While we’ve been watching Lake Mead and Lake Powell drop during the last decade, the hidden reservoirs of groundwater beneath the Colorado River Basin have been dropping even faster, according to new research by Stephanie Castle and her colleagues at U.C. Irvine. Using gravity data from NASA’s GRACE satellite (the new tool all the cool kids are using to study groundwater), Castle and colleagues have posted up some striking data.

In the time period, they studied, 2004-13, they identified a total of 12 million acre feet of water lost as we drew down reservoirs. That’s the white bathtub ring in Lake Mead, the drop in Lake Powell, and the reductions in storage in smaller reservoirs – the stuff we can see. But they also found a whopping 40.5 million acre feet of loss in aquifer storage, as folks pumped groundwater to make up for surface water shortfalls.

This has significant policy implications, because essentially all the basin-scale policy discussions focus on surface water, while dealing with residual problems left by groundwater shortfalls left to state governance:

While the need to exploit groundwater resources to meet Basin water demands has long been recognized,  withdrawals required to meet current demands remain undocumented and are uncertain in the future. In particular, water management under drought conditions focuses on surface water  resources without a regulatory framework to manage  groundwater withdrawals outside of “river aquifer” systems. At question is  the potential impact of solely managing surface water allocations and diversions in the Basin,  without regard to groundwater loss, on meeting future water demands.

(I don’t have a link yet, will add one as soon as it’s available.)

Here’s a link to the paper, and Jay Famiglietti, one of the authors, has a writeup over at National Geographic. Also, AGU news release.