Groundwater is a critically valuable resource for water supply around the world. But the development of groundwater resources has consequences – its stock or reserves (i.e., the volume in storage in the aquifer) can be reduced and groundwater withdrawal can deplete surface-water flows and resources and have other environmental impacts. Specifically, new groundwater withdrawals through wells will have to be balanced by some combination of (1) a reduction in the volume of groundwater in storage in an aquifer, (2) an increase in recharge to the aquifer, and (3) a reduction in the discharge from the aquifer. The latter two constitute “capture,” and capture often manifests itself largely as a reduction in streamflow. The balancing between pumping and the three listed factors is dynamic, and it changes over time. If the groundwater storage depletion over time becomes negligible, then groundwater withdrawals are maintainable indefinitely (as long as other factors do not affect the aquifer’s water balance). However, even if groundwater withdrawals are maintainable from a hydrologic perspective, one must also consider its impacts on surface-water resources (and the timing of those impacts) because the changes in groundwater recharge and discharge (capture) may diminish those surface-water resources, and thereby potentially have economic, legal, political, and environmental consequences.
Historically, large-scale groundwater development and its subsequent effects (including storage depletion, capture, and land subsidence) have occurred in many areas prior to any recognition of its impacts or any consideration of its acceptability. Today, hydrogeologists have the knowledge and tools to understand and predict the magnitude and timing of these effects. This understanding and quantitative assessments can provide reliable scientific input to water managers and policy makers.