USGS_Streamgages_NHD_Location

                                http://answers.usgs.gov/cgi-bin/gsanswers
                              

                                The positional accuracy of the NHD locations is very good. Most locations were reviewed by USGS Water Science Center personnel who knew the precise locations for the vast majority of all the streamgages. These reviewed locations have the REVIEWED field set to 'Y', which stands for "yes, these locations have been reviewed." In general, the positional accuracy of the active streamgage location and streamgages that were active in the past 30 years could be considered slightly more accurate then the older historical streamgages.

        Most NHD locations were reviewed by USGS Water Science Center personnel. The NHD locations of 17,482 are within a few meters of the delineated stream reach, since they were automatically snapped to the nearest latitude/longitude vertices in the NHD arc. The remaining 4,271 NHD locations were manually adjusted during the review and moved to the correct locations. Although the new locations were not snapped exactly to NHD vertices, reviewers were instructed to place the point so that it would be close enough to snap to the proper position on the NHD.

        The NHD locations not reviewed by USGS Water Science Center personnel are also included. These locations have the REVIEWED field set to 'N' which stands for "not reviewed. These locations were snapped to the NHD in an automated process, but no review has been done to ensure they snapped to the proper NHD reach. A total of 2,042 gages in the dataset were not reviewed."
                              

                                Point locations and hydrologic data
                              

                                Creating the streamgage dataset:
        The NWISWeb database was used exclusively to create the initial streamgage dataset. NWISWeb consolidates all the needed data from all USGS Water Science Center (WSC) NWIS databases in one place, making it easy to use without having to query each WSC's NWIS database individually. NWISWeb only contains information from NWIS db_01. Stations in other NWIS databases were not considered and are not part of the streamgage dataset.

        There are a total of 25,768 stations in the streamgage dataset for areas that are covered by NHD, which are, the conterminous US, plus Hawaii and Puerto Rico. Stations in Alaska, US Virgin Islands, Canada, and other US territories are not included because these areas do not have NHD coverage, at the time of the indexing.
        The following outlines how the streamgage dataset was constructed:
        1.	Retrieved all daily-values (DV) for the entire period of record from the NWISWeb database on April 20, 2004. The results are in 48 Water Science Center-specific files.
        2.	Each district DV file was examined. Stations containing at least 1 day of data associated with one or more of the following parameters were flagged:
         -	parm = 00060 - Streamflow, ft/s
        	-	parm = 00065 - Gage height, ft
        	-	parm = 00054 - Reservoir storage, acre-ft
        	-	parm = 00053 - Surface Area, acres
        	-	parm = 00055 - Stream velocity, ft/s
        	-	parm = 00059 - Streamflow rate, gal/min
        	-	parm = 00062 - Reservoir elevation, ft
        	-	parm = 50042 - Discharge, gallons per minute
        	-	parm = 50051 - Flow rate, instantaneous, million gallons per day
        	-	parm = 72022 - Reservoir contents, M/gal
        	-	parm = 99020 - Elevation above NGVD 1929, ft
        	-	parm = 99060 - Streamflow, m/s
        	-	parm = 99065 - Gage height, m

        3.	The flagged stations were appended and duplicate stations removed.
        4.	Stations with data in water-years 2003 or 2004 were identified as "active". (Not all daily-values for the current water-year (2004) are in the database, and that is why water-year 2003 data was also considered. This means a few stations that were recently discontinued will be incorrectly flagged as "active".)
        5.	All 25,768 stations in the streamgage dataset were batch indexed using the US Environmental Protection Agency's (EPA) medium-resolution NHD database.
        6.	EPA provided the results of the indexing, containing a latitude/longitude coordinates of point location on the NHD stream that is associated with each station; known as an "event point". This "event point" and the latitude/longitude coordinate of the station were converted to the Albers Equal Area projection so a distance, in meters, between the station and the "event point" could be computed and added as an attribute.
        7.	The final streamgage dataset is in geographic coordinates using the NAD83 datum.

        Water Science Center (WSC) personnel then reviewed the batch-indexing results using a provided streamgage dataset and GIS/ArcMap custom application. This process not only tied USGS streamgages to the NHD, but also represents an important opportunity to use an efficient process to correct station location errors in NWIS.

