Streamflow Depletion by Wells-Understanding and Managing the Effects of Groundwater Pumping on Streamflow
Groundwater provides drinking water for millions of Americans and is the primary source of water to irrigate cropland in many of the nation’s most productive agricultural areas. Despite the many benefits of groundwater development, connected streams and rivers may suffer from reduced flows through a process called streamflow depletion by wells. This report, just released by the USGS, summarizes what is known about streamflow depletion, highlighting common misconceptions, and presenting new concepts to help water managers and others understand the effects of groundwater pumping on surface water.
Groundwater and surface-water systems are connected, and groundwater discharge is often a substantial component of the total flow of a stream. In many areas of the country, pumping wells capture groundwater that would otherwise discharge to connected streams, rivers, and other surface-water bodies. Groundwater pumping can also draw streamflow into connected aquifers where pumping rates are relatively large or where the locations of pumping are relatively close to a stream.
The report’s major conclusions throw into relief the water management issues related to balancing groundwater uses with preservation of streamflows. The report concludes that individual wells may have little effect on streamflow depletion, but small effects of many wells pumping within a basin can combine to produce substantial effects on streamflow and aquatic habitats. Basinwide groundwater development typically occurs over a period of several decades, and the resulting cumulative effects on streamflow depletion may not be fully realized for years. Streamflow depletion continues for some time after pumping stops because it takes time for a groundwater system to recover from the previous pumping stress. In some aquifers, maximum rates of streamflow depletion may occur long after pumping stops, and full recovery of the groundwater system may take decades to centuries. The major factors that affect the timing of streamflow depletion are the distance from the well to the stream and the properties and geologic structure of the aquifer. Streamflow depletion can affect water quality in the stream or in the aquifer. For example, in many areas, groundwater discharge cools stream temperatures in the summer and warms stream temperatures in the winter, providing a suitable year-round habitat for fish. Reductions in groundwater discharge to streams caused by pumping can degrade habitat by warming stream temperatures during the summer and cooling stream temperatures during the winter. Sustainable rates of groundwater pumping near streams depend on the total flow rates of the streams and the amount of reduced streamflow that a community or regulatory authority is willing to accept.
The report supports its conclusions with a wealth of technical explanation and examples in understandable prose and attractive explanatory graphics. Although not light reading, this report could be used as a reference for anyone with an interest in streams and rivers and the impacts on them of groundwater pumping.
The report, a product of the USGS Groundwater Resources Program, is available in print and at http://pubs.usgs.gov/ circ/1376/.