Kansas Geological Survey, Open-File Rept. 96-1a
Proposed Management Areas--Page 7 of 16
Sustainability implies the attainment of a new dynamic equilibrium under conditions of widespread development. For equilibrium to occur withdrawals from the aquifer must induce either additional recharge to the aquifer, reduced discharge from the aquifer, or both (Figure 5). This occurs by increasing the hydraulic gradient into the aquifer when the hydraulic head within the aquifer is decreased. These decreases will continue until changes in recharge and discharge balance withdrawals from the aquifer. The most direct evidence of this new balance is long-term stability of hydraulic heads in the aquifer.
Figure 5. Changes in the water budget of a confined aquifer caused by water-resources development. The dashed line represents the trace of the potentiometric in the cross section. Under the steady-state, pre-development phase recharge (R) is balanced by discharge (D) from the aquifer with no change in storage. In the early stages of development the water produced by wells comes entirely from storage. Once the potentiometric surface has been lowered sufficiently to increase the hydraulic gradient near the recharde or discharge area, additional recharge (r) or reduced discharge (d) or both may occur, generating capture. In this later stage of development, the water produced by wells comes entirely from capture and there is no further loss of storage and a new equilibrium is created. The time required to generate significant capture depends on the diffusivity (the ratio of hydraulic conductivity to specific storage) of the aquifer.
The sustained yield depends on the rate at which the hydraulic head decreases propagate through the aquifer to the recharge or discharge area. The closer the pumping centers are placed to either the recharge or discharge areas, the more likely it is that additional recharge or reduced discharge can be realized by withdrawals (Figure 5). The rate of propagation is a function of aquifer diffusivity (the ratio of aquifer hydraulic conductivity and the specific storage). The higher the diffusivity, the faster the rate of propagation and the more likely it will be that pumping centers located farther away from either the recharge or the discharge areas will influence the amount of into and from the aquifer. Diffusivity values from near the central Kansas outcrop belt are generally higher and more variable than parameter values from southwestern Kansas and adjacent southeastern Colorado (Macfarlane, in review).
Water budgets are useful for examining the flow of water between the various compartments within the hydrologic cycle or a ground-water flow system. Under pre-development conditions within an aquifer, there is a dynamic equilibrium between recharge to and discharge from an aquifer (Figure 5A). There is also water in storage within the pores of the aquifer framework and under these conditions, the total volume in storage remains relatively constant.
With the introduction of pumping wells, the volume of water in each of these three categories changes depending on the location of the pumping wells in the aquifer with respect to sources of recharge and discharge, the rates of withdrawal, and the characteristics of the aquifer. All of the water withdrawn by a pumping well comes either from storage or capture (Figure 5B). Initially, all of the water produced by the well comes from storage in the aquifer adjacent to the well. As the well continues to pump, more and more of the aquifer experiences water-level declines. Given enough time, these declines propagate to either the recharge or the discharge area, or both, producing "capture" (Figure 5C). When this happens, additional recharge enters or discharge from the aquifer system is decreased, resulting in an additional source of water added to the aquifer to balance the withdrawal. Capture is the sum of the increase in recharge and the decrease in discharge that results from the pumping. As the well continues to pump, more and more of the water produced by the well is replaced by capture. The generation of capture by pumping does not create any "new" water, but is merely a reallocation of the amounts in the total hydrologic budget for a region. All other things being equal, the proportion of added recharge to or decreased discharge from the aquifer will depend on whether the well is located closer to the source of the recharge or the discharge area.
Eventually, a new dynamic equilibrium occurs when the amount of water withdrawn is balanced by capture. At this point, the hydraulic heads in the aquifer approach stability at levels that are lower than they were under pre-development steady-state conditions. Wells located within the aquifer discharge area may reduce the hydraulic head in the aquifer minimally because most of the water produced by the well may be natural discharge from the aquifer. Likewise, in the case where there are two hydraulically-connected aquifers and the well is located in the aquifer that naturally receives recharge from the other aquifer, there will be little change in hydraulic head due to pumping. This occurs because of the increase in leakage from the hydraulically-connected aquifer as pumping continues.
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