Stream chemical response is mediated by hydrologic connectivity and fire severity in a Pacific Northwest forest

Large-scale wildfires are becoming increasingly common in the wet forests of the Pacific Northwest (USA), with predicted increases in fire prevalence under future climate scenarios. Wildfires can alter streamflow response to precipitation and mobilize water quality constituents, which pose a risk to aquatic ecosystems and downstream drinking water treatment. Research often focuses on the impacts of high-severity wildfires, with stream biogeochemical responses to low- and mixed-severity fires often understudied, particularly during seasonal shifts in hydrologic connectivity between hillslopes and streams. We studied the impacts of the 2020 Holiday Farm Fire at the HJ Andrews Experimental Forest where rare pre-fire stream discharge and chemistry data allowed us to evaluate the influence of mixed-severity fire on stream water quantity and quality. Our research design focused on two well-studied watersheds with low and low-moderate burn severity where we examined long-term data (pre- and post-fire), and instantaneous grab samples collected during four rain events occurring immediately following wildfire and a prolonged dry summer. We analysed the impact of these rain events, which represent the transition from low-to-high hydrologic connectivity of the subsurface to the stream, on stream discharge and chemistry behaviour. Long-term data revealed total annual flows and mean flows remained fairly consistent post-fire, while small increases in baseflow were observed in the low-moderately burned watershed. Stream water concentrations of nitrate, phosphate and sulfate significantly increased following fire, with variance in concentration increasing with fire severity. Our end member mixing models suggested that during rain events, the watershed with low-moderate severity fire had greater streamflow inputs from soil water and groundwater during times of low connectivity compared to the watershed with low severity fire. Finally, differences in fire severity impacts on concentration-discharge relationships of biogenic solutes were most expressed under low catchment connectivity conditions. Our study provides insights into post-wildfire impacts to stream water quality, with the goal of informing future research on stream chemistry responses to low, moderate and mixed severity wildfire.
Keywords: concentration-discharge, end-member mixing, Holiday Farm Fire, hysteresis, mixed-severity fire, Oregon, rain events, wet forests