Time Base

occurrence of short-duration, high magnitude flow events that occur within the averaging period, while

over-representing effects of low flows. On large rivers the use of the mean daily values is acceptable

because the difference between the mean and peak daily discharges is negligible. However, on smaller

streams, flood events may last only a few hours, so that the peak daily discharge is much greater than the

corresponding mean daily discharge. Excluding the flood peaks and the associated high sediment loads can

result in underestimation of the effective discharge. Rivers with a high flashiness index, defined as the ratio

of the instantaneous peak flow to the associated daily mean flow, are most likely to be affected. To avoid

this problem it may be necessary to reduce the time base for discharge averaging from 24 hours (mean

daily) to 1 hour, or even 15 minutes on flashy streams. When a shorter time base is used for the discharge

data, it is necessary to use a corresponding time base for the sediment rating curve. For example, an

investigation of discharge data for eleven USGS gaging stations in the Yazoo River Basin, Mississippi

revealed that the annual yield of bed material calculated using mean daily discharge data was approximately

50% less than the yield calculated using 15-minute data (Watson et al., 1997). These are relatively small

basins (drainage areas less than 1,000 km2) with high rainfall intensities and runoff characteristics that have

been severely affected by land-use change and channel incision. Consequently, hydrographs are

characterized by steep rising and falling limbs, with events peaking and returning to base flow in much less

than 24 hours.

In practice, mean daily discharge data may be all that are available for the majority of gaging

stations and these data may be perfectly adequate. However, caution must be exercised when using mean

daily data for watersheds with flashy runoff regimes and short-duration hydrographs. The use of 15-minute

data to improve the temporal resolution of the calculations should be seriously considered whenever the

available flow records allow it.

Period of Record

The period of record must be sufficiently long to include a wide range of morphologically-significant

flows, but not so long that changes in the climate, land use or runoff characteristics of the watershed

produce significant changes with time in the data. If the period of record is too short, there is a significant

risk that the effective discharge will be inaccurate due to the occurrence of unrepresentative flow events.

Conversely, if the period is too long, there is a risk that the flow and sediment regimes of the stream at the

beginning of the record may be significantly different to current conditions. A reasonable minimum period

of record for an effective discharge calculation is about 10 years, with 20 years of record providing more

certainty that the range of morphologically significant flows is fully represented in the data. Records longer

than 30 years should be examined carefully for evidence of temporal changes in flow and/or sediment

regimes. If the period of record at a gaging station is inadequate, consideration should be given to

developing an effective discharge based on regional estimates of the flow duration as outlined below.

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