Humans bring another factor to the dynamic exchange of groundwater with the surface because they pump water from the subsurface using wells. Pumping lowers water levels near the well, causing water to flow out of storage and toward the well. This lowers water levels farther from the well, creating a cone shaped water level surface around the well. The difference of water level before and after pumping is called drawdown and the cone shaped water level surface is called a cone of depression, or a drawdown cone (Figure 26a and b). If pumping continues, the cone of depression grows until the inflows balance the volume pumped at the well. When the drawdown cone extends to a stream, stream water infiltrates through the stream bed into the groundwater system (Figure 26c).
Groundwater, unlike most other extractable resources, can be renewable. Thus, it is possible to develop a groundwater source that will last indefinitely, which is a highly desirable societal outcome. Groundwater, however, is a shared resource in which usage by one party may be highly beneficial to that party, but harmful to others and/or to the long‑term viability of the resource. Thus, societal management is necessary to prevent the “Tragedy of the Commons” (the situation where the collective action of individual users of a shared resource, acting in their own self‑interest, behave contrary to the common good of all users by depleting or spoiling the shared resource). The term sustainability has been proposed for such societal management. Sustainability in groundwater development is frequently interpreted as withdrawing less groundwater than is naturally recharged. This management approach is called safe yield. However, the expanding cone of groundwater depression can decrease existing evapotranspiration, induce infiltration from adjacent surface water, and capture water that would have discharged under natural conditions. If groundwater extraction is balanced by these sources, a new equilibrium is achieved and is referred to as sustained yield. Management based on sustained yield neglects the impact of extraction on the water resources that are connected to the system through the larger hydrologic cycle. A change to one part of the hydrologic cycle impacts other parts of the cycle and thus, may have cultural or legal implications.
Groundwater resources cannot be sustained if withdrawal of groundwater exceeds recharge to the groundwater system because groundwater levels will decline, and eventually the groundwater stored in the pores of aquifers and streams in the area will no longer be available. If the amount of water pumped exceeds the amount of recharge, the cone continues to expand and deepen until the aquifer is no longer a good source of water. An example of this is the excessive pumping of groundwater from the Ogallala aquifer in Kansas, United States. This withdrawal in excess of recharge is called groundwater depletion, groundwater mining, or groundwater overdraft. The overuse of groundwater in Kansas, caused water level declines such that streams in an area of about 20 million hectares went dry as shown in Figure 27.
Sometimes humans further complicate the hydrologic cycle by exporting water from one place to another. Occasionally this involves moving water through pipelines that transfers water from one drainage basin to another, but more often humans inadvertently move “virtual” water. Virtual water is the water contained in a product that arrives in one country but was grown, bottled or manufactured in another country, using water pumped from that country.