The second mechanism that contributes to balancing of groundwater pumpage is capture – a combination of increased recharge and decreased discharge of groundwater (Lohman et al., 1972; Bredehoeft and Durbin, 2009; Leake, 2011; Barlow and Leake, 2012; and Barlow et al., 2018).
The decrease in groundwater discharge typically represents a reduction in the discharge to surface water, such as streams, lakes, wetlands, springs, drains, estuaries, and/or along coastlines. However, evapotranspiration is also a type of discharge from a groundwater system that also can be captured. In fact, in areas where surface water is sparse or absent, as in closed desert basins, the reduction in groundwater discharge (and capture) can primarily encompass a reduction in evapotranspirative losses from the groundwater system as the water table declines over time.
Capture also includes increases in recharge in response to pumping and water-level declines. For example, if a lowland area has a water table that lies at or immediately below the land surface, precipitation falling onto that surface cannot infiltrate to recharge the aquifer because no pore space is available to absorb it ‑‑ all the pore spaces are already fully saturated. This potential recharge is “rejected.” However, if the water table drops because of pumping, then future precipitation onto this same land surface area will be able to infiltrate the soil and recharge the groundwater system. Also, increases in recharge in response to pumping can occur where surface water features intersect aquifers. If drawdown due to pumping is so large as to reverse the hydraulic gradient, then where groundwater flow was formerly directed from the aquifer to the stream it now becomes directed from the stream into the aquifer. This seepage loss from the stream is usually termed “induced infiltration.”
But in general, capture typically is composed mostly of streamflow (or other surface water) depletion. In the United States, groundwater discharge represents from 15 to 90 percent of total annual streamflow ‑‑ about 50 percent on average (Winter et al., 1998). Thus, any reduction in groundwater discharge supporting streamflow can have serious detrimental consequences. Streamflow depletion is most often observed as a reduction in the base flow (or low flows) of streams. In the extreme, streams can go dry (Figure 8). Such streamflow depletion is of great concern for water‑supply managers, for those with senior rights to use surface water, and because of potential environmental impacts. In fact, streamflow depletion due to groundwater pumping has been the subject of several United States Supreme Court cases in recent years, whose decisions have clearly recognized the relation between groundwater and surface water (Alley and Alley, 2017).