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<br /> <br />B <br /> <br />The Ground-Water Component <br />of Streamflow <br /> <br />Ground water contributes to streams in most physio- <br />graphic and climatic settings. Even in settings where streams <br />are primarily losing water to ground water, certain reaches <br />may receive ground~water inflow during some seasons. The <br />proportion of stream water that is derived from ground-water <br />inflow varies across physiographic and climatic settings, The <br />amount of water that ground water contributes to streams can <br />be estimated by analyzing streamflow hydrographs to deter- <br />mine the ground-water component, which is termed base flow <br />(Figure B-1), Several different methods of analyzing hydro- <br />graphs have been used by hydrologists to determine the <br />base-flow component of streamflow, <br />One of the methods, which provides a conservative <br />estimate of base flow, was used to determine the ground- <br />water contribution to streamflow in 24 regions in the contermi- <br />nous United States, The regions, delineated on the basis of <br />physiography and climate, are believed to have common <br />characteristics with respect to the interactions of ground <br />water and surface water (Figure B-2), Fifly-four streams <br />were selected for the analysis, at least two in each of the <br /> <br />Figure 8-1. The ground-water compo- <br />nent of streamflow was estimated <br />from a streamflow hydrograph for the <br />Homochitto River in Mississippi, using <br />a method developed by the institute of <br />Hydrology, United Kingdom. (Institute <br />of Hydrology, f 980, Low flow studies: <br />Wallingford, axon, United Kingdom, <br />Research Report No, 1.) <br /> <br /> <br />100,000 <br /> <br />10,000 <br /> <br />" <br />z <br />o <br />u <br />w <br />"' <br />a: 1,000 <br />w <br />0- <br />>- <br />W <br />W <br />~ <br />u <br />iii <br />a 100 <br />~ <br />'i <br />o <br />~ <br />~ <br /> <br />10 <br /> <br />24 regions, Streams were selected that had drainage basins <br />less than 250 square miles and that had less than 3 percent <br />of the drainage area covered by lakes and wetlands, Daily <br />streamflow values for the 30-year period, 1961-1990, were <br />used for the analysis of each stream, The analysis indicated <br />that, for the 54 streams over the 30-year period, an average <br />of 52 percent of the streamflow was contributed by ground <br />water. Ground-water contributions ranged from 14 percent <br />to 90 percent, and the median was 55 percent. The ground- <br />water contribution to streamflow for selected streams can <br />be compared in Figure B-2, As an example of the effect <br />that geologic setting has on the contribution of ground water <br />to streamflow, the Forest River in North Dakota can be <br />compared to the Sturgeon River in Michigan, The Forest <br />River Basin is underlain by poorly permeable silt and clay <br />deposits, and only about 14 percent of its average annual <br />flow is contributed by ground water; in contrast, the Sturgeon <br />River Basin is underlain by highly permeable sand and gravel, <br />and about gO percent of its average annual flow is contributed <br />by ground water, <br /> <br />Total streamflow <br />Base flow <br /> <br />, . ~~ <br /> <br />~. A <br /> <br /> <br />, <br />, 2'416'81101121'4'16118120'221W~1281~'~lMl~1 <br />TIME, IN DAYS <br /> <br />12 <br />