A lot of rain and afternoon thunderstorms are typical of the summer monsoon season here in Colorado. Monsoon? In Colorado? When you hear the word monsoon, you probably think of the heavy rains in India during the summertime. While The North American monsoon is not as strong or persistent as its Indian counterpart due to overall lower topography of the Mexican Plateau as compared with the Tibetan Plateau in Asia, it shares the same basic characteristics: seasonal reversals of wind direction due to temperature differences between land and ocean leads to big changes in weather patterns.
Winds blow from areas of high pressure to low pressure. The difference in temperature is what causes the difference in pressure. In the summer, the desert Southwest of North America is extremely hot compared with the surrounding areas. As hot air rises, low pressure is created over Arizona, New Mexico and northern Mexico. The adjacent air is relatively cooler, compared with the hot desert air, creating surrounding regions of high pressure. Air in these high-pressure regions, like from the Gulf of California, eastern Pacific and Gulf of Mexico, get drawn into the region of low pressure, bringing along plenty of moisture.
While monsoons play a role in reducing wildfire threat by providing moisture, this has been an abnormal year of flooding events in Colorado due, in large part, to last year’s historic wildfire season. It takes just half an inch of rain in an hour on steep terrain to cause flash flooding in burn scar areas. Lack of vegetation – which aids water absorption and distribution in the soil – is one factor. But there’s another that is not as obvious or well-known.
When organic materials – like plant leaves, twigs, branches and needles, collectively called “litter” – burn at very high temperatures, they can turn the underlying soil “hydrophobic”, making it repel water – like pavement – instead of absorbing it. Waxy water-repellant compounds derived from plant material that get burned during a hot fire initially get vaporized, with the gas forced into the soil by the heat of the fire. Soils with lots of pore space between the grains (like sand) are more susceptible to formation of hydrophobic layers because they transmit heat more easily than soils with much less pore space (like clay). These compounds penetrate into the soil as a gas, but they eventually re-solidify after cooling, forming a waxy water-repellant coating around soil particles a few inches down from the surface. During a rainfall event, the top layer of soil becomes saturated with water because it cannot penetrate the lower hydrophobic layer, leading to rapid and catastrophic mobilization of debris material downslope. Not all wildfires create water-repellent layers. The factors that lead to this formation include a thick layer of plant litter prior to the fire, high-intensity fires, prolonged periods of intense heat, and coarse soils. During the first few years after a wildfire, according to the U.S. Forest Service, rain can erode steep terrain between 24 – 40 tons per acre each year.
More reading here: https://www.fs.fed.us/rm/pubs_journals/2015/rmrs_2015_pierson_f001.pdf
photo: while driving in western Colorado, trying to get home via I-70, which was closed in parts due to mudslides.