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Changing meteorological conditions also strongly affect seepage patterns in surface-water beds, especially near the shoreline, The water table commonly intersects land surface at the shoreline, resulting in no unsaturated zone at this point. Infiltrating precipitation passes rapidly through a thin unsaturated zone adjacent to the shoreline, which causes water-table mounds to form quickly adjacent to the surface water (Figure 6), This process, termed focused recharge, can result in increased ground-water inflow to surface-water bodies, or it can cause inflow to surface-water bodies that normally have seepage to ground water, Each precipitation event has the potential to cause this highly transient flow condition near shorelines as well as at depressions in uplands (Figure 6), Surface water do s\lr'ace LOn Water table Water table d before recharge _ folloWing focuse ........ ....-------::.__ _ _ _ - - - -- _c......... rech8r~_::::"_--- ------ Figure 6, Ground-water recharge commonly is focused initially where the unsaturated zone is relatively thin at the edges of surface-water bodies and beneath depressions in the land surface, Transpiration by nearshore plants has the opposite effect of focused recharge, Again, because the water table is near land surface at edges of surface-water bodies, plant roots can penetrate into the saturated zone, allowing the plants to transpire water directly from the ground- water system (Figure 7), Transpiration of ground water commonly results in a drawdown of the water table much like the effect of a pumped well, This highly variable daily and seasonaltranspira- tion of ground water may significantly reduce ground-water discharge to a surface-water body or even cause movement of surface water into the subsurface, In many places it is possible to measure diurnal changes in the direction of flow during seasons of active plant growth; that is, ground water moves into the surface water during the night, and surface water moves into shallow ground water during the day, These periodic changes in the direction of flow also take place on longer time scales: focused recharge from precipitation predominates during wet periods and drawdown by transpiration predominates during dry periods, As a result, the two processes, together with the geologic controls on seepage distribution, can cause flow conditions at the edges of surface-water bodies to be extremely variable, These "edge effects" prob- ably affect small surface-water bodies more than large surface-water bodies because the ratio of edge length to total volume is greater for small water bodies than it is for large ones, Surface water -- " d. su",ace \..afl -- -- -- -- -- -- ---\ Water table during dormant season \ Water table during growing season Figure 7, Where the depth to the water table is small adjacent to surface-water bodies, transpiration directly from ground water can cause cones of depression similar to those caused by pumping wells, This some- times draws water directly from the surface water into the subsurface, t Phreatophytes along the Rio Grande in Texas. (Photograph by Michael Collier.) 8