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