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YOUR WATER
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Climate change and groundwater
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It’s no simple matter to figure out how regional changes in precipitation expected from global climate change may affect water supplies. However, a new analysis by Massachusetts Institute of Technology researchers has found that the changes in groundwater actually may be much greater than precipitation changes.
In places where annual rainfall may increase by 20 percent as a result of climate change, groundwater might increase as much as 40 percent. Conversely, in some cases just a 20 percent decrease in rainfall could lead to a 70 percent decrease in recharging of local aquifers — a potentially devastating blow in semi-arid and arid regions. Exact effects depend on soil type, vegetation and the exact timing and duration of rain.
The research was conducted by Gene-Hua Crystal Ng and professors Dennis McLaughlin and Dara Entekhabi of MIT’s department of civil and environmental engineering, along with Bridget Scanlon, a senior researcher at the University of Texas. The results were presented in December at a meeting of the American Geophysical Union.
The analysis combines computer modeling and natural chloride tracer data to determine how precipitation, soil properties and vegetation affect the transport of water from the surface to the aquifers below. This analysis focused on a specific semi-arid region near Lubbock, Texas, in the southern High Plains.
“It looks like the changes in recharge could be even greater than the changes in climate,” Ng says. “For a given percentage change in precipitation, we’re getting even greater changes in recharge rates.”
Among the most important factors, the team found, is the timing and duration of the precipitation. For example, it makes a big difference whether it comes in a few large rainstorms or many smaller ones, and whether most of the rain falls in winter or summer. “Changes in precipitation are often reported as annual changes,” Ng says, “but what affects recharge is when the precipitation happens, and how it compares to the growing season.”
The team presented the results as a range of probabilities, quantifying as much as possible “what we do and don’t know” about the future climate and land-surface conditions, Ng notes. “For each prediction of climate change, we get a distribution of possible recharge values.”
If most of the rain falls while plants are growing, much of the water may be absorbed by the vegetation and released back into the atmosphere through transpiration, so little percolates down to the aquifer. Similarly, rainwater from frequent small storms may be mostly soaked up by plants, whereas a few intense storms may be more likely to saturate the soil and increase the recharging effect.
“It’s tempting to say that a doubling of the precipitation will lead to a doubling of the recharge rate,” Ng says, “but when you look at how it’s going to impact a given area, it gets more and more complicated. The results were startling.”
The work was funded by a grant from the National Science Foundation.
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