Global Groundwater Depletion
In the drier areas of the world where surface water is scarce, groundwater is abstracted in large volumes in order to irrigate crops, and to a lesser extent for drinking water supply and industry. If groundwater abstraction exceeds groundwater recharge for extensive areas and long time, overexploitation or persistent groundwater depletion can occur. The resulting lowering of groundwater levels can have devastating effects on natural streamflow, groundwater fed wetlands and related ecosystems.
To assess where in the world groundwater depletion occurs and at what rate, the scientists combined several pieces of information. From a database of the International Groundwater Resources Assessment Centre (IGRAC), which was part of Deltares, they extracted country-based statistics on groundwater abstraction. By combining these statistics with estimates of water demand based on maps of population density and the location of irrigated areas, a global map of groundwater abstraction could be derived. Using a global water balance model, the researchers subsequently produced a global map of groundwater recharge. By subtracting groundwater abstraction from groundwater recharge they arrived at a global map of groundwater depletion.
The results show that the areas of greatest groundwater depletion are in India, Pakistan, the United States and China. Therefore, these are areas where food production and water use are unsustainable and eventually serious problems are expected. The hydrologists estimate that from 1960 to 2000 global groundwater abstraction has increased from 312 to 734 km3 per year and groundwater depletion from 126 to 283 km3 per year.
Because most of the groundwater released from the aquifers ultimately ends up in the world's oceans, it is possible to calculate the contribution of groundwater depletion to sea level rise. This turned out to be 0.8 mm per year, which is a surprisingly large amount when compared to the current sea level rise of 3.3 mm per years as estimated by the IPCC. It thus turns out that almost half of the current sea level rise can be explained by expansion of warming sea water, just over one quarter by the melting of glaciers and ice caps and slightly less than one quarter by groundwater depletion. Previous studies have identified groundwater depletion as a possible contribution to sea level rise. However, due to the high uncertainty about the size of its contribution, groundwater depletion is not included in the latest IPCC report. This study confirms with higher certainty that groundwater depletion is indeed a significant factor.
Figure 1: Average groundwater depletion (red to yellow-orange) and recharge (dark yellow to blue) in major basins across the world, expressed as a depth (or water level) change (millimeters). Modified from Richey et al. (2015). Quantifying renewable groundwater stress with GRACE. Water resources research, 51(7), pp.5217-5238.
Groundwater Depletion in US
Groundwater levels are dropping across a much wider swath of the United States than is generally discussed, according to a new report from the Columbia Water Center.
In addition to confirming alarming depletion in well-known hot spots such as the Great Plains and Central California, the study identifies a number of other regions, including the lower Mississippi, along the Eastern Seaboard and in the Southeast where water tables are falling just as rapidly. Overall, the report concludes, between 1949 and 2009 groundwater levels declined throughout much of the continental U.S., suggesting that the nation’s long-term pattern of groundwater use is broadly unsustainable.
Figure 2: Groundwater depletion in major regional aquifer systems of the United States over the last century (left) and recently (right), expressed as a depth (or water level) change (millimeters). From Konikow, L. F., 2015: Long‐term groundwater depletion in the United States. Groundwater, 53 (1), 2–9. doi:10.1111/gwat.12306.
Figure 3: Trends in groundwater levels observed between 1949 and 2009. Negative (red/orange) indicates decline in groundwater level, while positive (blue) indicates a rise in groundwater level. Source: Columbia Water Center
Figure 4: Groundwater level decline in a local bedrock aquifer, Ottawa County, Michigan, United States. Modified from Curtis, Liao and Li (2018), “Ottawa County Water Resource Study – Phase II Final Report”, Dept. of Civil and Env. Engineering, Michigan State University, East Lansing, Michigan, United States.
Groundwater Depletion in CHINA
Figure 5: Groundwater depletion in China, by province, in units of millions of cubic meters. Also shown are major rivers draining the basins. From Chinawaterrisk.org
Groundwater Depletion in INDIA
Figure 6: Groundwater withdrawals as a percentage of groundwater recharge, based on state-level estimates of annual withdrawals and recharge reported by India’s Ministry of Water Resources. Note that the three states that are labeled are using groundwater at a rate equal to (or above) the rate of groundwater recharge. Source: NASA/Matt Rodell.