David M. Wolock 2003 Unknown raster digital data U.S. Geological Survey Open-File Report 03-311 Reston, VA U.S. Geological Survey http://water.usgs.gov/lookup/getspatial?rech48grd This 1-kilometer resolution raster (grid) dataset is an index of mean annual natural ground-water recharge. The dataset was created by multiplying a grid of base-flow index (BFI) values by a grid of mean annual runoff values derived from a 1951-80 mean annual runoff contour map. Mean annual runoff is long-term average streamflow expressed on a per-unit-area basis. The concept used to construct the dataset is based on two assumptions: (1) long-term average natural ground-water recharge is equal to long-term average natural ground-water discharge to streams, and (2) the base-flow index reasonably represents, over the long term, the percentage of natural ground-water discharge in streamflow. The mean annual natural ground-water recharge grid was developed for use in the U.S. Geological Survey's National Water-Quality Assessment (NAWQA) Program (Gilliom and others, 1995). The dataset was produced to help NAWQA national synthesis teams interpret relations between land use and water quality in ground water. Purpose References: Gilliom, R.J., Alley, W.M., and Gurtz, M.E., 1995, Design of the National Water-Quality Monitoring Program--occurrence and distribution of water-quality conditions: U.S. Geological Survey Circular 1112, 33 p., available on the World Wide Web, accessed July 7, 2003, at URL http://water.usgs.gov/pubs/circ/circ1112/ Background on the base-flow index: The raster dataset of estimated mean annual natural ground-water recharge was created by multiplying a grid of base-flow index (BFI) values (Wolock, 2003b) by a grid of mean annual runoff values (Gebert and others, 1987). This approach is similar to those reported in some other studies (Holtschlag, 1997; Rutledge, 1998; Dumouchelle and Schiefer, 2002). Base flow is the component of streamflow that can be attributed to ground-water discharge into streams. The BFI is the ratio of base flow to total flow, expressed as a percentage. The BFI grid used to compute ground-water recharge was interpolated from BFI point values estimated for U.S. Geological Survey streamgages (Wolock, 2003a). These point values were computed using an automated hydrograph separation computer program called the BFI program (Wahl and Wahl, 1988; 1995). Regarding this computer program, 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 (Institute of Hydrology, 1980). 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." A subset of the streamgage BFI values was selected before the interpolation process was used to generate the BFI grid. The criteria for including a streamgage in the interpolation were (1) a period of record of at least 10 years of daily streamflow data, and (2) a maximum drainage basin area of 1,000 square miles (2,590 square kilometers). The first criterion selects streamgages with a reasonably long period of record, thereby averaging year-to-year variability in BFI values. The second criterion minimizes the effects of routing within the stream network on BFI values. Applying these selection criteria resulted in a point dataset of 8,249 streamgage BFI values. The mean period of record in the dataset was 33 years, and the mean drainage basin area was 204 square miles (528 square kilometers). The point dataset of streamgage BFI values was interpolated to a raster dataset using the ArcInfo inverse distance weighting interpolation method (ESRI, 2000). (The use of firm, trade, and brand names is for identification purposes only and does not constitute endorsement by the U.S. Geological Survey.) The grid-cell size was set to 1 kilometer, and default interpolation parameter values were used except for the "power" parameter, which was set to a value of 0.5. The "power" parameter controls the significance of surrounding points on the interpolated value; a higher value results in less effect from distant points. Using a "power" value of 0.5 produced a smooth interpolated grid that performed the best in statistical tests (Wolock, 2003b). Supplemental Information References: Dumouchelle, D.D., and Schiefer, M.C., 2002, Use of streamflow records and basin characteristics to estimate ground-water recharge rates in Ohio: Columbus, Ohio, Ohio Department of Natural Resources, Division of Water, Bulletin 46, 45 p. ESRI, 2000, ArcInfo, version 8.0.2: Redlands, CA, Environmental Systems Research Institute, variously paged. Gebert, W.A., Graczyk, D.J., and Krug, W.R., 1987, Average annual runoff in the United States, 1951-80: U.S. Geological Survey Hydrologic Investigations Atlas HA-710, 1 sheet, scale 1:7,500,000. Holtschlag, D.J., 1997, A generalized estimate of ground-water recharge rates in the Lower Peninsula of Michigan: U.S. Geological Survey Water-Supply Paper 2437, 37 p. Institute of Hydrology, 1980, Low flow studies: Wallingford, United Kingdom, Institute of Hydrology, Research Report 1, variously paged. Rutledge, A.T., 1998, Computer programs for describing the recession of ground-water discharge and for estimating mean ground-water recharge and discharge for streamflow records--update: U.S. Geological Survey Water-Resources Investigations Report 98-4148, 43 p. 