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 ground water actually may 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
ground water 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, 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 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, 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."
