WaterSubject

Ground water pumping from aquifer systems that are hydraulically connected to streams depletes streamflow. The amplitude and timing of stream depletion depend on the stream depletion factor (SDF_i) of the pumping wells, which is a function of aquifer hydraulic characteristics and the distance from the wells to the stream. Wells located at different locations, but having the same SDF and the same rate and schedule of pumping, will deplete streamflow equally. Wells with small SDF_i deplete streamflow approximately synchronously with pumping. Wells with large SDF_i deplete streamflow at approximately a constant rate throughout the year, regardless of the pumping schedule. For large values of SDF_i, artificial recharge that occurs on a different schedule from pumping can offset streamflow depletion effectively. The requirements are (1) that the pumping and recharge wells both have the same SDF_i and (2) that the annual total quantities of recharge and pumping be equal. At larger SDF_i values, it takes longer for pumping to impact streamflow in a wide aquifer than it does in a narrow aquifer. In basins that are closed to further withdrawals because streamflow is fully allocated, water-use changes replace new allocations as the source of water for new developments. Ground water recharge can be managed to offset the impacts of new ground water developments, allowing for changes in the timing and source of withdrawals from a basin without injuring existing users or instream flows.

The conjunctive management of ground water and surface water has been practiced in many parts of the world for nearly a century and has been advocated in the professional literature on water resources management for more than half a century. As water resources in basins around the world become fully allocated and the basins become institutionally closed to further development, water managers are increasingly reexamining policies of conjunctive management. Used creatively, conjunctive management can enable a change in water use that may allow new development without adversely impacting existing water users, including ecosystem needs for instream flows.

A typical pattern of change in both land and water use is from agricultural to residential and commercial uses. In many parts of the western United States, this change is accompanied by a shift in water use from surface to ground water. In other places, traditional agricultural water users are shifting from surface water to ground water sources as a means to save labor. Like surface water diversion, ground water pumping depletes streamflow. Much of the earlier work on this topic has focused on the total quantities of water diverted from the stream by pumping induced depletion.

Although the quantity of water depleted may be the same, the timing of that depletion can vary significantly when withdrawals move from surface to subsurface sources. It is important to consider the timing of stream depletion in designing mitigation schemes to prevent adversely impacting down stream water users and aquatic ecosystems.

In this article, we provide guidelines to water managers and hydrogeologists for mitigating the impacts of ground water development that take into account the timing of stream depletion caused by cyclical pumping. This article explores the impact of seasonal pumping and recharge on the flow of a nearby stream. Using a two-dimensional ground water flow model of a hypothetical alluvial system, we show how ground water pumping and recharge, aquifer properties, and well locations affect the timing of streamflow depletion. We relate various scenarios to their stream depletion factors (SDF_i) and examine how the SDF_i can be used to guide design strategies that offset surface water impacts due to ground water pumping.

Stream depletion caused by pumping varies according to the SDF of the well and the aquifer width. In a given aquifer that has more or less uniform properties, the stream depletion varies as the distance squared from the stream. Near the stream, seasonal pumping causes a large seasonal variation in the stream depletion. However, as the distance between well and stream increases, seasonal pumping tends to produce a constant depletion, with only a small seasonal fluctuation.

When a pumping well is in close proximity to the impacted stream, stream depletion can be offset by changing the use of an existing surface water right from irrigation to instream flow. When the well is far from the stream, streamflow depletion can be offset by artificially recharging water from an existing surface water right into the aquifer. The width of the aquifer plays an important role in determining how quickly the system reaches a new dynamic equilibrium in which the depletion is the same in successive years.

The fact that at some distance from the stream, depletion from seasonal pumping becomes relatively constant, provides an interesting opportunity for mitigation. Seasonal recharge at some distance from the stream also causes a constant return flow to the stream, with only a small seasonal fluctuation. This means that if the SDF_i of a recharge facility were of the order of 1000 d or larger, recharging a large quantity of water during a short period, at any time during the year, would supplement the stream with a more or less constant flow throughout the entire year. This opens up a wide range of opportunities to conjunctively manage streamflow and offset stream depletion.

For example, in jurisdictions that allocate water rights according to prior appropriation, an individual with an acquired surface water right that is sufficiently senior to be consistently in priority for high spring flows can recharge the water in the period of high spring runoff (from the acquired seasonal surface water right) and offset streamflow depletion due to year-round or seasonal ground water pumping. There are caveats. A scheme that recharges water during the period of peak runoff depletes runoff that may be important for geomorphologic and/or ecological integrity. It goes without saying that care must be taken in assessing the impacts of both pumping and mitigation.