The GRACE satellites measure monthly changes in total earth water storage by converting observed gravity anomalies measured from space into changes of equivalent water height this was a method developed by Matthew Rodell & James S. Famiglietti in 1999. The GRACE mission has collected more than ten years of data and the scientists have just completed their analysis of all the data from January 2003 to December 2013. While some preliminary reports have been published in the last couple of years, these two new reports represent a complete analysis of the first 10 years of data. The scientists raise the very real possibility of some groundwater basins running out of water in the near future.
The scientists found that more than one third of Earth's 37 largest groundwater basins are using up their groundwater faster than it is being replaced. Though the GRACE satellites can be used to see the rate of net water consumption, there is little accurate data about how much water actually remains stored in the earth for future us. Eight of the earth’s 37 major groundwater basins were classified as "overstressed," by the scientists. These basins have nearly no natural recharge of the groundwater to offset the ongoing and increasing consumption. Another five groundwater basins were found to be "extremely" or "highly" stressed by the scientists. Those aquifers were still being depleted but had some water flowing back into them. Management of the resource could restore groundwater resilience.
The GRACE data found that the Arabian Aquifer System, an essential water source for more than 60 million people in the Middle East, is the most overstressed aquifer on earth. The Indus Basin aquifer of northwestern India and Pakistan is the second-most overstressed, and the Murzuk-Djado Basin in northern Africa is third. Though California's Central Valley basin is used heavily for agriculture and suffering rapid depletion during the current drought it was found to be slightly better off, but highly stressed. The same was true for the North China Aquifer System and the Tarim Basin. The major stressors for these systems are irrigation and population. The Ganges, the Indus Basin, the Californian Central Valley Aquifer System, and the North China Aquifer System, have the four highest levels of irrigation demand and among the highest levels of population density.
Surface water has throughout history served as the principal freshwater supply used by mankind. However, the importance of groundwater has increased in recent decades as mankind’s demand for water has surpassed surface supplies and our ability to access groundwater has increased with technology. Fresh surface water can no longer support the needs of mankind. Accessing groundwater allowed populations to increase, and provide reliable water as surface water has become less reliable and predictable as weather patterns change and regions experience extended droughts. Regions of the earth have come to rely more heavily on groundwater as a dependable water supply source. Groundwater represents almost half of all drinking water worldwide, though a lesser proportion of irrigation water and is currently the primary source of freshwater for approximately two billion people [Famiglietti, 2015].
Groundwater is a renewable resource, but not in the way that sun light is. Groundwater recharges at various rates from precipitation. To recharge groundwater, it must rain (or snow) and the soil must absorb the water. Changes in rainfall patterns and the actions of man can impact the recharge rate of groundwater. It is known that some groundwater is quite ancient and other groundwater only days old. However, very little actual knowledge exists about global groundwater supplies. Groundwater storage estimates commonly cited in global groundwater assessments were traced to decades-old heuristic estimates. These largely uncertain estimates have been cited as fact so often in the global groundwater literature, and although they were originally only working speculative estimates or assumptions, they have become commonly accepted as fact.
Although there is no measured basis, it is commonly accepted that groundwater comprises 30% of global freshwater calculated from “the upper estimate of global groundwater storage” from a 1978 paper which assumed uniform groundwater supply across the entire global land area. This is not likely to be accurate, but has been used to estimate groundwater supplies in critical regions. Groundwater is an essential portion of the water supply and ecology-providing fresh water and stream baseflow in times of drought. For groundwater to be available to provide in times of drought indefinitely there must be a balance between the volume of water that enters a groundwater system and the volume that leaves the system over time.
The climate of the planet has continually changed over the millennia and some groundwater aquifers are legacies of an earlier climate and are not being recharged. There are some groundwater systems that have no natural recharge; unless they are artificially recharged they have a limited life span. The problem is we do not know how much water is available in the aquifer. If the water from a groundwater basin is used faster than it is recharged, it is being used up and ultimately it will run out. The scientists conclude that significant segments of Earth's population are consuming groundwater more quickly than it is recharging without knowing when it might run out.
Worldwide groundwater is largely unregulated and unmanaged. These two studies highlight regions that may be vulnerable to tipping points to higher ecological, economic and political stress. Potential consequence when an overused aquifer such as the Arabian Aquifer System can no longer supplement declining water supplies are starvation, war and death. Alexandra Richey is the lead author on both studies, conducted the research as a doctoral student and says: "We're trying to raise red flags now to pinpoint where active management today could protect future lives and livelihoods."
These studies highlight regions that may be vulnerable to tipping points toward higher levels of stress driven by a range of factors including conversion to intensified agriculture, or population pressures that increase the demand for water. The lack of ground-based measures of total groundwater availability will prevent a full characterization of aquifer stress and resilience, and the ability to predict critical water stress. To improve groundwater estimates would require a significant investment in regional monitoring and measuring systems to better characterize saturated thickness and soil properties within an aquifer. Water and water availability will drive the political and economic events of the next fifty years.
All this information is from a recently published articles “Quantifying Renewable Groundwater Stress with GRACE” and “Uncertainty in Global Groundwater Storage Estimates in a Total Groundwater Stress Framework” by Alexandra S. Richey, Brian F. Thomas, Min-Hui Lo, John T. Reager, James S. Famiglietti, Katalyn Voss, Sean Swenson, and Matthew Rodell, and published in Water Resources Research in 2015. Like all scholarly, peer reviewed articles this one took several years to go from data gathering to publication so the data collection was from January 2003 through December 2013. These are open access articles.