Groundwater Loss: The Colorado River’s Unseen Threat
I’ll never forget my first encounter with the Colorado River: I was hiking down the South Kaibab Trail of the Grand Canyon, amazed at how the setting sun colored the canyon walls, when suddenly the River came into view and the panorama changed entirely. In that moment of humbling beauty, and even now, it’s hard to imagine that a river so strong could be in such danger. What’s even more difficult to imagine is that the one of the gravest threats to the River is the most unknown and underestimated: groundwater depletion.
Last week, hydrologists at NASA and the University of California, Irvine published a study illuminating the extent of groundwater loss within the Colorado River Basin. Because withdrawals on a Basin-level are not documented or well-understood, this is the first study to present a comprehensive picture of groundwater depletion within the region. NASA’s innovative GRACE technology is able to measure changes in the Basin’s total water storage by analyzing fluctuations in Earth’s gravitational pull. The research team was able to break down this data to examine changes in groundwater levels over the 2004-2013 study period, and the results are shocking.
The data shows that total Colorado River Basin Reserves have dropped 52.5 million acre-feet (maf) over the past nine years, 40.6maf of which was groundwater – an amount of water equivalent to 1.5 times the storage capacity of Lake Mead. This rate of depletion is much greater than experts had expected, and far exceeds the rate of surface water decline on the Colorado’s largest reservoirs.
Groundwater has commonly been viewed as “backup” water, reserves to fill the gap between supply and demand when drought and reduced snowmelt limit surface water availability. However, as the West faces its fourteenth consecutive year of drought, groundwater pumping has become the norm – utilized heavily for irrigation and, increasingly so, for public water supply.
Aside from contributing to an overall decline in water supply, groundwater depletion has direct impacts on the flow and health of the Colorado River. As they are hydraulically interconnected, much of a river’s flow can be attributed to “baseflow,” groundwater that naturally discharges from the aquifer and contributes to a river or stream. Excessive pumping within the Colorado River Basin reduces aquifer levels, leading to a reduction in this natural discharge and, over time, a reduction in the flow of the River itself.
Despite this now well-understood concept of hydrology, many states in the West manage their water resources independently of each other – refusing to acknowledge the interconnectedness of these water systems. For the health of the Colorado, and all of our Nation’s rivers, it is crucial that the law reflect our heightened understanding of groundwater. Sustainable pumping and the conjunctive management of surface and ground water is vital to establishing a balanced water system, protecting our beloved rivers and maintaining awe-inspiring vistas, like those from the South Kaibab Trail.