There’s bad news but also some good news about the impending impacts of climate change on Southwest Colorado and the Four Corners, according to a researcher who gave a presentation in Cortez on April 11.
The bad news is that the area is going to grow warmer in coming decades even under the most conservative scenarios for climate change.
The good news is that this may not mean less overall precipitation. In fact, there could be more.
Seth Arens, a research integration specialist with Western Water Assessment, told the audience that data specific to Montezuma County indicates that by 2035 the area will be an average of approximately 4 degrees Fahrenheit warmer than it is now. That’s under either high or low-emissions scenarios for greenhouses gases. By 2050 the area is projected to be 5 to 6 degrees warmer, and by 2100 it could be 6 to 9 degrees hotter.
“We are currently on a high-emissions trajectory unless we take seriously reducing emissions of greenhouse gases,” he said.
Climate models are complex and although very reliable at projecting temperatures into the future, they are much less certain about predicting precipitation, especially in the Four Corners, Arens said.
“Climate models are a little less certain about precipitation in general but here in this region specifically they are even a little less certain,” he said.
While the Northwest is reliably expected to grow wetter and southern Arizona to grow drier, the Four Corners is “in one of these zones where it’s in between so precipitation is little less certain for this region, as it is for Utah and the intermountain West.”
He said so far there is “no discernible trend” in precipitation for Montezuma County. Historically it has varied widely from year to year and that continues to be the case.
2018, for instance, was a year of historic drought. A typical year in the upper Dolores River Basin sees about 30 inches of precipitation, Arens said, 18 inches of which typically leaves through evapotranspiration (evaporation and plant transpiration), leaving 12 inches as runoff.
During a drought year there is less precipitation and increased evapotranspiration; a severe drought year may have just 20 inches of precipitation, with 14 of that leaving through ET, meaning just six inches left for runoff, or half of normal. This is a common occurrence, Arena said. Even though the area is receiving two-thirds of its normal precipitation, just half the typical moisture is available as runoff because soils are extremely dry.
In 2018 in the Four Corners, he said, 14.8 inches of precipitation fell – less than half of normal – and 12.5 inches was lost to ET, meaning a scanty 2.3 inches of runoff. That represented less than a fifth of the normal amount.
“By all historical records it was the most extreme drought. . . since about 1900,” he said, the severest drought on record for this region.
Now, the area is coming off a bountiful winter that had numerous snowstorms. McPhee Reservoir is set to fill, there may even be a small managed release for boaters, and for the first time in its history, Purgatory Ski Resort is going to be open in May.
But that’s just a blip in time, and people have to consider the big picture, he said. Southwest Colorado has been in a drought since about 2000.
“It’s been an exceptionally wet winter and I think that’s great from our perspective,” Arens said, “but that doesn’t change the fact that there has been nearly 20 years of drought.”
Precipitation is only part of the picture, he said. Temperature is a factor, and as it increases, so does ET. Since 2000, the area has been about 1.6 degrees warmer than its 20th Century average. The rising temperatures, of course, aren’t just happening locally, but around the world.
Arens said greenhouse gases, which trap heat in the atmosphere, are at their highest levels in at least 800,000 years. “Man is making an impact on climate in this world,” he said.
As a young graduate student in 2001, he and others had to measure carbon dioxide in the atmosphere. At that time it was about 362 parts per million. Now it’s at 412 ppm. All this warming will affect the precipitation picture.
“Warming alone has an impact on drought and water availability,” Arens said. It means more rain but less snow will fall, especially during the “shoulder seasons.” Peak runoff arrives earlier and there are reduced annual flows in the Dolores River Basin.
Arens said there is enormous variability in streamflows in the Dolores River from year to year, but since 2000 they have been 21 percent below the 20th Century average.
However, climate modeling can’t predict with any reliability what that will mean for overall moisture in the area, he said. There is expected to be an overall decline in snowpack of 10 to 30 percent over the next 30 years, he said. Runoff will decrease and there will be a change in the timing of runoff.
However , warmer air can hold more moisture, so it’s possible annual precipitation will increase. But weather extremes and severe events are also going to be increasing.
Over the next 30 years there will probably be another one to four severe droughts lasting multiple years, he said.
Heat waves are likely to grow in severity and frequency along with droughts, and there will be more wildfires. And more extreme precipitation events are likely, leading to flooding, debris flows and landslides.
In answer to a question, Arens said groundwater levels in the Colorado River Basin are likely to fall, but he doesn’t expect people’s wells to dry up entirely. “Groundwater is an understudied part of the picture,” he said.
He was also asked about the dust-on- snow phenomenon that causes early melting in the San Juan Mountains. Arens said this is one of just one or two places in the world “where dust deposition on snowpack has a tremendous impact on water availability and timing of runoff.”
“The very high mountain range here acts as a catcher’s mitt for anything blowing from the west,” he said. The Colorado Plateau in Utah has a great deal of terrain with loose dust that can be picked up by winds, partly because of livestock-grazing over the last century. “That makes those soils more available for transport by wind, and the San Juan Mountains provide a place to catch that.”
Dust “completely changes the energy balance of that ecosystem” because it doesn’t reflect light as well as snow. Just as wearing a black shirt on a 100-degree day is hotter than wearing a white shirt, a blanket of dust increases heat.
“Dust isn’t black but it’s much darker than snow so much more radiation from the sun can be absorbed,” Arens said. That means the snowpack melts sooner and overall there is less water available in the rivers.
That’s certainly a concern for Cortez, which has no source of water but the Dolores River and McPhee Reservoir. The city brought together a group of elected officials, citizens, and staff “to promote the long term, sustainable development and wise use of water,” according to a statement on its website, and partnering with Western Water Assessment is part of that effort. Arens’ presentation was one of several presentations the city offered in April on water and wise water usage.
City Public Works Director Phil Johnson said at the forum that the city has no target for reducing water consumption, but would like to whittle down per-capita usage, currently at 200 gallons per day. In contrast, the city of Santa Fe, N.M., uses about 90 gallons per capita per day.
Johnson said any reductions will be mainly in use for lawns and landscaping, not in water used in the home.
Arens was asked whether there will still be snow for skiing by the year 2100.
“I sure hope so,” Arens said, adding that he is teaching his 8-year-olds to ski. He said there should be snow at higher elevations by the end of the century, but lower-elevation sites will be less viable.
The city has more information on its website, www.cityofcortez.com. Click on the Water Is Our Future tab.