Introduction Small watersheds have been the focus of research on the hydrology and biogeochemistry of various types of land-use landscapes, including agricultural, forested, urban, and natural (Hewlett et al. 1969; Winter 1997; McCammon et al. 1998; Baedecker and Friedman 2000). Small water- sheds commonly are selected for intensive research because they are believed to be areas that are well defined by the watershed boundary. By being hydrologically well defined, an area lends itself to detailed accounting of gains, losses, and storage of water and solutes. The need to examine watershed processes in order to understand flow and transport in surface waters has long been recognized (Leopold 1974; Hynes 1975; Likens 1984). Some detailed studies of surface waters that included an accounting of water and solutes were in basins that had negligible contributions from or losses to ground water (Likens et al. 1967; Schindler et al. 1976; Hemond 1980). In other cases, although contribution of water from ground water was small, such contribution delivered sub- stantial solutes (Schwartz and Gallup 1978; Hurley et al. 1985). Subsequent examination of watershed processes in a variety of landscapes has shown surface water can receive considerable water and solute contributions from ground water (LaBaugh et al. 1995; Moore 1999; Holmes 2000). Thus, there is growing recognition that watershed process studies should include examination of ground water (Goodrich and Woolhiser 1993). A watershed is the topographic demarcation that defines a surface water drainage basin, and by doing so, separates surface drainage basins from one another. Tradi- tionally, the term “watershed” is used with respect to sur- face water. Ground water hydrologists use the term less often because they traditionally deal with aquifer systems. However, ground water systems also have watersheds, at flow-system divides. Although it would be convenient for water- and chemical-budget studies of watersheds if ground water divides underlay surface divides, they commonly do not. This is especially true for small watersheds. Further- more, as natural resource managers move increasingly toward managing water supply, water quality, and other Abstract Surface water and ground water watersheds commonly do not coincide. This condition is particularly relevant to understanding biogeochemical processes in small watersheds, where detailed accounting of water and solute fluxes commonly are done. Ground water watersheds are not as easily defined as surface watersheds because (1) they are not observable from land surface; (2) ground water flow systems of different magnitude can be superimposed on one another; and (3) ground water divides may move in response to dynamic recharge and discharge conditions. Field studies of relatively permeable terrain in Wisconsin, Minnesota, and Nebraska indicate that lakes and wetlands in small watersheds located near the lower end of extensive ground water flow systems receive ground water inflow from shallow flow systems that extend far beyond their surface watershed, and they may also receive ground water inflow from deeper regional flow systems that pass at depth beneath local flow systems. Field studies of mountain- ous terrain that have low-permeability deposits in New Hampshire and Costa Rica also indicate that surface water bodies receive ground water inflow from sources beyond their local surface watersheds. Field studies of lakes and wetlands in North Dakota, Nebraska, and Germany indicate that ground water divides move in response to changing climate conditions, resulting in a variable source of ground water inflow to those surface water bodies. 989 Where Does the Ground Water in Small Watersheds Come From? by Thomas C. Winter 1 , Donald O. Rosenberry 1 , and James W. LaBaugh 2 1 U.S. Geological Survey, Box 25046, Denver Federal Center, Denver, CO 80225 2 U.S. Geological Survey, Mail Stop 411 National Center, 12201 Sunrise Valley Dr., Reston, VA 20192 Published in 2003 by the National Ground Water Association. Vol. 41, No. 7—GROUND WATER—Watersheds Issue 2003 (pages 989–1000)