Temporal And Spatial Controls On Post-Wildfire Water Quality In The Colorado Front Range
Murphy, Sheila F. 1 ; Writer, Jeff 2 ; McCleskey, Blaine 3 ; Martin, Deborah 4
2 Presenting Author
1 USGS
2 CU
3 USGS
4 USGS
Hydrologic processes can be dramatically altered post-wildfire. However, the high temporal and spatial resolution data needed to effectively quantify post-wildfire linkages between hydrology, water quality and ecological response in Colorado Front Range watersheds has been limited. After the September 2010 Fourmile Canyon Fire near Boulder, Colorado, precipitation, stream-discharge, water quality (including turbidity, suspended sediment, dissolved organic carbon [DOC], and nitrate), and stream ecosystem response (biofilm growth) were evaluated. Different classes of runoff events (snowmelt, frontal storms, and high-intensity convective storms) were evaluated for a three-year period, including the most recent September2013 flood event. Heterogeneity in the amount, timing, and intensity of rainfall was found to be critical in determining post-wildfire impacts on water quality. For the first year following the wildfire, hydrologic conditions during baseflow and snowmelt indicated that discharge, turbidity, nitrate, and DOC concentrations were not significantly different upstream and downstream from the burned area. However, thunderstorms caused substantial short-term (days) increases in turbidity, nitrate, and DOC levels at locations downstream from the burned area relative to pre-storm levels. For example, turbidity increased from 3 NTU to 23,000 NTU, nitrate increased from < 0.02 mg/L to 9 mg/L, and DOC increased from 1.5 mg/L to 71 mg/L. A substantial water quality response to thunderstorm events continued for 3 years post-fire. Following these thunderstorms, statistically significant increases in nitrate and DOC concentrations were consistently observed down-gradient from the burned area during base-flow and spring snowmelt; these changes were likely due to leaching of nitrate and DOC from sediment left in the stream channel during runoff from thunderstorms. Increased nutrient export from the burned watershed following the thunderstorms significantly increased the primary productivity of stream biofilms. DOC export from the burned watershed was higher relative to the unburned watershed; as an example, two thunderstorms in July 2011 represented nearly 10% of the total carbon exported from the watershed that year. However, drought conditions observed in 2012 had a more substantial effect on seasonal DOC export than did wildfire. The impact of the September 2013 flood has not been quantified, but clearly had the greatest hydrologic impact on water quality and the stream ecosystem.