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 Wahl, K.L., and Wahl, T.L., 1995, Determining the flow of Comal Springs at New Braunfels, Texas, in Proceedings of Texas Water '95, 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 Wolock, D.M., 2003a, Flow characteristics at U.S. Geological Survey streamgages in the conterminous United States: U.S. Geological Survey Open-File Report 03-146, digital dataset, available on the World Wide Web, accessed June 30, 2003, at URL http://water.usgs.gov/lookup/getspatial?qsitesdd Wolock, D.M., 2003b, Base-flow index grid for the conterminous United States: U.S. Geological Survey Open-File Report 03-263, digital dataset, available on the World Wide Web, accessed July 8, 2003, at URL http://water.usgs.gov/lookup/getspatial?bfi48grd 1951 unknown 1980 unknown ground condition Complete None planned -127.887748 -65.346810 51.608329 22.860749 None Ground-water recharge Base-flow index inlandWaters None Conterminous United States None The mean annual natural ground-water recharge dataset was generated by multiplying a grid of base-flow index (BFI) values (Wolock, 2003) by a grid of mean annual runoff values (Gebert and others, 1987). Mean annual runoff is long-term average streamflow expressed on a per-unit-area basis. Natural recharge estimated in this way is very uncertain. The sources of uncertainty in the following list should be carefully considered before the dataset is used. 1. The approach used to create the natural recharge dataset is based on two main assumptions: (1) long-term average natural ground-water recharge is equal to long-term average natural ground-water discharge to streams, and (2) the BFI reasonably represents, over the long term, the percentage of ground-water discharge in streamflow. The extent to which these assumptions are valid determines, in part, the degree to which the mean annual natural recharge estimates can be considered reasonable. Users of the dataset should assess whether these assumptions are valid on the basis of knowledge of the local hydrologic system. Qualifications regarding the first assumption (ground-water recharge and discharge to streams are equal) that should be considered include the following: a. The natural recharge dataset is likely to underestimate "true" natural recharge in areas where ground-water evapotranspiration or near-stream ground-water pumping is significant. Ground-water evapotranspiration and near-stream ground-water pumping reduce ground-water storage and, thereby, also can reduce ground-water discharge to streams. The net result is that recharge to ground water will exceed the discharge of ground water to streams. Ground-water evapotranspiration can be high in arid regions. b. Ground-water discharge to streams does not occur in "losing" streams, which by definition "lose" water to the local ground-water system instead of "gaining" water from the ground-water system. Losing streams are more common in arid regions than in humid regions. c. Ground-water discharge to small streams will be less than total ground-water recharge if some of the recharge flows to deep, regional ground-water systems. Qualifications regarding the second assumption (the BFI reasonably estimates the percentage of ground-water discharge in streamflow) include: a. The BFI may be higher than the true percentage of ground-water discharge in streamflow for snowmelt-dominated streams. Snowmelt generally occurs gradually over time. Whether it enters the stream as ground-water discharge or overland flow cannot be distinguished by the BFI hydrograph separation technique. b. The BFI may be higher than the true percentage of ground-water discharge in streamflow for regulated streams because regulation dampens rapid temporal changes in the hydrograph (see Supplemental_Information). About one-quarter of the streamgages used to make the BFI grid have been identified as being regulated. The BFI values for these streamgages, however, were not different from the BFI values for streamgages identified as being unregulated. 2. Natural recharge may be only a small component of total recharge. Irrigation can be a significant component of recharge to ground water that greatly exceeds natural recharge. 3. The two grids (the base-flow index and runoff grids) multiplied by each other to make the natural recharge grid are highly generalized in space. The lack of spatial detail in these grids is reflected in the natural recharge grid. Although the natural recharge dataset likely reflects general patterns across broad geographic regions, recharge values at specific sites are unlikely to be accurate. 4. The two grids (the base-flow index and runoff grids) multiplied by each other to make the natural recharge grid are highly generalized over time. The runoff grid represents the 1951-80 mean annual runoff, and the base-flow index grid is interpolated from streamgages with an average record length of 33 years. The mean annual natural ground-water recharge values, therefore, are also long-term average estimates. Additional information about the accuracy of the base-flow index is given in the Supplemental_Information section and in Wolock (2003). Use Constraints References: Gebert, W.A., Graczyk, D.J., and Krug, W.R., 1987, Average annual runoff in the United States, 1951-80: U.S. Geological Survey Hydrologic Investigations Atlas HA-710, 1 sheet, scale 1:7,500,000. Wolock, D.M., 2003, Base-flow index grid for the conterminous United States: U.S. Geological Survey Open-File Report 03-263, digital dataset, available on the World Wide Web, accessed July 8, 2003, at URL http://water.usgs.gov/lookup/getspatial?bfi48grd David M. Wolock U.S. Geological Survey Research Hydrologist mailing and physical address

