When a well is pumped, it introduces a new discharge from the system. As elucidated by Theis (1940), water to supply the well comes from (or is balanced by) three potential sources: (1) an increase in the recharge to the aquifer caused by the pumping, (2) a decrease in groundwater discharge from the aquifer caused by the pumping, and (3) a reduction in groundwater storage in the aquifer system, or some combination of the three. This principle simply means that water mass is conserved, and that additional (new) discharge by pumping must be balanced or compensated by changes in other elements of the aquifer’s water budget (as graphically illustrated in Figure 3). Based on the equivalency expressed in Equation 2, one can simplify the quantitative global water balance statement for the system (Equation 1) to:
|∆Rt – ∆Dt – ∆V/∆t = Qt||(3)|
We define a new quantity (∆Rt – ∆Dt) as the capture (Lohman et al., 1972). That is, capture encompasses the changes in recharge and discharge caused by drawdown and changes in hydraulic gradients resulting from pumping a well, and represents water “captured” by the well that otherwise would not have entered the groundwater system or otherwise would have discharged from the groundwater system naturally. When the change in the volume of water in storage, ∆V, is negative, the change represents a depletion of the volume (or mass) of groundwater stored in the aquifer. The balance depicted in Figure 3 holds regardless of whether the individual terms are all represented as rates or as cumulative volumes. It is important to distinguish capture from capture zone, which represents the three-dimensional, volumetric portion of a groundwater flow field that discharges to a well (Anderson, et al., 2015; Barlow et al., 2018). The capture zone may or may not include the area where capture is occurring.