Water used for agriculture is primarily obtained from three sources: local precipitation contributing to soil moisture available for root water uptake (green water), irrigation water taken from rivers, lakes, reservoirs, wetlands and renewable groundwater (blue water) and irrigation water abstracted from non-renewable groundwater (fossil water).
The vast majority of the Earth’s freshwater resides underground, stored within pores and cracks in the soil and rock. Groundwater moves slowly through highly permeable soils and rocks called aquifers. Groundwater is the largest storage of freshwater available for use by humans and replenished by precipitation in a form of groundwater recharge. Fossil water is groundwater that has stored in an aquifer for a long period of time (thousands to even millions of years) and is hardly or not replenished by recharge.
In recent decades, to sustain growing food demand and increasing standard of living, humans extract vast amounts of water from surface water and groundwater resources over various regions of the world. Global water use has increased by nearly 8 times from ~500 km3 yr-1 to ~4000 km3 yr-1 since the beginning of the 20th century, over the last 100 years, with an acute increase between 1960 and 2010. Irrigated agriculture is the principal user of water and accounts for ~70% of the total and accounts for 40% of the global food production, sustaining the livelihood of billions of people.
Intense water use occurs over India, Pakistan, China, the USA, Mexico, southern Europe, north Iran, and Nile delta, where more than 90% of the global irrigated areas are present. Over the period 1960-2010, groundwater abstraction shows a consistent increase and nearly tripled. Importantly, as long as groundwater abstraction is smaller than groundwater recharge, it will only reduce the groundwater discharge to surface water (base flow). However, if groundwater abstraction exceeds the groundwater recharge over extensive areas for prolonged periods, persistent groundwater depletion occurs. The extraction of water from such non-replenishing groundwater reserves is considered as water mining over regions where groundwater reserves still exist, leading to falling groundwater levels such as the Ogallala aquifer, the California’s Central Valley, the North China Plain, northwest India and northeast Pakistan, and the Tigris-Euphrates. In that case fossil groundwater, not being an active part of the current hydrological cycle, is used as an additional, albeit nonrenewable, source of water supply, that may have devastating effects on natural streamflow, groundwater-fed wetlands and related ecosystems. Increasing water mining also raise a large concern for the future sustainability of food production and economic development over regions where local farmers rely on non-renewable groundwater for irrigated agriculture.
Since most of the groundwater released from storage ends up in the ocean, partly by river runoff and, as most of the groundwater use is for irrigation purposes, predominantly through evapotranspiration and then precipitation, water mining contributes to sea-level rise through the persistent removal of groundwater from subsurface storage (depletion) as a net transfer of freshwater to active hydrological cycle at the earth’s surface. Current rate of global groundwater depletion is responsible for 10-20% of recently observed sea-level rise.
The sustainable use of global water resources is a key issue to economic development and food production, however, the dependence on nonrenewable water resources likely increases the vulnerability of water supply. The degree of current groundwater depletion casts significant doubt on the sustainability of regional water supply and associated food production, and will poses a serious threat to future well-being of our society.