Expect it to rain less but when it does, watch out

By Phoebe Sweet

Thursday, Jan. 10, 2008 | 2 a.m.

Residents of the Southwest have heard the refrain that droughts caused by global warming will worsen the region’s already serious water shortage.

But hotter, drier weather won’t be the only way that climate change affects the water supply, according to a report released last month by the nonprofit advocacy group Environment America.

It also will mean stronger, albeit less frequent, storms rather than light rain throughout the year.

The study found that extreme downpours have increased 29 percent in Nevada in the past 60 years, and 25 percent overall in the Mountain West.

Scientists say these increased deluges will hurt, not help, the long-term water picture.

Not only could they mean more flooding, soil erosion and pollution runoff into waterways, but also less ground-water recharge, the process in which water soaks into the ground and moves to deeper layers to replenish aquifers.

So an increase in the number of downpours does not necessarily mean more water will be available.

“Scientists expect that extreme downpours will punctuate longer periods of relative dryness, increasing the risk of drought,” the report said. “In the Southwest, for example, total annual precipitation is projected to decline amplifying the impact of periods of little rainfall between heavy storms.”

Exacerbating the condition in Las Vegas, which gets most of its water from Rocky Mountain snowpack via the Colorado River, as temperatures rise, precipitation will become increasingly likely to fall as rain rather than snow, the report said.

“That will mean the flows of rivers like the Colorado are likely to become more variable, with spring runoff coming sooner and less snowpack available to keep river flow consistent over time,” said Travis Madsen, a policy analyst for the research and policy think tank Frontier Group and an author of the report “When It Rains It Pours.”

A spokesman for the Southern Nevada Water Authority said the agency could not comment on the report before analyzing its underlying data. He did say, however, that the authority “is keenly aware of the implications of climate change not only on the amount of precipitation our watershed receives, but the frequency, form and intensity of that precipitation.”

“We applaud research efforts that help us better understand, prepare for and to the extent possible avert the consequences of climate change.”

Madsen said 40 states showed statistically significant increases in deluges over the past 60 years, meaning this problem will be faced nationwide.

“Climate change means we have to worry not only about how much water we get, but how we get it,” said Peter Gleick, president of the Pacific Institute, a nonpartisan think tank in California. “If we got more water where it’s dry and more droughts where it’s really wet, that might not be a bad thing. But that’s not what’s going to happen. The last thing we want in the West is more extremes.”

Gleick said global warming and accompanying extreme weather will mean droughts and floods.

The increase in extreme rains is caused by two factors, according to the report. Increasing temperatures of land and oceans cause water to evaporate faster, and rising air temperature allows the atmosphere to hold more water vapor. Clouds become richer with water, making downpours more likely.

Kenneth Kunkel, acting chief of the Illinois State Water Survey, whose research was used in the report, said it is not certain that the increase in extreme rains is linked to global warming or that water supplies will be affected by the increase in extreme rains.

But he is certain the increase could mean more loss of life and property from floods, like one near Reno this week that forced hundreds of people from their homes after an irrigation canal gave way.

“Our exposure to the risk of flood is there and hasn’t gone down over time,” he said.

During the 20th century, floods caused more property damage and loss of life than any other natural disaster in the United States, the report said.

Kunkel said the report’s findings that extreme rains have increased and will continue to do so echo conclusions from the Intergovernmental Panel on Climate Change and are consistent with those across the globe.