4821 Quail Crest Place

Lawrence KS 66049 USA 1-888-275-8747 785-832-3500 dwolock@usgs.gov http://water.usgs.gov/GIS/browse/reach48grd.jpg Graphic representative of the estimated mean annual natural groundwater recharge in the conterminous United States. JPG None Unclassified None Microsoft Windows 2000 Version 5.1 (Build 2600) Service Pack 1; ESRI ArcCatalog 8.3.0.800 None None David M. Wolock 2003 Unknown raster digital data U.S. Geological Survey Open-File Report 03-263 Reston, VA U.S. Geological Survey http://water.usgs.gov/lookup/getspatial?bfi48grd online 2003 unknown publication date bfi48grd The grid of base-flow index values was multiplied by the grid of mean annual runoff values to estimate natural ground-water recharge. Warren A. Gebert 1987 Unknown vector digital data U.S. Geological Survey Hydrologic Investigations Atlas HA-710 Madison, MI U.S. Geological Survey http://water.usgs.gov/lookup/getspatial?runoff online 1951 unknown 1980 unknown ground condition runoff The contour map of 1951-80 mean annual runoff for the conterminous United States was interpolated to produce a grid of mean annual runoff values. The grid of base-flow index values then was multiplied by the grid of mean annual runoff values to estimate natural ground-water recharge. The digital 1951-80 mean annual runoff contour map (Gebert and others, 1987) was interpolated to produce a 1-kilometer resolution grid of 1951-80 mean annual runoff. This step was accomplished using the ArcInfo TOPOGRID command (ESRI, 2000). (The use of firm, trade, and brand names is for identification purposes only and does not constitute endorsement by the U.S. Geological Survey.) The DATATYPE subcommand argument was set to CONTOUR, the CONTOUR subcommand argument was set to the runoff line coverage name, and the ENFORCE subcommand argument was set to OFF. The mean annual runoff contour map (Gebert and others, 1987) was produced from mean annual runoff computed at U.S. Geological Survey streamgages. Gebert and others (1987) selected 5,951 streamgages to represent tributary flow free from effects of upstream reservoirs or diversions. The mean annual flow was divided by the drainage basin area for each streamgage to compute mean annual runoff. The mean annual runoff value then was plotted at the center of the drainage basin of each streamgage, and contour lines of equal mean annual runoff values were drawn. Process Description References: ESRI, 2000, ArcInfo, version 8.0.2: Redlands, CA, Environmental Systems Research Institute, variously paged. Gebert, W.A., Graczyk, D.J., and Krug, W.R., 1987, Average annual runoff in the United States, 1951-80: U.S. Geological Survey Hydrologic Investigations Atlas HA-710, 1 sheet, scale 1:7,500,000. runoff 2003 runoffgrd David M. Wolock U.S. Geological Survey Research Hydrologist mailing and physical address

4821 Quail Crest Place

Lawrence KS 66049 USA 1-888-275-8747 785-832-3500 dwolock@usgs.gov The mean annual natural ground-water recharge grid was generated by multiplying the base-flow index grid by the runoff grid and dividing by 100. Division by 100 is required because the base-flow index grid units are percentages. runoff bfi48grd 2003 rech48grd David M. Wolock U.S. Geological Survey Research Hydrologist mailing and physical address

4821 Quail Crest Place

Lawrence KS 66209 USA 1-888-275-8747 785-832-3500 dwolock@usgs.gov Raster Grid Cell 2905 4614 1 Albers Conical Equal Area 29.500000 45.500000 -96.000000 23.000000 0.000000 0.000000 row and column 1000.000000 1000.000000 meters North American Datum of 1983 Geodetic Reference System 80 6378137.000000 298.257222 rech48grd Mean annual natural ground-water recharge in millimeters per year Wolock ObjectID Internal feature number. ESRI Sequential unique whole numbers that are automatically generated. VALUE Mean annual natural ground-water recharge Wolock 0 2150 millimeters per year 1 COUNT Number of grid cells with corresponding "VALUE" ESRI 1 306332 Number of grid cells 1 grid cell The "VALUE" in the grid is the mean annual natural ground-water recharge value in millimeters per year. Wolock U.S. Geological Survey Ask USGS - Water Webserver Team mailing

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Reston VA 20192 1-888-275-8747 (1-888-ASK-USGS) http://answers.usgs.gov/cgi-bin/gsanswers?pemail=h2oteam&subject=GIS+Dataset+rech48grd Although this data set has been used by the U.S. Geological Survey, U.S. Department of the Interior, no warranty expressed or implied is made by the U.S. Geological Survey as to the accuracy of the data and related materials. The act of distribution shall not constitute any such warranty, and no responsibility is assumed by the U.S. Geological Survey in the use of this data, software, or related materials. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government. Winzip Full coverage zipped 3.540 meg http://water.usgs.gov/GIS/dsdl/rech48grd.zip tar gunzip Full coverage zipped 2,958 http://water.usgs.gov/GIS/dsdl/rech48grd.tgz tar compressed Full coverage zipped 2.958 http://water.usgs.gov/GIS/dsdl/rech48grd.tz None. This dataset is provided by USGS as a public service. 20041108 U.S. Geological Survey Ask USGS -- Water Webserver Team mailing

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Reston VA 20192 1-888-275-8747 (1-888-ASK-USGS) http://answers.usgs.gov/cgi-bin/gsanswers?pemail=h2oteam&subject=GIS+Dataset+rech48grd FGDC Content Standards for Digital Geospatial Metadata FGDC-STD-001-1998