4.3 Magnitude of Storage Depletion
Groundwater storage depletion is becoming recognized as an increasingly serious global problem that threatens sustainability of water supplies (e.g., Schwartz and Ibaraki, 2011). Long-term cumulative groundwater storage depletion, both in the United States and globally, was estimated by Konikow (2011; 2013) using calibrated groundwater models, analytical approaches, gravity-based analyses, and/or volumetric budget analyses for multiple aquifer systems. Estimates were derived from bringing together information from the literature and from new analyses.
Long-term cumulative depletion volumes in the United States were assessed for 40 separate aquifers or areas as shown in Figure 5 and one broader land-use category (agricultural and land drainage where the water table has been permanently lowered).
Estimated total groundwater depletion in the United States during the 20th century was approximately 800 km3, increasing by about 25 percent during the next 8 years for a total of 1,000 km3 during 1900-2008. The rate of groundwater depletion has increased markedly since about 1950 (Figure 6), with maximum rates occurring during the recent period (2001–2008) when the depletion rate averaged almost 25 km3 per year (compared to 9.2 km3 per year average for the 1900-2008 timeframe). Two large aquifer systems in the northwestern United States showed long-term negative depletions, in other words, water-table rises thus increased groundwater storage. This is attributable primarily to the diversion and application of surface water for irrigation purposes, which increases recharge above rates that occur naturally.
Depletion volumes for individual aquifer systems vary substantially across the United States (Figure 5). The three aquifer systems having the largest depletion volumes are the High Plains aquifer (341 km3), the Mississippi Embayment aquifer system (182 km3), and the Central Valley of California (145 km3). However, this does not tell the whole story. For example, because the High Plains aquifer encompasses a very large area (~450,000 km2), the average depletion over the entire area is less than in some other systems, and depletion is nonuniform – being much greater in its southern part. Another way to assess the magnitude of aquifer depletion is to normalize the depletion volume by the area of the aquifer, yielding a measure of depletion intensity (Konikow, 2015). During the 20th century, the highest depletion intensities occurred in three relatively small basins in southern California. However, during the beginning of the 21st century, the highest depletion intensity was in the Central Valley of California, which has an area of about 52,000 km2. The aquifer-wide depletion intensity there averaged 0.075m/yr during 2001-2008 (Konikow, 2015). Consistent with this measure, the Central Valley has been experiencing increasing water shortages, additional water-level declines, and accelerated land subsidence in the most affected parts of the valley since 2000 (Faunt et al., 2016).
Groundwater storage depletion and capture can be measured in terms of nondimensional fractions relative to pumpage (Konikow and Leake, 2014). Reliable estimates of cumulative pumpage in the United States are available for the United States as a whole for 1950-2005, during which the cumulative withdrawals were approximately 5,340 km3 (Kenny et al., 2009). During that same time period, the total net groundwater storage depletion was about 812 km3 (Konikow, 2013). Thus, about 15 percent of the total pumpage was derived from a reduction in the volume of groundwater in storage; that is, the long-term depletion fraction is about 0.15 and the capture fraction is about 0.85. But depletion fractions vary widely throughout the United States. There are adequate withdrawal and depletion data available for 31 specific areas or aquifers within the United States. The mean depletion fraction for these areas is 0.39 and the mean capture fraction is 0.61 (Konikow and Leake, 2014). Overall, even though groundwater storage depletion is a serious problem in many places, it is evident that over periods of years to decades that capture is generally larger than depletion and constitutes an even more serious concern.
To assess the potential contribution of groundwater depletion to sea-level rise, one can make a bounding calculation by assuming that the oceans represent an ultimate sink for essentially all depleted groundwater. Then the contribution of groundwater storage depletion to sea-level rise can be estimated by spreading the volume of depletion across the surface area of the oceans (a total area of about 3.61×108 km2). On this basis, groundwater depletion in the United States alone would account for (or balance) as much as 2.2 mm of sea-level rise during the 20th century. During this 100-year period, the observed rate of sea-level rise averaged about 1.7 mm/yr. Thus, groundwater depletion in the United States alone can explain about 1.3 percent of the observed global sea-level rise during the 20th century.
Groundwater storage depletion is a global problem. Konikow and Kendy (2005) note that excessive groundwater depletion affects major regions of North Africa, the Middle East, South and Central Asia, North China, North America, and Australia, as well as localized areas throughout the world. Konikow (2011) estimated the cumulative global groundwater depletion, as well as its equivalent sea-level rise for 1900-2008 (Figure 7). Obtaining data on water-level changes from many parts of the world is extremely difficult, so the resulting depletion estimates have a greater uncertainty than those for just the United States. The total depletion volume is on the order of 4,500 km3, which could explain about 12.6 mm of sea-level rise. However, during the most recent part of this evaluated time period (2001-2008), the rate of global groundwater depletion had increased from an average of 33.7 km3/yr during the 20th century to approximately 145 km3/yr (equivalent to a sea-level rise contribution of 0.40 mm/yr). For these same reference time periods, the rate of sea-level rise had increased from about 1.7 mm/yr to about 3.1 mm/yr. Therefore, during the first part of the 21st century, global groundwater depletion can explain almost 13 percent of the observed rate of sea-level rise.
