Multi-decadal Post-fire Water Quality Trends in California Lakes and Reservoirs

Abstract:

Wildfires can have a significant effect on water quality. Immediate impacts on lakes and reservoirs used for drinking water supply during active burning include ash contamination. Longer-lasting impacts include increased watershed flooding and erosion potential post-burn, and changes in the mobilization and reactivity of organic matter, sediments, nutrients, and other materials in. In California, just under 6 million acres of public and over 7 million acres of private land have a high fire risk and a high water supply importance. Remote sensing via satellite imagery is the ideal method of recording long-records of both water quality and fire history because it can cover large geographic extents with high temporal coverage with the use of multiple satellites, including in areas that are typically inaccessible. This study quantifies the multi-decadal trends in water quality trends in California lakes and reservoirs and trends in the occurrence, frequency, and intensity of wildfires in their adjoining watersheds. This study focuses on turbidity, chlorophyll-a and cyanobacteria derived from multiple satellite datasets including Landsat, MERIS, Sentinel-2/3 with a combination of publicly available GIS data. Satellite remote sensing products were used to assess trends in watershed wildfire occurrence, duration and frequency, fuel type, wildfire burn area, and intensity, as well as other important watershed characteristics such as topography, tree cover, impervious surfaces, soil type, and post-burn recovery. Trends in algal blooms and lake clarity vary by region and there is some correspondence between water quality trends and wildfire occurrence. An influx of sediment and charred vegetation from post-fire rain events can increase the nutrients in freshwater systems, inducing phytoplankton blooms. Of the 3,920 lakes detectable from Landsat (30m) over California HUC 8 basins, 96% of the watersheds had a wildfire since 1984, with the least active fire repeat interval having 3 and the most active 300 for the observed time period. Monitoring and understanding the water quality responses to watersheds is an important source of information for water managers to develop and prioritize mitigation strategies aimed at ensuring water quality and water supply.