[Editors] MIT: Climate change could affect water supplies dramatically

[Elizabeth Thomson]

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MIT finds climate change could dramatically affect water supplies
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For Immediate Release
WEDNESDAY, DEC. 17, 2008

Contact: Elizabeth A. Thomson, MIT News Office
E: thomson at mit.edu, T: 617-258-5402


CAMBRIDGE, Mass.--It’s no simple matter to figure out how regional  
changes in precipitation, expected to result from global climate  
change, may affect water supplies. Now, a new analysis led by MIT  
researchers has found that the changes in groundwater may actually be  
much greater than the precipitation changes themselves.

For example, in places where annual rainfall may increase by 20  
percent as a result of climate change, the groundwater might increase  
as much as 40 percent. Conversely, the analysis showed in some cases  
just a 20 percent decrease in rainfall could lead to a 70 percent  
decrease in the recharging of local aquifers — a potentially  
devastating blow in semi-arid and arid regions.

But the exact effects depend on a complex mix of factors, the study  
found — including soil type, vegetation, and the exact timing and  
duration of rainfall events —  so detailed studies will be required  
for each local region in order to predict the possible range of  
outcomes.

The research was conducted by Gene-Hua Crystal Ng, now a postdoctoral  
researcher in MIT’s Department of Civil and Environmental Engineering  
(CEE), along with King Bhumipol Professor Dennis McLaughlin and  
Bacardi Stockholm Water Foundations Professor Dara Entekhabi, both of  
CEE, and Bridget Scanlon, a senior researcher at the University of  
Texas. The results are being presented Wednesday, Dec. 17, at the  
American Geophysical Union’s fall meeting in San Francisco.

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.

Predictions of the kinds and magnitudes of precipitation changes that  
may occur as the planet warms are included in the reports by the  
Intergovernmental Panel on Climate Change (IPCC), and are expressed as  
ranges of possible outcomes. “Because there is so much uncertainty, we  
wanted to be able to bracket” the expected impact on water supplies  
under the diverse climate projections, Ng says.

“What we found was very interesting,” Ng says. “It looks like the  
changes in recharge could be even greater than the changes in climate.  
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 rainfall occurs in winter or summer. “Changes in  
precipitation are often reported as annual changes, but what affects  
recharge is when the precipitation happens, and how it compares to the  
growing season,” she says.

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 says. “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 very little percolates down to  
the aquifer. Similarly, it makes a big difference whether an overall  
increase in rainfall comes in the form of harder rainfalls, or more  
frequent small rainfalls. More frequent small rainstorms may be mostly  
soaked up by plants, whereas a few more intense events 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,  
as part of the Information Technology Research Program.

--END--

Written by David Chandler, MIT News Office
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