        After the review was completed, the results were then consolidated creating this product. The reviewers examined 23,945 streamgages and found
        -	17,482 to be batch indexed correctly and the event point was not moved. ~ 73%.
        -	962 gages were flagged as "Do Not Use" and removed from this dataset, gages that do not contain any meaningful information.  ~ 4%
        -	1380 gages were flagged as "No 100k NHD Reach" and removed from this dataset ~ 5%
        -	4271 gages have modified/moved event points - moved, on average, 795 meters. ~ 18%
        -	4111 gages have modified/moved site locations (gage house) - moved, on average, 1123 meters ~ 17%

        8. The non-reviewed streamgages were added to the final dataset and these streamgages were flagged as "not reviewed" and the reviewed streamgages flagged as "reviewed".

        9. The streamgages that were not included in the initial streamgages database but had streamflow characteristics were also added to the final streamgage database. The nearest NHD reach was automatically determined and the nearest point on the nearest NHD reach was calculated. This nearest NHD point became the NHD location. These streamgages have not been reviewed.

        The final streamgage database contains the location of 23,426 streamgages. Started with 25,768 streamgages, 962 were removed because they were identified during the review as "do not use". Another 1380 streamgages were removed because they were located on streams not delineated on the 100K NHD. Of the total 23,426 streamgages, 21,384 were reviewed and 2,042 were not. 


        Estimating the flow characteristics:

        1. Daily streamflow data for all streamgages were copied from the files located in the subdirectories of nadww01.er.usgs.gov:/www/htdocs/nwisweb/data/discharge on June 15, 2005.

        2. The Perl program qstats.pl (Xiaodong Jian, USGS, written commun., 2005) was used to compute the time period and flow statistics for each streamgage.

        3. Base-flow index estimation:

        A base-flow index (BFI) was computed for the streamgages using a Fortran program written by Tony Wahl (Bureau of Reclamation. U.S. Department of the Interior) and Ken Wahl (U.S. Geological Survey) (http://www.usbr.gov/pmts/hydraulics_lab/twahl/bfi/index.html). The computer program name is bfi41.f and was the most current version available at the time. The computer program was compiled on a Sun Unix system and run with the default parameter values. 

        The BFI Web page (http://www.usbr.gov/pmts/hydraulics_lab/twahl/bfi/index.html) states:

        "The BFI program was developed to make the base-flow separation process less tedious and more objective. The program implements a deterministic procedure proposed in 1980 by the British Institute of Hydrology. The method combines a local minimums approach with a recession slope test. The program estimates the annual base-flow volume of unregulated rivers and streams and computes an annual base-flow index (BFI, the ratio of base flow to total flow volume for a given year) for multiple years of data at one or more gage sites. Although the method may not yield the true base flow as might be determined by a more sophisticated analysis, the index has been found to be consistent and indicative of base flow, and thus may be useful for analysis of long term base-flow trends. Users should be very cautious about using methods such as this for short-term storm events or for locations where streamflow is affected by upstream regulation, such as reservoir releases. In general, the method interprets most regulated releases as base flow. If the program is used for regulated streams, the effects of regulation must be carefully accounted for through manual adjustment of the program output."

        BFI references:

        Wahl, K.L., and Wahl, T.L., 1995, Determining the flow of Comal Springs at New Braunfels, Texas, In Proceedings of Texas Water '95, American Society of Civil Engineers, August 16-17, 1995 San Antonio, Texas: American Society of Civil Engineers,  p. 77-86, information available on the World Wide Web, accessed March 25, 2003 , at URL http://www.usbr.gov/pmts/hydraulics_lab/twahl/bfi/texaswater95/comalsprings.html

        Wahl, K.L., and Wahl, T.L., 1988, Effects of regional ground-water declines on streamflows in the Oklahoma Panhandle, In Proceedings of Symposium on Water-Use Data for Water Resources Management: Tucson, Arizona, American Water Resources Association, p. 239-249, information available on the World Wide Web, accessed March 25, 2003, at URL http://www.usbr.gov/pmts/hydraulics_lab/twahl/bfi/bfi_beaver_river.pdf

        The use of firm, trade, and brand names is for identification purposes only and does not constitute endorsement by the U.S. government.
                              

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                                Dataset copied.
                              

                                This dataset was subsetted (clipped) from the original nationwide dataset.  It was clipped to a California-only extent using the outer boundary of a California 24K county boundary dataset.  It has also been projected to California Teale Albers NAD83, and one field name was renamed (NWISWEB to LINK) for compatiblity with California Department of Fish and Game online map viewers. Portions of this metadata file may refer to the original nationwide dataset.