Short-term effects of a boreal wildfire on water quality

Gustaf Granath\({}^{\star}\), 11Gustaf Department of Ecology, Swedish University of Agricultural Sciences Box 7044, SE-750 07 Uppsala, Sweden.’
Christopher D. Evans, Centre for Ecology and Hydrology, Bangor, UK.
Anna Landahl, Department of Aquatic Sciences and Assessment, Swedish University of Agricultural Sciences Box 7050, SE-750 07 Uppsala, Sweden.
Jens Fönster, Department of Aquatic Sciences and Assessment, Swedish University of Agricultural Sciences Box 7050, SE-750 07 Uppsala, Sweden.
Stephan Kölhler, Department of Aquatic Sciences and Assessment, Swedish University of Agricultural Sciences Box 7050, SE-750 07 Uppsala, Sweden.

\(\star\) Gustaf


In 2014 Sweden experienced the largest and most severe wildifre in modern time. The burnt area is intensively managed for forestry which includes extensive drainage of peatlands in some catchments. The immediate effect of fire on the downstream water quality is rarely quantified and possible interactions with land-use and land cover charect eristics is not well known. We studied water quality (No3, NH4…..etc) in 9 catchments from 3 weeks to one year after the fire occurred. A simple burn severity map was produced with remote sensing techniques. Maps on land cover and drainage (ditches) were extracted from data bases. Our results show that an extremely high sulfate pulse (1-2 months after fire) were buffered by the release of base cations, and likely presence of organic matter, which largely suppressed surface water acidification in the area. In line with earlier results, the increased nutrient export returns quickly to normal values and is not detectable one year. Differences in the water chemical response were linked to differences in land cover and drainage. Ditched and forested wetlands burn deeper and may contribute ……Lakes and and natural wetlands are more fire resistant… Nutrient leaching is generally higher in areas with high wetland cover where organic matter is oxidized by the fire. In conclusion,……

*Corresponding author: Target journal: Biogeosciences/Ambio Rough draft deadline: christmas


Wildfires are a natural phenomenon but human activities are altering both the driving factors (climate) and the vulnerability (land-use factors) of ecosystems, increasing both frequency and severity of fire impacts. The boreal zone has experienced an intensified fire regime over the last decades and this trend is predicted to continue (Flannigan 2009). This is an issue of concern given that wildfires play a major role in altering nutrient status of soils and waters. Yet, few studies have investigated the impact boreal wildfires have on water quality and how this is altered by land-use and landscape characteristics. Here we address this topic by capitalizing on a wildfire in Sweden that occurred in a managed area with multiple catchments and an ongoing water quality monitoring program.

Postfire water quality is determined by what is hydrologically exported to streams and lakes. After fire there is an increase of available nutrients in the soil that can leech out, mainly caused by an increase in soil pH which is associated with an increase in exchangeable cations (e.g.Ca2+,Mg2+, and K+, and the anion SO42) in soil (González-Pérez 2004). These ions are easily exported to streams and lakes and studies have shown post-fire peaks in sulphate (SO42-), chloride (Cl-) and base cation concentrations ( (Carignan 2000) (Mast 2008) (Bladon 2008) (Bladon 2014)). If acid anions (NO3-, SO42- and Cl-) dominates over base cations a acidity effect is observed in downstream waters ( (Lydersen 2014)). This acidification effect is enhanced in areas which have higher concentrations of stored S from acid rain or have a high proportion of peatlands (Bayley 1992). Post-fire acidification can also result in high aluminum concentrations (Lydersen 2014) and possibly other metals (e.g. Fe, Mn, and Cu) (Certini 2005). However, a high base cation concentration may counterbalance the acididyt effect ((Carignan 2000)). There is also a an increased availability of dissolved P in the soil post-fire (Certini 2005) and in boreal Canada burned watersheds exported more phosphorus per unit area than reference watersheds even after 4 years post-fire (Burke 2005).

