CSU Department of Atmospheric Science: less water gathering in Colorado reservoirs due to higher temperatures

The year 2012 was confirmed as the warmest year on record. Have rising temperatures had significance in the lack of snowfall this year and what does the lack of snow mean for Colorado?

“It meant a slow start to the 2012-2013 winter recreation season and a bunch of nervous urban and agricultural water managers concerned about the coming irrigation season — spring and summer,” Professor Nolan Doesken, state climatologist at the Colorado Climate Center and professor in the Department of Atmospheric Science at CSU, said in an email to the Collegian.

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“In Northern Colorado we are at 25 to 89 percent of the normal (30 year average) snowfall, as of January,” said Steven Fassnacht, a professor in the CSU Department of Atmospheric Science.

“The Cache la Poudre is at 70 to 89 percent, but other nearby areas have less snow; the headwaters of the South Platte have less than 50 percent,” Fassnacht said.

According to Doesken, December snows were enough to blanket the western and southern mountain valleys of Colorado, resulting in a mid-winter freeze.

“Heat and drought go hand in hand during the summer months, but that association is more complicated during the winter,” Doesken said. “What heat does do — and we saw that clearly in 2012 — is hasten the melt of our mountain snowpack.”

“Last year, considerable amounts of snow were already gone by later March weeks before the high mountain snow melt usually begins,” Doesken said. “That means an earlier and longer wildfire season. It also means the potential for higher evapotranspiration rates.”

Colorado is home to five million people who rely on snowmelt to survive. Warming temperatures cause the snowpack in the mountains to melt earlier in the season, leading to more of the water being evaporated back into the atmosphere and less water accumulation in the reservoirs.

“Two things: One is you could have more or less snowfall out of the sky but the other thing is really the warming is more about how much of that snowpack winds up getting into the reservoirs and staying in the reservoirs,” said Scott Denning, professor in the CSU Department of Atmospheric Science.

“If it’s a lot warmer more of the snow will evaporate or melt and run off earlier so it doesn’t fill the reservoirs in the spring,” Denning said.

As the climate warms, the result is less water even if there isn’t less snow because the extra warming evaporates the water before it ever gets to us.

“Last year was a poor snowpack year and it was very warm,” Denning said. “But the year before that was a huge snowpack year.”

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Denning went on to relate that in the future, “it’s going to get warmer because of all the extra CO2 and that will tend to melt and evaporate more of the snow before it gets into our reservoirs.”

Although winter is still upon us, the chance of reaching an average water accumulation by spring is becoming less likely.

“We’re only at about the halfway point of the winter snow accumulation season now, but in terms of water supply, we’re far enough below average, only about 60 percent of average statewide, that the chances of recovery to normal by April when our high elevation snowpack usually reaches its maximum water content are now slim — one chance in 10,” Doesken said.

One look at Horsetooth Reservoir and the impact of last year’s dry, hot climate becomes evident. According to Doesken, the water levels have dipped below average throughout most of Colorado.

“Since last year was dry and hot, our reservoirs have fallen below average over most of the state,” Doesken said.

“We don’t necessarily need a really wet year to get back to normal — but it would be good if we could at least be close to ‘average.’ But with another week of dry, sunny weather forecast, the chances of catching up are getting lower.”

Doesken said we simply cannot tell if we’ll continue dry, revert to near average or experience wetter than average conditions.

“The climate is naturally highly variable and often swings back and forth. Many drought periods in the past ended with floods or very wet weather,” Doesken said. “So while we don’t know what will happen this year, from past experience we can tell it could be interesting.”

Collegian Writer Cassandra Whelihan can be reached at news@collegian.com.