Understanding the Interactions Between Post-Fire Landscapes and Water Quality Over Time and Space

In the western US, roughly 65% of water supply originates in forested headwaters. Thus, increasing wildfire frequency and severity in this region can threaten the sustained delivery of clean drinking water. For example, stream nutrient concentrations commonly increase following wildfire and can remain elevated for decades. However, we currently lack a mechanistic understanding of the temporal and spatial linkages between landscape structure and water quality in severely burned watersheds. This research fills that gap by leveraging temporally and spatially extensive datasets following the 2002 Hayman fire. First, we compared a time series of spectral vegetation indices to a long-term water quality record (1 to 17 years post-fire) that includes watersheds with variable burn extent. Post-fire watersheds with a high burn extent (>60%) had lower mean catchment normalized difference vegetation index (NDVI) values and higher stream nitrate (NO₃) concentrations throughout the 17 year post-fire record. However, the time series also demonstrate gradual recovery whereby the return of stream NO₃ lags behind the return of vegetation to pre-fire baseline. We also conducted synoptic sampling throughout 8 watersheds with unique burn patterns to generate a distributed water quality dataset 16 years post-fire. We then utilized geospatial data to characterize both natural gradients in topography and hydrology and superimposed gradients of fire severity and vegetation mortality. Linear mixed effects models identified mean catchment NDVI and tree cover as strong predictors of stream NO₃ concentrations, across spatial scales. Furthermore, incorporating the spatial distribution of wildfire (i.e. concentrated burning in headwaters, uplands, or convergent landscape positions) improved stream NO₃ predictions. Together, these time series and synoptic datasets demonstrate that the reestablishment of vegetation over space and time is a dominant driver of post-fire stream nutrient concentrations.