For hydrologist Dennis Hallema, a recent conference presentation in Kelowna, British Columbia, turned into an opportunity to speak about an urgent research issue in front of an even larger audience. Following his talk at the 4th International Conference on Forests and Water in a Changing Environment, Hallema (an Oak Ridge Institute for Science and Education fellow working with the Eastern Forest Environmental Threat Assessment Center) was approached by a producer for CBC Radio One. The following morning, Hallema spoke live on air with CBC host Chris Walker about the effects of wildland fires on water supply in the United States. Their discussion focused on a collaborative study that began in October 2014 with funding from the Joint Fire Science Program. The interview, which was broadcast July 8 on CBC Radio One Daybreak South, is summarized below.
Have we seen more fires in recent years?
Our project funded by the Joint Fire Science Program focuses on the continental U.S., where, since 1984, we have regionally observed more stand-replacing fires in, for example, the Sierra Nevada, Southern Cascades, and Nevada. Burned area has increased in the Southern Rockies and Colorado Plateau. In the eastern U.S., on the other hand, fire severity often is lower due to a transition toward closed-canopy forests with more shade-tolerant and fire-adapted species, and fires are smaller due to stricter suppression requirements of the dense population living in this area.
What do forest fires have to do with water?
Approximately 50 percent of our fresh water comes from forests, so if wildfires impact water supply, we need to know. The persisting drought in the western states has added the question, how does drought affect the link between wildland fires and water supply?
So how do fires affect water? Were you able to identify a relationship between wildland fires and water supply?
One early study in the 1950s in a southern California watershed found a more than 100 percent increase in storm runoff in the year following the 1953 Barrett high-severity fire, and a number studies followed in different areas; however, there is no general picture of how this works and whether there are national patterns in fire effects on water. We know that fire removes the canopy and temporarily reduces forest transpiration. Normally trees use water and transpire water through their leaves, but fire largely eliminates these mechanisms so that more water runs off and increases streamflow for some time, and erosion and landslides may be the result. The situation is, however, more complex than that since high-intensity fires can render the upper part of the soil water-repellent, which can in turn lead to either more or less runoff depending on the conditions of the area. We have found different conclusions for different areas.
How are you studying these areas? What are some of the conclusions?
We used an approach in which we combined large-scale climate data, burn severity data, streamflow data and evapotranspiration data coming from satellites. First we identified candidate watersheds based on burned area and available stream flow data, and after that we analyzed more than 30 watersheds in different parts of the U.S. in more detail. It looks like streamflow was higher than expected based on precipitation and snowmelt in some 15 watersheds (13 of these have had wildfires, one had a prescribed burn, and another had both a wildfire and prescribed burns). In 12 watersheds, we found no change in streamflow, and streamflow was lower than predicted in nine watersheds. We are now using a computer model to explain the mechanisms that cause these differences.
What can British Columbia do to reduce fire impacts on water?
Our project aims to establish the long-term impact of fire on U.S. watersheds by using local observations and large-scale computer models, and this will eventually lead to a scientific report. Policy makers can use this report to decide what strategies to follow, while our team focuses on the scientific questions.
For more information, email Dennis Hallema at email@example.com.