Groundwater resources and climate variability
|Title||Groundwater resources and climate variability|
|Publication Type||Feature Articles|
|Full Text|| |
An END InSight participant recently asked an interesting question about a vital issue in the Southwest, which this article will address: “What is the impact of the recent drought on groundwater supplies?”
First, a bit of context: groundwater constitutes more than 25 percent of the nation’s water supply, equal to 65 gallons per day for every man, woman, and child in the country for domestic purposes alone. Worldwide, 1.5 billion people, or one-quarter of the world’s population, depend on groundwater for drinking water (1). Groundwater is by far the most important water source in the Southwest, providing drinking water for 90 percent of New Mexico’s population and 60 percent of Arizona’s (2). Overpumping of this limited resource not only threatens human water supplies, but can also lead to land subsidence, the loss of springs, streams, and wetlands, and decreases in water quality (3).
Groundwater Monitoring Challenges
Linking groundwater levels to climatic variability is a complex task for several reasons. Some aquifers are shallower and more sensitive to climatic fluctuations, while others are deeper and slower to respond. An area’s geophysical characteristics, as well as rainfall patterns and seasonality, can determine how much precipitation is absorbed into the aquifer and how much runs off and evaporates before it can replenish groundwater supplies. Human activities can also simultaneously influence aquifer levels, making it difficult to isolate climatic impacts. Thus there is no simple formula for equating the amount of precipitation that falls directly with recharge rates. However, some up-todate groundwater monitoring tools are available.
Groundwater Monitoring Information
Interestingly, it appears easier to monitor longer-term groundwater changes in New Mexico, while shorter-term fluctuations are easier to track in Arizona. The U.S. Geological Survey (USGS) provides a very useful map of New Mexico at http:// nm.water.usgs.gov/drought/ gwbasin.html. Clicking on any of the 34 monitoring wells pictured will produce a hydrograph of water level changes over several decades. While the hydrographs can be correlated with records of past climatic events, they do not distinguish which fluctuations are actually climatically induced and which are due to human activities and land-use change.
Arizona and several other states (not including New Mexico!) have realtime well monitoring systems that monitor water level changes on time scales of one week to one month (see http://waterdata.usgs.gov/nwis/current/?type=gw for a list of stations). Users can click on one of 12 site locations (see accompanying map) and see a hydrograph of well level changes. Again, this information does not reflect human activities, such as drilling new wells or shutting down existing ones, and does not provide information for all areas of the state. However, it does make it easier to see the effects of specific periods of precipitation on localized groundwater supplies.
Groundwater in the Southwest
Depending on the information you examine, it may be easy to see that groundwater declines are a serious problem in many Southwestern cities. For example, groundwater is Albuquerque’s sole source of municipal water supplies, and on average less than half of what the city currently pumps is naturally recharged on an annual basis (6). Like many rapidly growing Southwestern cities, Albuquerque is seeking to supplement its dwindling groundwater supplies with greater use of surface water. The city intends to divert surface water from the Rio Grande River and via the San Juan-Chama Diversion Project, but the project is not expected to be completed until 2025 (6). The city faces a more immediate obstacle to using Rio Grande River water to which it holds the rights, but which is also needed to provide habitat downstream for the silvery minnow. Because this species is federally protected under the Endangered Species Act, the city may be prohibited from impeding the river’s flow by storing water upstream, behind the Heron Dam in northern New Mexico (7).
Flagstaff, Arizona, on the other hand, utilizes both surface and groundwater, but groundwater supplies are quite limited, and surface water is highly sensitive to climatic variability. The city currently taps Lake Mary to cover increased water demand on hot summer days, but several consecutive years of below-average rainfall have left the lake nearly dry (8). The city is seeking to increase its groundwater pumping, although drilling new wells is expensive and contributes to Flagstaff’s comparatively high water rates (9).
Both cities, as well as other locations in the Southwest, were forced to restrict municipal water use during the summer of 2002 to cope with ongoing drought conditions and could face even more severe supply problems if the drought continues through the summer of 2003. While groundwater has been viewed as a hedge against severe drought, mounting human needs are severely testing its ability to compensate for the lack of precipitation in many areas.
(1) Glennon, R. 2002. Water Follies: Groundwater Pumping and the Fate of America’s Fresh Waters. Island Press, Washington. p 2–3.
(2) U.S. Geological Survey Ground- Water Resources Program. 2001. U.S. Geological Survey Fact Sheet 056-01. Accessed at http://water.usgs.gov/ogw/pubs/fs01056/ on Jan. 14, 2003.
(3) Leake, S. A., A. D. Konieczki, and J. A. H. Rees. 2000. Desert Basins of the Southwest. U.S. Geological Survey Fact Sheet 086-00. Accessed at http://water.usgs.gov/ogw/pubs/fs00086/ on Jan. 8, 2003.
(4) U.S. Geological Survey. Selected Groundwater Wells in New Mexico. Accessed at http://nm.water.usgs.gov/drought/gwbasin.html# on 1/13/03.
(5) U.S. Geological Survey. Real-Time Data for Groundwater. Accessed at http://waterdata.usgs.gov/nwis/current/?type=gw on Jan 17. 2003.
(6) City of Albuquerque. 2000. Albuquerque 2000 Progress Report. Accessed at http://www.cabq.gov/ on Jan. 15, 2003.
(7) Jehl, D. 2003. Albuquerque case pits thirst against fish, New York Times, National section, 1/19/03.
(8) Flagstaff’s water choices deserve wide input. Arizona Daily Sun, 8/28/ 02. Available at http://www.azdailysun.com .
(9) Flagstaff Online. Flagstaff 2020 report, water. Accessed at http://www.flagstaff.az.us/ on Jan. 10, 2003.