JFSP PROJECT ID: 14-1-06-18
Ge Sun, Research Hydrologist, Eastern Forest Environmental Threat Assessment Center, Southern Research Station, U.S. Department of Agriculture Forest Service
Dennis Hallema, Hydrologist, Eastern Forest Environmental Threat Assessment Center, Southern Research Station, U.S. Department of Agriculture Forest Service
Fire-prone regions face concerns about the amount and quality of surface water after wildfires, and are confronted with the increased cost of water treatment and flood mitigation. The effect of any particular fire on surface water is often difficult to detect over a long timeframe due to a variety of simultaneous landscape responses and climatic variability. Scientists at the Southern Research Stations of the US Forest Service combined the hydrometeorological and fire data for 168 fire-affected areas in the contiguous United States collected between 1984 and 2013. This enabled them to determine when wildland fires can affect the annual amount of flow in rivers, and to create a suite of climate and wildland fire impact models adapted to local conditions. A number of discoveries were made:
- Large wildland fires that burn more than 19% of the watersheds enhanced annual river flow for at least 5 years, even in areas affected by recurring drought.
- Fires increased river flow relatively the most in the Lower Colorado, Pacific Northwest ,and California regions. In the Lower Colorado and Pacific Northwest regions, flow increased (+128% and +24%, respectively) despite post-fire drought. In southern California, post-fire drought effectively obscured the flow enhancement (>50%) caused by wildfire, meaning that annual yield declined but not as much as expected based on the decline in precipitation.
- Not all fires affected annual river flow. Prescribed burns in the Southeastern United States for example were relatively small in area, and characterized by low burn severity, so the effect on river flow was virtually undetectable.
The continuity of water supply is a challenge in fire-affected watersheds because fire can alter the timing and magnitude of river flow. More river flow is not automatically beneficial to water supply–other JFSP studies show that wildfires can severely degrade water quality for at least several months. Therefore, these new insights into interactions between wildland fire, river flow, climate, and landscapes will need to be integrated into local water resource plans to properly reflect the local needs in each region.