Shifting rain could be lowering groundwater

The most recent satellite data is showing a drop in Minnesota’s groundwater in recent years, and at least one scientist is suggesting that a main reason may be a change in when rain falls.

The National Oceanic and Atmospheric Administration’s Climate Watch magazine earlier this month showed what a pair of satellites measuring tiny changes in the earth’s gravity say about the nation’s groundwater levels, combining that with groundwater use. The headline was the very obvious drop between 2003 and 2012 in California, New Mexico, Texas and other parts of the South where drought has been persistent and pumping has been vigorous.

Brown areas show groundwater declines from 2003 to 2012. Blue areas show increases. Small circles show heavy pumping. Source: Climate.gov, National Oceanic and Atmospheric Administration.

But Minnesota stands out as well, not so much because of high pumping but because of a contrast with nearby regions in water drawdowns. Most northern tier states have experienced rising groundwater levels. (As we reported in our “Beneath the Surface” project, three quarters of Minnesota households get their water, not from rivers or lakes, but from wells that tap groundwater.)

Minnesotans have become more aware of pressures on the state’s groundwater availability and quality, but a problem is the difficulty in understanding what’s going on down where you can’t see anything. Satellite data is the newest tool to add to the measurement of water levels with monitoring wells, albeit not the most precise one. (One of the lead researchers analyzing the NASA data, Jay Famiglietti, was at the University of Minnesota in September as part of a Freshwater Society’s speaker series. )

For more on how the satellites, known as GRACE for Gravity Recovery and Climate Experiment, measure groundwater, see this.

So why would Minnesota see a decline in groundwater levels? One answer could be the increase in farm irrigation in parts of the state. In the sandy soils of west central Minnesota, farmers have increased their pumping substantially, and in two places the Minnesota Department of Natural Resources is trying a different approach to getting farmers, businesses and others to come to agreement on how much groundwater is used and who gets to use it.

But Don Rosenberry, a U.S. Geological Survey research hydrologist in Denver who studies Minnesota lakes, said scientists in his office have been looking at lower groundwater in wells in Cass County “and we’ve been wondering if it might be due to a change in the timing of rainfall.”

Minnesota rainfall — the source of recharging groundwater supplies — has been increasing over recent decades. But rain has increased, particularly in big storms, in the summer and decreased in spring and fall, Rosenberry said. “The big summer events rarely turn into groundwater recharge. . .So there might be a story here in addition to irrigation.”

It’s an idea he said the USGS looked at a few years ago and decided it needed more data or statistical power. It might be worth returning to the notion now, he said.