Daily Values

General Information

For decades the streamflow readings were gathered by gage readers who recorded staff gage observations. To correlate the staff gage readings to the discharge and establish a rating curve for that station, they surveyed the channel profile several times a year by suspending themselves from cables in carts and lowering sounding weights (a sort of leaden torpedo) with sediment bottles and current meters attached, down to the bed of the channel. By repeating these soundings at regular intervals as they winched across the stream, the gage readers divided the stream below the cable into a series of cells. The width of each cell is determined by the sampling interval; 10 feet per cell on a large stream is common. The depth, of course, is established by noting the depth at which the sounding weight reaches bottom.

Because discharge is equivalent to width times depth times velocity, a discharge measurement requires velocity readings. By counting the clicks on the current meters, the hydrologists measured velocity in the cells defined by their survey of the bed perimeter. The velocity readings in each cell were typically taken at 0.8 depth and 0.2 depth, though it was soon found that a single reading at 0.6 depth provided a velocity for a cell that was very close to the average for that cell. Summing the discharge per cell gave the stream's discharge.

Later came automatic stage recorders, concealed in the familiar corrugated- pipe stilling wells. These recorded the stage by marking a tape that was read about every 6 weeks. Most recently has come telemetry, with readings of the stage taken automatically every 15 or 30 minutes (other observation intervals are possible), beamed to a satellite in geosynchronous orbit, and thence eventually to a mainframe in Reston, VA. The readings in the course of a day are written to WATSTORE's unit values file. The mean of a day's flow readings is written to the daily values file on magnetic disk, and after 2 years, to tape. Any updates and corrections to the historical record are written to tape as necessary, approximately twice a year.

Loading the data into WATSTORE is the responsiblity of the district office that takes the measurements. The amount of time that passes from when the measurements are taken until the data are loaded into the WATSTORE database varies among the districts. In fact, some stations have never been loaded into WATSTORE even though they are currently being operated and their observations appear in the Water Data Reports published by the District office. Data for other stations may be one or more years out of date. In a few cases data in WATSTORE are different than those published in the Water Data Reports. We don’t know why this happens.

In the midst of this state-of-the-art technology the discharge measuring system evokes its own past -- even telemetered gages have to have the bed perimeter resurveyed frequently to adjust the stage/discharge correlation as the river scours and fills. The resurvey is carried out as it always was: from cable cars or (nervously, during floods; and at stations where cable is impractical) from the bow of johnboats or even tugboats.

What is perhaps unexpected in browsing through the Daily Values data is the volume of non- streamflow data. Many climate stations, lake stations, and water quality measurements at regular stream gages swell the dataset. The non-flow measurements that were taken on a "continuous" (daily) basis go in the daily values file, and are found in Hydrodata. But many of the water quality measurements are "discontinuous" (taken at random times during the year); these are written to the Q - W file (Quality of Water) and are published in Hydrosphere’s STORET database.

Provisional Data

Provisional data are those observations that have not yet been formally blessed or published by the agency charged with their collection and management. While all observations of natural phenomena must be used with care, particular caution should be exercised when using data marked as provisional. They are subject to change as the agency reviews them.

The data that is currently available from the website does not specify if it is provisional or not. We assume this means that no provisional data will be posted to the website.

There exists in WATSTORE data that have been marked as provisional ever since they were posted. These data, when retrieved from the website, are the same. It is unclear if this means that the data are now considered "valid" and no longer provisional, or what.

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Last Updated March 10, 1998
Copyright © 1997 Hydrosphere Data Products, Inc.