility FOIA Privacy Policies and Notices
U.S. Department of the Interior | U.S. Geological Survey URL: http://mn.water.usgs.gov/durationHydrographs/hydrographExplanation.html Page Contact Information: Webmaster Page Last Modified: Thursday, 09-Aug-2007 15:43:34 EDT