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Any initial computation of discharge will result in some form of "provisional data"; however, there are classes of "provisional" that represent differing qualities of values. Provisional data in any form are subject to change prior to publication. Because of the variety of factors that can introduce errors in provisional streamflow data, users are cautioned to consider carefully the nature of the information before using it for decisions that concern personal or public safety or the expenditure of money.
Additional information about streamflow measurement protocols is available at the U.S. Geological Survey website. Questions about the Water Resources Department streamflow gaging program may be addressed to the Department´s hydrographics staff.
After the conclusion of the water year (October 1 to September 30), the streamflow data, if preliminary, are again reviewed or, if discharge has not yet been determined, the data development and review process is initiated. During this process, any previous computations of streamflows are evaluated and adjustments are made as necessary to reflect changes in the stage-discharge curve that may have occurred during the year. The streamflow data from stations in the drainage basin are compared to those from the other stations in the same or nearby, to weather records to determine if the flow patterns are consistent. Estimates of streamflows are verified or developed where the data are missing or appear to be erroneous. All data are considered to be provisional and are subject to change until the review process has been completed.
Streamflow data for a gaging station are developed by regularly recording the height of the water surface (stage) at the station. Streamflow is calculated by converting the stage data to a discharge rating developed from measurements made at the site.
A rating is a simple model that predicts how stream stage relates to discharge. It is developed by directly measuring flow at various stages. Flow measurements are performed by sampling velocity at certain depths and width intervals across a section of the stream. The flow rate in each interval is found by multiplying its width, depth, and velocity. The discharges for all the intervals are added together to get the total discharge of the stream.
Erosion and deposition in the stream channel near a gage can have significant and permanent effects on the cross-section of a stream. The cross section can also be affected temporarily by fallen trees, ice, and aquatic vegetation. Changes in the cross-section of a stream alter the relationship between stage and discharge. The development of accurate records requires that the stage-discharge curves be verified by periodically repeating the discharge measurements and defining the effects of changes on the stream channel.
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