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            <rdf:value>["Use Constraints: 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", "Access Constraints: None"]</rdf:value>
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