Streamflow depletion arises from both a reduction in the discharge of groundwater into a stream and an increase in recharge from the stream to the aquifer as gradients are reduced or reversed between the aquifer and the stream. Both constitute capture, and both result in a depletion of streamflow downstream from the capture area.
An idealized representation of the sequence of changes for stream‑aquifer interaction and its response to pumping is depicted in Figure 9. The simplified schematic illustration shows that under natural (predevelopment) conditions (Figure 9a) the average recharge to the saturated zone equals the average discharge to the stream (assuming no evapotranspiration from the water table and ignoring any short‑term variations in precipitation that may affect recharge rates or stream stage). After a well is drilled and begins pumping (Figure 9b), a cone of depression develops and water is removed from storage. The drawdown reduces the gradient towards the stream and flow from the aquifer to the stream (i.e., groundwater discharge) is decreased. After more time, in some cases the hydraulic gradient at the stream‑aquifer boundary may be reversed, which locally reverses the flow direction and induces water in the stream to flow into the aquifer, thereby increasing recharge (Figure 9c). When pumping becomes fully balanced by capture, the heads will have stabilized and there will be no additional drawdown or removal of water from storage. The time it takes for this to occur is called the “time to full capture” (Bredehoeft and Durbin, 2009). At this time, the system will have attained a new equilibrium condition, and the pumping rate will be sustainable (or maintainable) from a hydraulic perspective. However, the streamflow depletion may have detrimental effects on downstream users and ecosystems, and may not be acceptable. If the pumping is turned off (Figure 9d), this sequence is reversed and groundwater levels begin to recover, and groundwater discharge to the stream increases. Given sufficient time, groundwater heads may return to their original levels, and recharge and discharge in the system will again achieve a long‑term equilibrium condition (Figure 9e).
The capture and streamflow depletion can be manifested in several ways. During low‑flow periods in a stream, the groundwater discharge constitutes a larger fraction of the total streamflow than during high‑flow periods, so a given reduction in groundwater discharge would be more easily detectable. So, where groundwater pumping and storage depletion is affecting streamflow, we would expect to see the clearest signal in the low‑flow records for the stream. For example, this is indeed evident in the records of a stream gage on the Sunflower River, Mississippi, USA, located within the area of the heavily pumped Mississippi Embayment regional aquifer system (Figure 10). The data show a significant decline in the minimum daily mean flow for each year shortly after groundwater use and storage depletion noticeably accelerated.
Another type of low‑flow characteristic is how often (or for how long) a stream goes dry. If streamflow depletion is affecting the flow of the stream, then the frequency of days during which there is no flow in the stream might increase or the lengths of dry stream reaches might increase. The Cache River in northeastern Arkansas, USA, also lies within the boundaries of the Mississippi Embayment aquifer, and the number of zero‑flow days first increased after 1980, shortly after groundwater depletion had increased (Figure 11). Another aspect of this phenomenon is that stream reaches that were perennially flowing prior to groundwater development can go intermittently dry so that those stream reaches are no longer considered to be perennial, as seen in western Kansas (Figure 12).