|From Castle et.al.|
Observing the groundwater buried beneath layers of soil and rock was almost impossible until, the twin satellites known as the Gravity Recovery and Climate Experiment, or GRACE, were launched in March 2002. At the time few believed the satellites could measure changes in groundwater, but thanks to work of Dr. Jay (James S.) Famiglietti and his graduate student (at the time) Matt Rodell, who were then working at the University of Texas at Austin the techniques for measuring groundwater using the GRACE satellites were developed and proven. Expanding on that work is this new paper by Stephanie L. Castle, Brian F. Thomas, John T. Reager, Matthew Rodell, Sean C. Swenson, and James S. Famiglietti.
While the need to use groundwater resources to meet Basin water demands has long been recognized, the quantity of available groundwater and the sustainable rate of groundwater use are not known. As the drought in the western states has persisted for most of this century, water management under drought conditions has focused only on surface water resources- the flow of the Colorado, the levels in Lake Mead and Lake Powell. There is neither enough data nor a regulatory framework to fully manage groundwater. However, as this study shows us, by only managing the water withdrawals from the reservoirs (Lake Mead and Lake Powell) water use may not have been reduced at all, but instead groundwater may have made up more of the shortfall in water.
|From Castle e.t al.|
The study found that the Colorado River Basin lost almost 53 million acre feet of water over the study period with 12 million acre feet coming from the falling level in the two reservoirs and the remaining 41 million acre feet being pumped out from groundwater. The scientists estimated changes in groundwater storage during the 9-year drought period, when reservoir volumes were intensively managed to maintain hydropower production, maintain water levels above the public supply water intake pumps, and to meet surface water allocations to the Basin states using the methods developed by Drs. Jay Famiglietti and Matt Rodell.
The total water storage in a region as seen by the satellites is comprised of soil moisture, snow water equivalent, surface water (including river flow and reservoirs), and groundwater. Accessible water is assumed to be surface water reservoir storage and groundwater storage. They assumed Lakes Mead and Powell accounted for the majority of the observed surface water change as they comprise approximately four times the annual flow of the river and make up 85% of surface water in the Basin at any time. So the flow of the river was ignored introducing an error of 5%-15%. USGS and ADWR monitoring wells in the Colorado Basin showed good agreement with the GRACE-based estimates further confirming the methodology.
A brief recovery in groundwater storage was observed in the data from June 2009-March 2010, when moderately wetter conditions provided a combination of potential groundwater recharge and temporarily alleviated the need to augment surface water supplies, but the overall observed trend is not good. As the Bureau of Reclamation more tightly controlled and limited surface withdrawals, groundwater reserves were tapped to make up the loss, demonstrating the close connection between surface water availability and groundwater use. As available water in the west has been diminished by an extended drought and demand for water has actually increased over these years, solely managing surface water in the Colorado Basin, without regard to groundwater loss, has resulted in the 41 million acre foot reduction in ground water reserves which predominately occurred from April 2010 to November 2013.
Groundwater is typically used to augment the limited surface water supplies in the arid, Lower Colorado Basin and across the entire Basin during drought. Groundwater represents the largest supply of water for irrigation within the Basin and against all reason irrigated acreage in the Colorado Basin increased during the study period. Furthermore, according to Drs. Famiglietti and Rodell, the prolonged drought across the southwestern region of the United States has resulted in overreliance on groundwater by public water to minimize impacts of the drought on public water supply. The decrease of an average of 4.5 million acre feet of groundwater each year may merely reflect the problems with the Colorado River Compact, the regulatory framework already in place to manage surface waters. The Compact which allocates the flow of the Colorado River to Colorado, Utah, Wyoming, New Mexico, Arizona, Nevada and California and through a 1944 treaty to Mexico promised what turned out to be more than 100% of the water available at the time and the current researchers believe that the over allocation of the Colorado River’s water was 30% during the study period based on the groundwater loss of 4.5 million acre feet a year from the groundwater reserves.
Specifically, the amount of water allocated under the Colorado Compact was based on an expectation that the river's average flow was 16.5 million acre feet per year. According to the University of Arizona, a better estimate would have been 13.2 million acre feet at the time of the Colorado Compact and the records going back to Paleolithic times (more than 10,000 years ago) indicates periods of mega-droughts in the distant past and climate forecasts for the future are dire. The political hurdles the Colorado River Compact may need to be renegotiated and include groundwater resources. During the drought of 2001-2006 the Colorado River flow was estimated at 11 million acre feet and hit a low of 6 million acre feet in 2002. The situation was critical bordering on regional rationing when the drought ended.
More than 23 million people of the lower basin are at least partially dependent upon the water resources of the Colorado River. Almost 74% of them reside in the greater Los Angeles and San Diego areas. The current drought in California has only emphasized the need for more active and enforceable groundwater management throughout the Basin, in particular, during drought. During the study period the scientists observed that groundwater is already being used to fill the gap between Basin demands and the annual, renewable surface water supply.
Managing groundwater is a daunting task. Even today groundwater sustainability is still not fully understood. In addition, there are droughts, climate changes; water draws from surface water changes the recharge rate of the groundwater. The U.S. Geological Survey did not begin quantitative analysis of the major groundwater systems of the United States until 1978 and since that time there has been tremendous evolution in the understanding of and ability to model groundwater systems. Before the groundwater basin is irreparably overdrawn, we need to understand what sustainable water use in the region is and embrace it. Otherwise the groundwater will be pumped until it is gone.