Evaluating the factors responsible for post-fire water quality response in forests of the western USA

Abstract

Wildfires commonly increase nutrient, carbon, sediment and metal inputs to streams, yet the factors responsible for the type, magnitude and duration of water quality effects are poorly understood. Prior work by the current authors found increased nitrogen, phosphorus and cation exports were common the first 5 post-fire years from a synthesis of 159 wildfires across the western United States. In the current study, an analysis is undertaken to determine factors that best explain post-fire streamwater responses observed in those watersheds. Increased post-fire total nitrogen and phosphorus loading were proportional to the catchment extent of moderate and high burn severity. While post-fire dissolved metal concentrations were correlated with pre-fire soil organic matter. Total metal concentration increased where post-fire Normalised Difference Vegetation Index, a remote sensing indicator of live green vegetation, was low. When pre-fire soil field capacity exceeded 17%, there was a 750% median increase in total metals export to streams. Overall, the current analysis identified burn severity, post-fire vegetation cover and several soil properties as the key variables explaining extended post-fire water quality response across a broad range of conditions found in the western US.

Megafire affects stream sediment flux and dissolved organic matter reactivity, but land use dominates nutrient dynamics in semiarid watersheds

“…While megafire had a clear and dominant influence on sediment dynamics, its effects on carbon and nutrients were more complex. Following observations from similar megafires [ 42 , 65 , 146 ], we hypothesized that increased nutrient availability and decreased nutrient demand in the terrestrial environment would result in higher lateral nutrient fluxes in burned watersheds. However, in contrast with previous research, which primarily focuses on burned-unburned comparisons in pristine watersheds [ 42 , 72 , 73 , 147 ], we found that the presence of urban and agricultural activity exerted a much greater influence on nutrient status than the megafire.…”

“…The hydrological changes described above interact with the biological and physical changes caused by wildfires to affect solute and particulate chemistry in soil, surface water, and groundwater [ 42 , 57 , 65 ]. The loss of vegetation and soil organic matter during a wildfire can destabilize soils, leading to increased erosion rates that typically last around 5 years [ 56 , 66 , 67 ].…”

The influence of wildfire on water quality and watershed processes: new insights and remaining challenges

“…Slow post-fire vegetation recovery likely dampens nutrient demand and retention and has been credited for persistent, elevated stream nutrients (Rhoades et al 2019). Here, Rust et al (2019) use publicly available stream water and wildfire data from more than 150 fires in the western US, and identified fire severity, post-fire vegetation recovery and site-specific soil properties as key contributors to post-fire stream nutrient and metal responses. The study by Williams et al (2019) examining the impacts of wildfire in the southern Canadian Rockies further demonstrates that the rate of vegetation recovery determines how long wildfires will alter precipitation inputs, which in turn drive the hydrologic processes that mobilise and transport potential contaminants.…”

“…They also found that eroded C and N declined within a few years. There is, however, growing evidence that post-fire stream nutrient changes may persist longer where revegetation is slow (Rhoades et al 2019;Rust et al 2019) and there has been little effort to date to rehabilitate lingering wildfire effects using in-stream, riparian or upland rehabilitation treatments.…”

“…Links between post-fire vegetation recovery and persistent post-fire water quality responses highlight the value of considering these disturbances from an ecosystem perspective. For example, both short- (Riggan et al 1994;Rhoades et al 2011) and longer-term wildfire effects on stream nutrients (Rust et al 2019) relate to the extent of high-severity wildfire, and these conditions, in turn, influence post-fire vegetation recovery (Chambers et al 2016;Malone et al 2018). Advances in understanding of the coupled hillslope, stream-reach and catchment-scale biogeochemical processes that regulate plant and soil nutrient and C uptake and release are needed to determine the threats of potential post-fire contaminants to downstream resources.…”