In Reply Refer To:
Mail Stop 415
January 29, 2013

OFFICE OF SURFACE WATER TECHNICAL MEMORANDUM 2013.02

SUBJECT: Requirement for 90-day notification period prior to gage datum changes at streamgages

Policy Summary: Prior to any change to the gage datum at U.S. Geological Survey (USGS) streamgages, the responsible USGS Water Science Center (WSC) is required to give notice to the appropriate streamgage stakeholders and cooperators, including the National Weather Service (NWS),at least 90 days prior to making the datum change. Among those to be notified at NWS include the appropriate NWS River Forecast Center (RFC) and NWS Weather Field Office (WFO). Exceptions to the 90-day notification time period will be considered upon appeal to the Chief of the Office of Surface Water.

Background: Gage datum is defined as the zero elevation reference plane at a streamgage to which all gages are set. Changes to the gage datum are sometimes made at USGS streamgages. Often times, the gage datum is lowered to prevent reporting of negative stage values. Although the USGS software packages NWIS and NWISWeb can process and accurately display negative values without problem, datum changes to eliminate negative values may be a matter of preference by the WSC, particularly to avoid confusing public users. At NWS forecast locations, however, a change in gage datum likely requires NWS to change the numerical value for flood stage or other public warning trigger points. NWS is required to give the public a 75-day notice prior to making the changes.

OSW Technical Memorandum 2013.02 is intended to allow NWS adequate time to post its public notices on changes to various trigger point stages in their forecast products.

NWIS Tracking of Datum Changes: Note that changes to gage datum can now be tracked in NWIS. When the current datum is updated in the NWIS Site File the historical datum should be entered into the NWIS datum history table as a separate step. See http://nwis.usgs.gov/nwisdocs5_0/gw/gwcoding_Sect2-9.pdf for details.

/signed/

Robert R. Mason, Jr.
Acting Chief, Office of Surface Water

Distribution: GS-W All