More than half the flow of rivers in the upper Colorado Basin is derived from groundwater, according to a study by the U.S. Geological Society. The study’s authors hope it will compel state water managers to ask important questions, since rivers are a key source of irrigation and drinking water across the west. For instance, should a farmer’s use of a nearby river be limited if he or she is also pumping large amounts of groundwater?
“Because we now have numbers on [the connection between groundwater and surface water], we have a better understanding of the importance of groundwater as a contributor to our surface water supply, and anything impacting the groundwater system will also impact flow in rivers,” Matthew Miller, a USGS scientist and the lead author of the study, told High Country News.
Historically, only surface water was regulated in the U.S. But in recent years, states have attempted to control groundwater use despite some public resistance. California was the last state to do so in 2014, with the passage of the Sustainable Groundwater Act.
“In some places, over-pumping has caused the water table to drop hundreds of feet, creating giant cracks in the land as it sinks and drying up streams,” says HCN. “In 2015, a team of NASA scientists determined that some 13 trillion gallons of groundwater had been lost from the Colorado River Basin since the NASA satellites began collecting data in late 2004 — equivalent to roughly one and a half times the total capacity of Lake Mead and as much water as the state of Arizona uses in six years.”
As temperatures rise, experts expect more precipitation in the west will come from rain rather than snow. But rain washes over the surface of the soil, meaning less water will make it into the ground. And forest fires, which are predicted to be more frequent, also make soil less absorbent.