The burning question: How do wildfires impact watersheds?
Article: Maina, F. Z., & Siirila‐Woodburn, E. R. (2020). Watersheds dynamics following wildfires: Nonlinear feedbacks and implications on hydrologic responses. Hydrological Processes, 34(1), 33-50. https://doi.org/10.1002/hyp.13568
For people in the western United States, images of devastating wildfires have become a terrifyingly familiar part of the annual news cycle. While the fires themselves can be dangerous and damaging to the communities where they occur, the impacts are not limited to when the flames are burning. With long-lasting changes to the landscape, fires can also change the way in which water moves through the landscape for months and years afterwards. When thinking about how water moves through an area, scientists often think in terms of a watershed, which is the area of land for which any rain that falls on that surface will eventually drain to the same point, such as a large lake or river.
Understanding processes happening within watersheds and how they could be affected by fires is important since changes to these processes impact water supplies, both for communities that lie within the bounds of the watershed, as well as communities further downstream. Some of the main processes within a watershed involve water entering through both rainfall and snowfall. Water can then go down into the ground and be stored as groundwater, or become runoff at the surface and end up in streams and rivers. Under normal conditions, plants also take up some of the water from the ground in a process called transpiration. This uptake of water, along with the loss of water from the ground that occurs from evaporation as soils dry out, is together referred to as evapotranspiration. However, if there are no plants, such as after a fire, the amount of evapotranspiration will be a lot less.
A model approach
In a study by Fadji Maina and Erica Siirila-Woodburn from Lawrence Berkeley National Laboratory, they explored how a watershed could be impacted by wildfires. Specifically, the scientists investigated the Cosumnes River watershed in California. It is a watershed that is very typical of many in California, extending from higher elevations forested areas in the Sierra Nevada mountains down to the lower elevation agricultural lands in the Central Valley. The Cosumnes River watershed and others like it supply around 70% of California’s water supply.
To understand how the watershed could be affected by wildfires, the scientists used a computer model of the watershed and tested what would happen if fires were to occur in different areas of the watershed. They tested the impacts of these fires for both a dry year and a wet year following the fire. Each simulation produced estimates of how evapotranspiration, snow levels, and groundwater levels would change throughout the watershed following the fire.
Overall, the scientists found that wildfires resulted in lower evapotranspiration, greater snow levels throughout the winter, and higher runoff and groundwater levels during the summer. In general, this means there was more water flowing into the Cosumnes River in the summer and also more water being stored underground. However, these results were also variable across the different areas of the watershed. For example, some areas downslope of burned areas actually showed higher rates of evapotranspiration. The impacts also differed depending on where within the watershed the fire occurred. If the fire occurred in lower elevation areas in the Central Valley, there was not much of a change in evapotranspiration, snow levels, runoff, or groundwater levels. If the fire occurred in the higher elevation areas of the Sierra Nevada mountains, there was much larger change in the metrics.
A fiery future
As we continue to feel the impacts of climate change more and more, studies such as this will be increasingly important. The future will likely hold longer droughts, higher temperatures, and more frequent and bigger wildfires. However, large-scale experiments of impacts from events like wildfires are not things that can be tested experimentally, so studies like this that use computer models are a valuable tool for trying to determine what could happen in the coming years. All of this could be putting our watersheds and water supplies at risk, so the more we can understand about the impacts the more we can do to mitigate and adapt to them. In places like California where most of our water supply comes from places that will likely be hotter, drier, and likelier to experience wildfires in the future, we need to take action to ensure a sustainable water future. Understanding how the amount of water flowing out of watersheds and the timing of those flows will help us to make better decisions about how we use and store water in the future.