Note that the data published in the 2002 State of the Nation’s Ecosystems Report as well as the 2003 and 2005 Web-Only Updates have been superseded by the 2008 Report and thus should be used with caution. For the most recent data, purchase the 2008 Report from Island Press.

The Indicator

Shallow aquifers, or deeper regional aquifers where shallow aquifers do not exist, are often the water source for the maintenance of riparian and wetland ecosystems (Dawson and Ehleringer 1991, Flanagan et al. 1992). Shallow groundwater is being increasingly withdrawn for agriculture, urban expansion, and mining. Reduction in stream flows, which maintain shallow alluvial aquifers, by dams or other activities also reduces the level and availability of this important water source (Shafroth et al. 2000). In addition, deep-rooted plants, such as pinyon-juniper and Western juniper, are capable of lowering shallow aquifers in the process of transpiration.

Declining groundwater has been shown to affect riparian ecosystems through a reduction of (1) the shallow water table necessary for recruitment of riparian species and (2) long-term maintenance of established woody riparian vegetation. Urban development may tap shallow groundwater associated with river basins, which can cause a gradual decline in associated riparian forests (Stromberg et al. 1992). Gravel mining may alter the natural gravel deposits along rivers, causing shallow groundwater to recede, affecting established riparian vegetation (Scott et al. 1999).

Streams in arid climates, affected by withdrawal of groundwater inputs, also show declining vigor of riparian vegetation as both the alluvial groundwater level declines and stream flow is reduced (Stromberg et al. 1996). Shallow groundwater decline is often a long-term phenomenon because it is usually caused by a gradual withdrawal of water from the shallow aquifer which may continue to be recharged, although inadequately, by stream inflows or from deeper aquifers. If the source of water replacement is affected, shallow aquifers, which are the primary water source for springs, seeps, wetlands, potholes, and riparian areas and which in some cases support declining ecosystems, will thus not be replenished.

Shallow groundwater depths are often used to determine long-term cumulative effects of groundwater withdrawal by agriculture, mining, or urban expansion. Urban expansion in the Great Basin has resulted in water claims on both shallow and deep aquifers. Modeling of this potential withdrawal shows that the shallow water table may decline by 1–3 m (Schaefer and Harrill 1995), a result that would drastically impact the isolated desert springs, the only water source for domestic livestock and wildlife in these areas. Decreasing aquifer volumes and dropping water tables also add to energy costs of water withdrawal, sufficiently so to cause decline or termination of regional agriculture in arid regions of the United States.

The technical note for Number and Duration of Stream Flow (immediately preceding this technical note) also provided relevant perspective on the interaction between groundwater, surface water, and land use.

The Data Gap

Although depth to deep groundwater or the regional aquifer is regularly measured in monitoring and functioning wells across the country and the data are reliable and maintained by appropriate agencies, these data have not been integrated either for the grassland/ shrubland region or nationally (see groundwater indicator in freshwater chapter; and USGS 1997).

Data on shallow aquifers are quite limited. Depths for shallow aquifers (e.g., groundwater under riparian communities) and deeper regional aquifers are usually treated separately. The limited shallow aquifer data from the U.S. Geological Survey and many academic and agency research projects dealing with rivers and adjacent floodplains (see citations above) may also be good sources for regional shallow groundwater data.

References

Dawson, T.E, and J.R. Ehleringer. 1991. Streamside trees that do not use stream water. Nature 350:335–227.

Flanagan, L.B., J.R. Ehleringer, and T.E. Dawson. 1992. Water sources of plants growing in woodland, desert, and riparian communities: Evidence from stable isotope analysis. US Forest Service Tech. Report INT-289:43–47.

Schaefer, D.H., and J.R. Harrill. 1995. Simulated effects of proposed ground-water pumping in 17 basins of east-central and southern Nevada. USGS Water-Resources Investigations Report 95-4173.

Scott, M.L., P.B. Shafroth, and G.T. Auble. 1999. Responses of riparian cottonwoods to alluvial water declines. Environmental Management 23:347–358.

Shafroth, P.B., J.C. Stromberg, and D.T. Patten. 2000. Woody riparian vegetation response to different alluvial water table regimes. Western North American Naturalist 60:66–76.

Stromberg, J.C., R. Tiller, and B. Richter. 1996. Effects of groundwater decline on riparian vegetation of semiarid regions: The San Pedro, Arizona. Ecological Applications 6:113–131.

Stromberg, J.C., J.A. Tress, S.D. Wilkins, and S. Clark. 1992. Response of velvet mesquite to groundwater decline. Journal of Arid Environments 23:45–58.

United States Geological Survey. 1997. Ground water atlas of the United States—Segment 1 California Nevada. Online data at http://water.wr.usgs.gov/gwatlas/ and http://water.usgs.gov/ogw/.