From Colorado Public Radio (Michael Elizabeth Sakas):
The North American monsoon has returned to Colorado, and the rain has brought some much-needed relief to some of the driest parts of the state — after multiple back-to-back years of almost no summer rain…
The seasonal moisture from the tropics creates afternoon cloud cover that protects his drought-stricken creeks from baking in the sun. The rain helps lower the risk for wildfires.
The timing of the monsoon is vital to Colorado’s ecosystem, which evolved on its schedule. Science suggests that climate change is making these rains less helpful.
Colorado’s Western Slope has been labeled a climate hot spot, where average temperatures are increasing faster than the global average. A climate change-fueled megadrought has plagued the state and parts of the West for 20 years…
[Bill] Trampe said hotter days and four years of almost no monsoon rain turned into “horribly dry” conditions during the summer of 2020. Many ranchers in Gunnison couldn’t grow enough hay to feed their cattle, which meant they had to buy it — a pricey added expense, he said.
Trampe was worried the land was too dry to feed the number of cattle in his herd, so he was forced to slaughter more animals in the fall than he planned. He also sold cattle over the winter, something he doesn’t usually do. Trampe said he was fearful 2021 would be another dry year…
While Trampe can grow hay on his private land using irrigated water from the Gunnison River, his cattle graze on federal land. Trampe said the federal rangeland is running out of water. “These rains are super important,” he said.
Monsoon rains are getting less reliable and less effective. Recent research from the Desert Research Institute in Nevada has found that less rain is making it into rivers as the climate warms.
This deficiency starts with snowpack, which collects in the mountains during the winter, said Rosemary Carroll, an associate professor of hydrology and the lead author of the study. If a big snowpack lasts into the summer, the soil can stay moist if temperatures are cooler.
Those conditions would mean rainfall would more likely reach a stream. If the snowpack is low, temperatures are higher and the soil is dry, less monsoon rain makes it into the stream, Carroll said. With climate change, an average water year can look more like a moderate drought.
Carroll stood on the edge of a high dirt road in Gothic, Colo., overlooking a winding tributary that eventually feeds water to the Colorado River, which supplies 40 million people in the Southwest. The nation’s two largest reservoirs are on this river — Powell and Mead. Those levels are dwindling to record lows, and climate change means less water is making it into the system.
Carroll is worried about where things are headed.
“It would be hard not to be,” she said. “With decreasing snowpack, warmer temperatures and now the volatility of the monsoon itself, this is just not great news for everybody.”
Next, the team explored the ability of these summer rains to produce streamflow during cool years with high snow accumulation, and during warm years with less snow accumulation. During cool years with more snow, soil moisture levels were higher going into summer, and greater streamflow was generated by the monsoon rains. During warmer years with low snowpack, dry soils absorbed much of the monsoonal rains, and less runoff made it to the streams.
“You can think of the soil zone as a sponge that needs to fill up before it can allow water to move through it,” Carroll said. “So, if it’s already depleted because you had low snowpack, the monsoon then has to fill it back up, and that decreases the amount of water you actually get in the river.”
As the climate warms, snowpack in the Rocky Mountains and other mountain systems is expected to decline, leading to reduced streamflow. Rising temperatures also lead to increased soil evaporation and increased water use by plants. According to the results of Carroll’s study, these changes will reduce the ability of water from the monsoon to make it to the river as streamflow.
“Our results indicate that as we move toward a climate that is warmer and our snowpack decreases, the ability of monsoon rain to buffer these losses in streamflow is also going to go down,” Carroll said. “So, the monsoon is not some silver bullet that is going to help mitigate those changes.”
Research from the University of Arizona finds dry periods are lasting longer across the West as the climate changes. When rain does fall, it often comes in fewer and larger storms, which hurts grasses with shallow roots that need a steady supply of moisture.
Even if the total amount of rainfall is the same during a growing season, it’s not as helpful if it comes in the form of occasional large rainstorms, said Joel Biederman, a research hydrologist with the U.S. Department of Agriculture who co-authored the paper.
Without consistent rain, dry plants become fuel for wildfires.
Separate research from the University of Arizona suggests that wildfires are harder to rein in without monsoon rains, which effectively end the fire season after July. Tree ring studies in neighboring states like Arizona and New Mexico link years that have weak monsoons to widespread fires in mountainous areas.