Geoscience Reference
In-Depth Information
increased flood risks
Ironically, even as drought has become the most pressing concern in the
American West, climate scientists and hydrologists are warning about
increased l ood risks throughout the region. Climate researchers Tapash
Das, Mike Dettinger, Daniel Cayan, and Hugo Hidalgo have described
how, even in a West with reduced precipitation, warming conditions
can lead to more frequent l ooding in mountain watersheds. Only a few
degrees in temperature can determine whether precipitation falls as rain
or snow over a watershed. In the mountains, as we have seen, these few
degrees translate to a shit of several hundred feet in snow-line elevation.
As temperature across the region rises, the total area above the snow line is
reduced, with a greater area receiving precipitation as rain. h e increase in
rain during the short winter months generates runof that will overwhelm
l ood control systems and overtop the banks of river channels, leading to
l oods. A particularly hazardous combination includes early winter storms
that drop snow in the mountains followed by early spring storms that bring
warm rains in these higher elevations, resulting in rapid and catastrophic
snowmelt.
Das's research team has simulated l oods on the western slope of the Sierra
Nevada, using projected temperature and precipitation patterns and assum-
ing that greenhouse gas emissions will increase throughout the twenty-i rst
century—the so-called “A2” scenario. h eir results indicate that, between
2050 and 2100, both the northern and southern Sierra Nevada will experi-
ence larger and more frequent l oods. h e causes include increased frequen-
cies of storms, greater amounts of precipitation, and a greater proportion of
precipitation falling as rain rather than as snow.
Mike Dettinger has also independently explored the possible ef ects that
warming in the twenty-i rst century could have on atmospheric river storms.
His analysis shows that, under the A2 greenhouse gas emissions scenario, the
nature of atmospheric river storms may change, with increasing frequency
of such storms and increases in water vapor transport rates as compared to
the historical record. Predicted storm temperatures will be higher, meaning
more rain will fall during these events. Dettinger is concerned that these
changes will lead to more frequent and larger l oods in the future. He pre-
dicts a lengthening of the atmospheric river storm season, meaning l oods
could potentially occur over a longer portion of the year. h is will make it
dii cult for water managers to operate reservoirs, where they are balancing
