Drought-Proofing Groundwater.

Abstract

Groundwater is characterized by high storage capacity relative to inflows. As such, it is commonly taken for granted as a buffer storage that can assure water availability during times of drought. Terms such as “insurance policy,” “water savings account,” and “resilience to climate variations and change” are commonly applied to groundwater. The reality can be different, however, with groundwater management failing to adequately consider the natural cycles of wet years and dry years, let alone potential long-term climate change. As a result, groundwater may fail to meet its expected role in drought mitigation and droughts simply intensify the overexploitation of groundwater resources. The recent drought in California is Exhibit A of this phenomenon, with groundwater levels falling to new lows and renewed land subsidence. Public and media attention to groundwater also undergoes a climate-induced periodicity, tending to diminish during wet periods and increase after droughts are well underway. For example, Congress funded the U.S. Geological Survey (USGS) Regional Aquifer-System Analysis Program in response to the 1976–1977 drought. Hydrogeologist Harold E. Thomas famously quipped, “The science of hydrology would be relatively simple if water were unable to penetrate below the earth's surface” (USGS Water-Supply Paper 2220, p. 1), yet surface-water hydrology offers some useful lessons on how to improve management of groundwater systems that are sensitive to climate variations. Surface-water reservoirs are designed and operated on the basis of historical inflows with potential adjustments for climate change scenarios. Uncertainty and variability resulting from climate are central features of these analyses. Reservoir operating strategies must contend with multiple competing purposes, such as flood control versus water supply. Hydrogeologic analyses should likewise incorporate explicit recognition of the effects of climate variability on groundwater resources and how these affect the multiple uses of groundwater systems for both humans and nature. In so doing, groundwater systems would be analyzed for their resilience and vulnerability to climate perturbations rather than just assuming groundwater is a convenient backup supply. This requires building climatic variations in models and projecting groundwater conditions over years to decades with a range of possible future climate conditions. Such projections should be part of overall efforts to raise awareness about maintaining groundwater as a reserve. Monitoring of water use and water levels is a key component of such efforts, as is recognition of managed aquifer recharge as a potentially critical element of drought mitigation planning. Current laws and regulations contribute to the problem. Because groundwater was developed after surface water, it has low priority in prior appropriation states. Shutting down wells during droughts, as dramatically illustrated several years ago in the South Platte Basin in Colorado, clearly works against the maximum beneficial use of groundwater. At the same time, overexploitation of groundwater during wet periods can lead to a drilling “arms race” during droughts that no one can afford to lose and damages the long-term viability of the resource. An alternate approach would increase groundwater rights during droughts, with more restrictive use during the intervening periods when surface water is more available. None of this is of course easy, and solutions require cooperative arrangements tuned to local conditions. Simply ignoring the reality of droughts for climate-sensitive groundwater systems, however, is a recipe for problems down the road when groundwater is most needed. Making use of the large volume of groundwater in storage through a conjunctive groundwater and surface-water system has the potential to greatly enhance the efficient use of water and help the world meet its growing food and water demands. Unfortunately, conjunctive use has not lived up to its promise. It's time to try new approaches, including greater emphasis on climatic effects and uncertainties in future scenarios of pumping impacts, maintaining awareness during wet periods of the importance of groundwater as a backup resource as a part of good groundwater governance, and working toward laws, regulations, and incentives that encourage use of surface water during wet periods and prepare for increased groundwater use during droughts.