Warm spring renews threat of megadrought
LOVELAND PASS» Here at 12,000 feet on the Continental Divide, only vestiges of the winter snowpack remain, scattered white patches that have yet to melt and feed the upper Colorado River, 50 miles away.
That’s normal for mid-June in the Rockies. What’s unusual this year is the speed at which the snow went. And with it went hopes for a drought-free year in the Southwest.
“We had a really warm spring,” said Graham Sexstone, a hydrologist with the U.S. Geological Survey. “Everything this year has melted really fast.”
The Southwest has been mired in drought for most of the past two decades. The heat and dryness, made worse by climate change, have been so persistent that some researchers say the region is now caught up in a megadrought, like those that scientists who study past climate say have occurred here occasionally over the past 1,200 years and lasted 40 years or longer.
Even a single season of drought is bad news for the Southwest, where agriculture, industry and millions of people rely on the region’s two major rivers, the Colorado and the Rio
Grande, and their tributaries for much of their water. Dry conditions also shrivel crops, harm livestock and worsen wildfires.
But droughts, even long ones, eventually end, when the natural variability of climate results in a few “good,” meaning wet, years in a row. So after a relatively cool and wet spring last year followed by a decent snowpack in the fall and winter, there was some optimism that 2020 might be remembered as the year the long Southwestern drought started to fade.
But then came April and May, which were warm and dry, leading to rapid melting and runoff.
Normally, Sexstone said, measurements of stream flow at gauges in the region would slowly climb to a peak and then drop off gradually as the season progressed.
“This year it seemed like it peaked and then plummeted,” he said.
Becky Bolinger, a drought specialist at Colorado State University and the assistant state climatologist, said the lack of new snow in late spring affected the rate of melting. As snow is exposed to the sun it warms and nears the melting point. If new snow falls, that lowers the temperature, stalling the process. But without any new snow, the melting continues unimpeded.
Couple that with soil that is exceptionally dry and vegetation that is thirsty, and the result is that less of the snowmelt at places like Loveland Pass ends up in the Colorado and, eventually, in reservoirs along the river, like Lake Powell on the Utah-Arizona border.
It’s not just the basins west of the Continental Divide that have experienced severe drought made worse by warming. A study published in May about the country’s largest river basin, the Upper Missouri, where snowmelt on the eastern side of the divide at Loveland Pass eventually ends up, showed that warming has affected runoff over the last few decades and increased the severity of droughts, including one from 2000 to 2010.
In its latest analysis, the monitoring group reported that the southern half of Colorado, northern and eastern New Mexico, northern Arizona and nearly all of Utah were in moderate to extreme drought, with varying degrees of water shortages and crop and pasture damage.
Drought can be complex, a function not only of high temperatures and lack of precipitation but also of factors like humidity, wind and cloud cover. Soil moisture and evaporation of water from the ground surface and from the leaves of vegetation, a process called transpiration, are important.
Dust that settles on snow can have an impact, by absorbing sunlight and warming, which speeds melting. And sublimation, by which a solid (snow) directly becomes a gas (water vapor), bypassing the liquid phase (water), plays a role as well.
But scientists are still learning how these various factors interact, and the relative importance of each. In some cases there is little data to analyze, and much of the research relies on computer models.
There are relatively few direct measurements of soil moisture, for example, even though it can greatly affect runoff as it likely did this year in the Southwest.
Soils were already very dry last fall, Bolinger said, because the annual latesummer rains in Arizona,
New Mexico and Southern Colorado largely failed to materialize.
As winter set in, the soil froze, remaining dry while the snow built up. Once the snow began to melt, the soil had to be replenished first, Bolinger said.
Sexstone’s work to better understand snowpack is part of an effort within the geological survey to more accurately quantify and forecast runoff, given uncertainty about water supplies in a warming and more drought-prone world.
“We’re looking at more intensive monitoring” within the Upper Colorado Basin, said Suzanne Paschke, who manages a project at the geological survey’s Colorado Water Science Center. Installation of advanced sensors to measure snow and other characteristics like soil moisture is expected to begin next year.
Most current snow measurements come from a network called SNOTEL, first established in the 1960s. It now includes hundreds of sites around the West.
While the SNOTEL network provides invaluable data about snow depth and how much water it holds, Sexstone said, the sites are below tree line and the system was developed when much less was known about what affects snowpack. Scientists have since realized that snowpack and runoff are a lot more complicated.
“Now we’re starting to say, OK, how do we account for all this other stuff?” he said.