1 Department of Physical Geography, Utrecht University, Utrecht, The Netherlands (firstname.lastname@example.org)
Water scarcity, caused by an existing regional imbalance of water availability and water demand, poses a serious environmental issue to the global society. Soaring human water use worsens water scarcity condition already prevalent in semi-arid and arid regions (e.g., India, Pakistan, North East China, the Middle East and North Africa), where available surface water is limited due to lower precipitation, increasing uncertainty for food production and economic development. Over these regions, the water demand often exceeds the available surface water resources due to intense irrigation which requires large volumes of water during crop growing seasons. Groundwater resources serve as a main source of such intense irrigation, supplementing the surface water deficit. Excessive groundwater pumping, however, often leads to overexploitation, causing groundwater depletion. Here, this study assesses and projects past and future development of water stress and groundwater depletion using a global hydrological and water resources model at a 0.5o global grid. Water demand is calculated for agricultural (livestock and irrigation), industrial and domestic sectors using the latest available data on socio-economic (e.g., population and Gross Domestic Product), technological (e.g., energy and household consumption and electricity production) and agricultural (e.g., the number of livestock and irrigated areas) drivers. This study explicitly accounts for groundwater abstraction based on available country statistics, and contrasts it to simulated groundwater recharge to estimate groundwater depletion worldwide. The results show that the number of people exposed to severe water stress tripled from 0.5 to 1.8 billion (17 to 30%) of the global population over the period 1960-2000, and is projected to increase further in coming decades. The results suggest that during the recent period 1990-2000 people have increasingly relied on groundwater resources as surface water has been extensively exploited during the past periods and the construction of new reservoirs has been tapering off. While readily accessible groundwater is an obvious choice to fill the gap between the increasing demand and limited surface water availability, the dependence on groundwater worsened groundwater depletion already reported in various regions. Over the Middle East and Northern Africa, more than half of irrigation water comes from nonrenewable groundwater abstraction (i.e., abstraction in excess of recharge). Over major irrigated countries, the contribution of nonrenewable groundwater abstraction to irrigation is equally substantial: ~20% for India, ~15% for China, and ~25% for the USA and Pakistan. Globally, nonrenewable groundwater abstraction contributes ~20% to irrigation water supply and has more than tripled in size over the last 50 years. Most of the groundwater released from storage due to groundwater depletion will end up in the ocean, the depleting groundwater resources were found to be an important contributor to global sea-level rise. Groundwater depletion contributed a considerable amount of 0.6 (±0.1) mm yr−1 to sea level rise for the year 2000, and the contribution is likely to rise by the 2050s.