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February
16, 2007 Even at the best of
times, the
West's water supplies are fraught with political, economic and
environmental
wrangling. When devastating droughts occurred in the 1970s and the
2000s,
farmers and fish alike suffered. Yet the ability to predict stream
flows in the
Forecasting models
that
incorporate high-powered computers and satellite data may soon
modernize the
way Western states manage freshwater supplies. Several such models are
currently under development. Dennis Lettenmaier, professor of civil and
environmental engineering at the UW, will describe the role of science
in
Western water management Friday in A half-century ago,
resource
managers would ski or hike to mountain stations and measure the amount
of water
stored in the snowpack. They took a metal tube and inserted it in the
snow,
then weighed the tube to calculate how much water it contained. Today's
electronic systems automate this process, but use a similar principle,
Lettenmaier said. "If you know how
much snow
is on the ground in the spring, you have a pretty good idea of how much
runoff
will occur during the spring and summer," Lettenmaier said. "That's
something that's been used for a long time. The question is: can we do
better
than that?" A new generation of
hydrologic
forecasting models integrate not only scattered, ground-based
measurements of
snow depth, but also satellite measurements of snow extent. The The overall aim is
to provide
computerized water forecasts equivalent to modern weather-prediction
models.
The new forecast methods incorporate a wealth of other climate
information to
produce results earlier in the season, more accurately and for
situations that
are outside the norm. These methods recalculate conditions every day by
incorporating satellite images of snow cover and computing the
influence of
that day's temperature and precipitation. Forecasts based on
physical
processes avoid the problems inherent in statistical forecasting
methods that rely
on historical patterns. For example, after unusually heavy snowfall in
the
Southwest in 2003, traditional forecast models predicted that the
spring and
summer runoff in "It's a classic
problem of
extrapolating a line out past the end of the observations," Lettenmaier
said. When current conditions don't look like anything previously seen,
methods
that are too closely related to historic patterns can fail. Water managers are
beginning to
feel a crunch related to climate change, Lettenmaier said. Springtime
melt now
starts some 20 days earlier than a half-century ago, which is "pretty
unequivocally" seen as a signature of climate change, he said. The
shift
results in a bigger gap between when the fresh water flows down from
the
mountains and when it actually is most needed in the height of summer.
Climate
change constitutes an additional challenge, on top of factors such as
population movement, agriculture changes and water use changes that
managers
must contend with. Knowing the amount
of water ahead
of time lets people prepare for droughts or flooding. Building more
reservoirs
would help, in particular to handle earlier runoff, but the West is
unlikely to
see any more dams built, Lettenmaier said. Instead, people can use
forecasts to
decide which crops to plant, whether to drain reservoirs to prepare for
flooding and how to allocate water resources early in the season.
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