Climate change will affect the local water supply, according to a forum Thursday in Cortez.
The forum was led by climate scientist Seth Arens, part of a group that partnered with the city in the fall. While it’s unclear exactly how climate change will affect Montezuma County’s precipitation levels, he said, rising temperatures directly affect water cycles and overall water supply.
“In a community, there are certain climate issues that might be more prevalent than others,” Arens said. “So in a community like Cortez, drought is certainly a big issue that might come into management issues.”
The presentation came after the city took part in an October workshop organized by Western Water Assessment, a research program based at the University of Colorado Boulder. The workshop and subsequent report focused on the localized impact of drought and rising temperatures, especially considering the dependency of the local economy on tourism and agriculture.
Arens emphasized that despite Southwest Colorado’s exceptionally wet winter this year, drought and rising temperatures remain a threat. Although it’s difficult to project climate change’s effect on precipitation in coming decades, water budget is more trackable.
Runoff – or streamflow – is the result of precipitation post-evapotranspiration, or “the combined loss of water vapor from the soil, water, snowpack and vegetation,” according to the report, “Vulnerability, Consequences, and Adaptation Planning Scenarios,” compiled by the researchers.
The city’s water supply primarily comes from runoff in McPhee Reservoir and the Dolores River. In a “normal” year in the upper Dolores River Basin, the region receives about 30 inches of precipitation; of that, 18 inches returns to the atmosphere due to evapotranspiration, and 12 inches remains as runoff, according to the report.
Last year, though, the region experienced an “exceptional drought” with lower precipitation and higher evapotranspiration levels. The upper Dolores River Basin received only 14.8 inches of precipitation. Of that amount, 12.5 inches returned to the atmosphere, leaving 2.3 inches of runoff – 19% of normal levels.
“It was an extremely low runoff year last year,” Arens said. “It was the most extreme drought in historical records.”
Montezuma County was not excessively warmer during the 20th century, according to the report. But since 2000, temperatures have been more than 1.6 degrees higher than last century’s average, and four of the five warmest years on record for the county have occurred in the past two decades.
Warming trends are expected to persist, according to the climate models Arens presented. Under a lower-emissions scenario, average temperatures, when compared with the late 20th century, are projected to be 4-6 degrees higher by 2050, and 5-7 degrees higher by 2080.
With a higher-emissions scenario, the projections are more drastic – 5-7 degrees higher by 2050 and 6-12 degrees higher by 2080.
The report was intended to be educational and to inform officials about possible measures to address the threat.
Arens said that conservation, rather than alternative water sources like grey water, might be the best option for Cortez.
The city’s per capita water usage has substantially dropped over the past three decades, from 325 gallons per person per day in 1990 to 200 gallons in recent years. However, further conservation is advised.
“This number, even if significantly lower than 30 years ago, is nevertheless still much higher than most cities across the U.S. and the rest of the world,” the report says. “The stated long-term goal of participants is to bring this number down to an average of 180 gallons per person per day by 2022 through conservation efforts and public outreach.”
Cortez Public Works Director Phil Johnson said conservation efforts needed to be applied to the city’s outdoor use, with methods like “xeriscaping,” or landscape design with limited irrigation.
“When we say conserve water, that doesn’t mean that we’re going to be trying to cut back on our healthy use of it,” Johnson said. “We need it to live, and we’re mostly water anyways. But we have to get better at using outdoor use.”