GRACE data has provided a global picture of water storage trends for over a decade and could be an invaluable tool for understanding water resource availability. The GRACE mission is able to monitor monthly water storage changes within river basins and aquifers that are 77,000 square miles or larger. While this area may be too large for community water management, it can be used on the regional and national scale, and to aide international policy discussions. This information could someday be used to develop a unifying principal of cross border water resource allocation. Now, though, the first use has been to study the trends on groundwater in various regions during this period.
|Stressed aquifers are in yellow, orange and red|
Dr. Famiglietti points out that groundwater represents almost half of all drinking water worldwide, though a lesser proportion of irrigation water. In the United States groundwater is an important natural resource, representing about 30% of all consumptive water use especially in those parts of the country that don't have ample surface-water sources, such as the arid West and in times of drought. Groundwater is a renewable resource, but not in the way that sun light is. Groundwater recharges at various rates from precipitation. Changes in rainfall patterns and the actions of man can impact the recharge rate of groundwater. Increasing the amount of impermeable area by paving or building and other changes to land cover can reduce groundwater recharge. 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.
To recharge groundwater, it must rain and the soil must be able to absorb the water. When you withdraw the groundwater from fine-grained compressible sediments and do not replace it, the land subsides. In the pursuit of wealth the ground water in the incredibly fertile Central Valley was pumped to such an extent that the ground subsided more than 75 feet in some places. The area was identified by the research efforts of Joseph Poland as the location of maximum subsidence in the United States due to groundwater mining. Once the land subsides, it loses its water holding capacity and will never recover as an aquifer. Groundwater mining in the Central Valley was believed to have slowed in the past few decades, but it continues as documented by the recent data from Drs. Famiglietti and Rodell’s work and the continual falling of the groundwater level.
Though, ten years of data may not be adequate to determine accurate changes in water availability and groundwater recharge. Using GRACE data, Drs. Famiglietti and Rodell identified what appear to be in this 10 year window water ‘hotspots’ in the United States, and these include the important food producing regions in California’s Central Valley, and the southern High Plains; large areas of the southeastern U. S. that has been plagued by persistent drought, including Houston, Texas, Alabama, and portions of the Mid-Atlantic region. Based on the data since 2003, the wetter, northern half of the U.S. has become wetter, while the drier, southern half has become generally drier.