Sub-humid to Arid Zones of the World Experiencing increasing Groundwater Depletion and Withdrawals

From 1960 until 2000, the amount of groundwater withdrawn and depleted from climate changes has increased in sub-humid to arid areas of the world. Measures of groundwater depletion and withdrawal were found with data of decreasing volumes of water percolating back underground, and the amount of groundwater withdrawals, to calculate groundwater lost per year that is not recharged. Water volumes in two soil layers and the groundwater layer were all measured in these regions of the globe, separated into multiple cells. The results showed an increase of around 57 km3 per year in groundwater depletion. This consists of 36 (±10)% of the global groundwater withdrawn annually, 2(±0.6)% of the annual recharge, 0.8(±0.1)% annual global runoff, and 0.4(0.06)% of global evaporation. Though uncertain, this decrease in groundwater supplies is likely to present an added sea-level rise of 0.8(±0.1) mm per year.  This study compiled by Wada et al. concerns only regions of sub-humid to arid climates. –Darien Martin

Wada, Y. Van Beek, L., Van Kempen, C., Reckman, J., Vasak, Slavek., Bierkens, M., 2010. Global depletion of groundwater resources. Geophysical Research Letters 37, 1–5.

          When performing this study, uncertainties in the data set results were caused by a number of variables. Wada et al. did not know the specific location of wells or irrigation systems, and it was assumed that they were nearby where the water was used. It was also assumed that demand for water correlated with water use. Demand was used to calculate estimated groundwater use.
          First, the volume of groundwater was measured using a model that determined the volume of groundwater recharge. Volumes of water were tracked among two soil layers, the groundwater layer below, evaporation, precipitation, and  snowmelt. two soil levels above the groundwater layer, evaporation, precipitation and snowmelt. Regions were organized into grids of  0.5°latitude by 0.5°longitude when measured. In each grid, soil types, vegetation, surface water body shapes, and groundwater area depth when measuring the cycling water volumes. Groundwater recharge was determined by the amount of water moving from the lower soil layer to groundwater storage.
          Next, the volume of water withdrawn in each grid was determined.  The international Groundwater Resources Assessment Centre’s online database was used which gathered the rate at which of groundwater was extracted in many countries.  Water was extracted at higher rates in Northeast China, much of Europe, the US, Iran, India and Pakistan. The rate of extracted groundwater in 2000, in all semi-humid to arid climates, is 734 (±82) km3 per year.
          Lastly, annual groundwater depletion from 1960 until 2000 was calculated by subtracting groundwater recharge from groundwater extraction.  A significant global trend in increasing groundwater depletion was found.  Most severely effected areas are in Iran, Yemen, Southeast Spain, the Central Valley of California, and Northeast China. 
Although uncertain due to offsets of reservoir storage, this increase in groundwater depletion is estimated to have caused 25(±3) % of the sea level rise per year. This was estimated using the volume of groundwater released from storage data and the amount of that will end up in the ocean through runoff and evaporation followed by precipitation; assuming that all other variables of climate stayed constant. Groundwater depletion trends will likely continue to increase in rates, and to cause sea levels to rise (Drouiche et al.).

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