In Reply Refer To: October 14, 2004
Mail Stop 415
OFFICE OF SURFACE WATER TECHNICAL MEMORANDUM 2005.01
SUBJECT: National depot for shift-adjusted ratings
The memorandum announces the availability of a national depot for serving shift-adjusted rating curves for streamgages shown on the National Water Information System Web (NWISWeb). This depot was developed to facilitate the smooth transition of current ratings to the U.S. Army Corps of Engineers and the National Weather Service. These agencies have had access to the depot since June 2004. Access will now be opened to other interested agencies and parties without restriction.
For each station serving stage and discharge data over NWISWeb, the current stage-discharge rating expanded to each hundredth of a foot, including any applicable shift, is retrieved daily from the District NWIS database in rdb format at 8:45 pm local time and transferred to the depot on NatWeb. Currently the depot is limited to stage-discharge ratings. For a slope station, only the base stage-discharge rating will be available and for an index-velocity station no rating will be available.
Ratings are available to agencies and other interested parties from the depot via http at the following URL: http://waterdata.usgs.gov/nwisweb/data/exsa_rat/XXXXXXXX.rdb where XXXXXXXX is the 8-15 digit USGS station number. Security restrictions will not allow a directory listing of the parent directory. The depot itself resides on NatWeb, so the redundant capabilities of that system will be available to users. The depot has "read-only" access so that the ratings cannot be altered by those accessing the depot.
The U.S. Army Corps of Engineers and the National Weather Service have already been informed of the depot; however, no public announcement is currently planned. Districts are welcome to notify cooperators and interested parties as they see fit. Those Districts that are interested or are currently serving ratings to the public via an alternative system are welcome to use the new depot for this purpose.
It is important to educate users of the depot that ratings will change, and sometimes frequently. All users should be reminded to check the depot frequently to ensure that the currently applicable shifted rating is used for the current data, and that the current rating cannot necessarily be used on historical data. A standard disclaimer statement at the head of each rating file itself will be added to help educate users.
For Districts that make links to the depot available to the general public, it is critical that supplemental information be provided explaining ratings and shifts and calling attention to their often transitory nature. An example of the type of wording that could be used is provided below.
"Stage-discharge relations (ratings) are usually developed from a graphical analysis of numerous discharge measurements. Measurements are made on various schedules and sometimes for different purposes. All discharge measurements are compiled and maintained in a data base. Each measurement is carefully made, and undergoes quality assurance review. Some measurements indicate a temporary change in the rating, often due to a change in the streambed (for example, erosion or deposition) or growth of riparian vegetation. Such changes are called shifts; they may indicate a short- or long-term change in the rating for the gage. In normal usage, the measured shifts (or corrections) are applied mathematically to a defined rating.
The tables being provided are shift corrected, incorporating the mathematical adjustments for ease of use by the user. The shift adjustments are applied to the individual ratings as measured data becomes available, resulting in an adjusted rating. Some ratings may change as often as weekly, others may not change for months or years.
Stephen F. Blanchard /signed/
Chief, Office of Surface Water
District and Regional SW Specialists,
District Database Administrators;
Cc: A, B, FO