In every case of groundwater development some water must be removed from storage in order to create a cone of depression that will create a local head gradient adjacent to the well that causes groundwater to flow into the well. However, commonly the intent is to end up with a system that will persist through time (i.e., where the pumping will be sustainable indefinitely, or at least can be maintained for some acceptably long period of time). Development that can persist through time is one in which eventually storage is no longer being depleted and pumping is balanced by capture:
|∆V/∆t = 0||(4)|
In this new equilibrium (or dynamic equilibrium) pumping is balanced entirely by capture. From this perspective the questions for an investigator or water manager are:
- What are the changes in recharge and/or discharge brought on by the pumping – what is the capture?
- Can the system that now includes the pumping reach a new equilibrium in which storage is no longer being depleted?
- If a new equilibrium can be reached, how long will that take to occur?
- If a new equilibrium cannot be achieved, for how long can the pumping be maintained?
- Is the capture acceptable from a water (and environmental) policy and management perspective?
The answers to these questions depend upon the particulars of the system being considered. For example, where pumping is situated relative to discharge often makes a difference – both in the feasibility of reaching a new equilibrium and in how long it takes. It makes a big difference where wells are located in a groundwater system.
At early times after a well starts to pump and the cone of depression begins to develop, the pumped water is derived exclusively from a reduction (or depletion) in the volume of water stored in the aquifer. As the cone of depression spreads out and begins to affect aquifer boundaries, more and more of the water pumped from the well will be balanced by capture (Figure 4). This time dependence is a key to understanding and predicting the effects of groundwater development. The time scale for the response curves depends on the hydraulic properties of the aquifer and the distance of the well from recharge and discharge locations. But as noted by Bredehoeft and Alley (2014), this transition period can be very long – perhaps years to decades to centuries – especially if the system under development is dominated by water-table conditions with large storativity. The pumping rate itself does not affect the relative (or fractional) responses shown in Figure 4, but will be proportional to the actual volumetric magnitude of the effects. A quantitative method to estimate the timing of capture is presented in detail in the section of this book titled “Estimating the Magnitude and Timing of Streamflow Depletion.”