DISCUSSION: Wildfires have a dramatic impact on the landscape. Obviously, fires clear vegetation, which can be clearly seen in the picture above which shows the landscape after the Woolsey Fire in Southern California (U.S.). The burn scar (brown area) is so large, it can be viewed from space.
The effects of fires on the landscape subsequently impacts how water interacts with the surface (i.e., hydrology). Without vegetation to hold soil in place, rains and their resulting runoff tend to erode more soil away. This can deposit more sediment into streams and rivers, reducing their capacity to carry water, and increasing flood risk. Under normal conditions, some rain soaks into the soil. This reduces the amount of runoff and reduces downstream flood risk. However, the intense heat from fires can actually cause soil grains to fuse together, reducing the amount of water that can soak into the soil. This effect increases runoff and flood risk. A recent study found that not all fires cause soil to repel water to the same degree. This water repellency depends on fire characteristics (e.g., intensity, duration), how much water is already in the soil, and the composition of the soil itself. For example, a fire that burns hotter and/or longer may increase the water repellency of soil after a fire. In addition, the study found that the soils can still be water repellent after more than a year. Thus, it may take a while for the soil to recover to pre-fire conditions.
Fires can also impact local precipitation. For example, less vegetation after a fire results in less transpiration (evaporation from plants), potentially less water vapor in the air, and thus, less precipitation. However, another recent study found that measured snowfall at the ground actually increases after a fire. Prior to a fire, some snow is intercepted by trees. A large fraction of this intercepted snow sublimates (turns from solid to gas) before reaching the surface. In contrast, after a fire, more of the snow reaches the surface, and less sublimates. In the western U.S., a large fraction of water resources comes from melting snow in the mountains. Thus, a beneficial impact of fires is an increase in available fresh water from snowmelt.
In summary, wildfires can exert both positive and negative impacts on local hydrology and water resources. They can increase the amount of fresh water available from snowmelt. But, they can also increase runoff, erosion, and flood risk. It is likely that the negative consequences of fires outweigh the positive in this case. Hence, it is probably NOT a good idea that we start fires in the mountains in an attempt to increase available water.
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©2019 Meteorologist Dr. Ken Leppert II
DISCUSSION: On the night of October 23, 2019, a fire broke out in Sonoma County in California near the town of Geyserville. The cause of the fire is still yet to be determined but it has grown to 25,000 acres by October 26 before exploding to over 75,000 acres over the weekend. The fire was officially contained on November 6 after burning 77,000 acres. Among the main reasons the fire grew rapidly was due to a ridge of high pressure that was dominant over the Pacific Coast which brought dry and warm air into Northern and Central California. In addition to the dry warm air, the fire was aided by strong winds from the mountainous regions of the Sierra Nevada due to the sinking air that is often affiliated with ridges of high pressure.
The fire led Pacific Gas & Electric (PG&E) to implement power outages in the mountainous and hilly regions as a precaution due to prior fires involving their electric lines. PG&E took these steps as the National Weather Service (NWS) in Monterey issued red flag warnings over most of the Bay Area due to wind speeds that were expected to be up to 30 miles per hour with gusts up to 55 mph in some spots as well as the lingering dry and warm air. A red flag warning is issued to alert fire agencies that the weather will likely help spark and develop fires due to wind, low humidity and high temperatures.
The winds decreased over the weekend and the temperature followed suit during the week, however, the humidity remained low due to a trough of low pressure that came in from the polar regions. The smoke of the fire also has led to the air quality in the Bay Area to worsen that Spare the Air alerts have been issued by the Bay Area Air Quality Management District since the smoke reaching and remaining in the lower levels of the atmosphere due to the winds. The decrease in wind also helped with the containment of the fire. In California, major fires often occur during the fall season with recent ones including the October 2017 outbreak with the Nuns, Tubbs, Atlas and Redwood Valley Complex fires and the Camp Fire in November 2018.
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© 2019 Meteorologist JP Kalb