Spatiotemporal patterns of water quality to wildfires in the western United States
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Multiple drought-related wildfires burned forests across the western United States (U.S.) during the 2020 fall season. Wildfires can significantly affect hydrologic processes such as streamflow and nutrient transport to stream, however, the natural variability of its behavior combined with spatially complex landscape patterns and hydrologic factors create uncertainties about the postfire effects on aquatic ecosystems. Because wildfire effects on water quality are less commonly reported than the effects on ecosystems, we investigated the spatiotemporal relationship between landscape patterns and water quality before and after the 2020 wildfire season in the pacific northwest. Specifically, we sought to examine the effect of fire on normalized difference vegetation and water indices (NDVI and NDWI), enhanced vegetation index (EVI), streamflow, and water quality (phosphorus (P), nitrogen (N), organic carbon (OC)) from 2001 to 2022 in two watersheds in Oregon (Archie Creek and South Obenchain). Landscape parameters were based on MODIS remote sensing products, and the water quality data was provided by the Department of Environmental Quality and Environmental Protection Agency in Oregon, US. We used flow exceedance probability, load duration curves, and breakpoint analysis to evaluate the seasonal patterns between nutrient loads and landscape characteristics (NDVI, NDWI, and EVI) for the pre- (2001 – 2020/10) and post-fire (2020/11 - 2022) periods. Post-fire streamflow and nutrient load responses varied seasonally. Streamflow significantly decreased after the fire during fall and summer in both regions. However, increases in winter and spring streamflow were observed in the area with the highest variability in flow regime (Archie). Nutrient loads significantly increased after fire in the winter season in both areas. While increases in N and OC loads during the fall season were observed at Archie creek, a relatively small load increase was observed for N during spring and for OC during summer in South Obenchain. Load duration curves and flow exceedance probabilities suggested a shift towards higher values for OC loads after the fire, with higher load values exceeding drinking water systems treatment capability for TOC in the western U.S. (>4 mg/l). Breakpoint analysis suggested significant shifts in NDVI and EVI after the fire and a considerable declining trend in NDWI starting ~ 2 years prior to the 2020 fire season. The breakpoints for the landscape metrics also aligned with the changes in nutrient loads in both regions. These findings are critical to a more holistic understanding of the spatiotemporal effects of fire in vulnerable areas such as forested headwaters that provide habitat for numerous aquatic species and are the primary source of drinking water in the western US. Our results provide insights into the spatial variability of postfire effects and may support future regional management strategies.
The views expressed in this abstract are those of the authors and do not necessarily represent the views or policies of the U.S. Environmental Protection Agency.