Nitrogen levels can increase dramatically post-fire (eg (Bladon 2008) and (Carignan 2000)). Following fire soil organic nitrogen is either volatilised or largely converted into inorganic forms (i.e. NH+4 and NO-3) (Certini 2005). Nitrite is mainly formed from NH+4 through nitrification up until months after the fire (Certini 2005). Both NH4+–N and NO3–N are available to plants, but with non-existing vegetation cover after a severe fire, these compounds are leached out (Smith 2011). Nitrite concentrations may peak shortly after the fire and return to reference values within 2-3 years (eg (Bladon 2008) and (Carignan 2000)). However, other studies in have reported high concentrations of nitrite up to 5-9 years post-fire ((Hauer 1998) (Mast 2008)). In contrast to nitrite, ammonium is expected to be held by the soil to a higher degree because it adsorbed onto negatively charged surfaces of soil particles (Mroz 1980). However, a study observed a NH+4 pulses that lasted over 2 growing seasons (Grogan 2000). ONLY N. Am.!! Add TURNER ref?

Variation in surface water quality at the catchment scale in the boreal landscape is mainly controlled by landscape heterogeneity (Humborg 2004). A major influence on surface water pH is proportion of peatlands in the catchment through the release of organic acids (Buffam 2007) . Peatland cover also reduces the nitrite concentration in surface waters (Sponseller 2014). Despite the clear effect of landscape characteristics on water chemistry this aspect has received little attention when examining the effect of wildfire on water chemistry. For example, wildfire can cause severe disturbance to peatlands and potentially increase oxidation of S with subsequent leaching as a result. Similary, nitrite may not be retained in peatlands after fire.

A wildfire in Sweden in 2014 created the oppertunity to study the effect of wildfire on water chemistry in a managed landscape with a high cover of peatlands. To quantify the effects of wildfire on water quality, and to understand the drivers behind variation in water quality responses to fire, before-after data is needed and catchment needs to be replicated. The burnt area consists of multiple catchments allowing us to investigate local variation in post-fire responses. One of the catchments is included in a national water monitoring network enabling comparison with long-term trends in water chemistry. This before-after approach is complimented by comparing data with nearby long-term monitored catchments. Hence, compared to most studies, our study does not rely on only post-fire data and a few reference sites (see Mast 2013 and Betts and Jones, 2009 for other before-after studies).

The overarching goal of this study is to investigate the short-term (2 years) effects of a boreal wildfire on stream and lake water chemistry. Downstream data from five burned watersheds are presented together with data from ten lakes. Post-fire data are contrasted with: 1) pre-fire data for one stream and lake within the burned area, and 2), reference lakes and streams in the surrounding with similar land-use characteristics. Together this is a unique opportunity to quantify the impact of the wildfire on water chemistry and to test how current trends in water chemistry are altered by a fire. Furthermore, we want to explore if catchment characteristics is associated with the post-fire water chemistry. In particular we tested if the changes in water chemistry were associated with the proportion of peatlands and catchment size. Finally, we use stream flow data to estimate fluvial exports of S and base cations for two catchment during the first year after the fire.


The wildfire started on the 31 of July 2014 in the county of Västmanland, located in the central parts of Sweden. The forest fire lasted for 12 days and a total of 14 000 ha were consumed by the fire. During the initial days (31/7 – 3/8) the spreading of the fire were of moderate intensity but the 4th of august the wind and fire intensity increased which drastically enlarged the fire affected area. Five streams and ten lakes, larger than 1 ha, located inside the affected area and three lakes adjacent to the area were sampled. One of the measured streams (Gärsjöbäcken), and one of the lakes (Märrsjön) are part of the Swedish regional monitoring program (RMÖ) since 1995. Sampling points and catchment characteristics are presented in Figure \ref{fig:figure1} and table \ref{tab:table1}.

Sampling and chemical analysis

The first post-fire measurements of the streams were made on the 21 of August (2014). During the first months the streams were frequently sampled but logistic constraints made it hard to sample all streams the same day. The streams were in general measured every second to fourth week, except parts of the winter. Synoptic measurements (what is this?) were made at XX occasions during measurement period. The lakes were sampled at three occasion: 2014 October 28th, 2015 October 28th, and 2016 December 1st. A few lakes were sampled more frequently. The water sampling procedure and water chemistry analysis were made according to the Swedish monitoring program (Fölster 2014) using SWEDAC accredited methods at the geochemical laboratory at the Department of Aquatic Sciences and Assessment at the Swedish University of Agricultural Sciences (Sonesten, 2015).

Stream flow data were modelled for two catchments that are included in the Swedish Swedish Meteorological and Hydrological Institute’s (SMHI) catchment database for which their S-HYPPE model produces daily flow data. Model outputs were compared to pressure data (transducers XX model etc).

Catchment delineation and peatland cover

The catchments of the sampled streams were produced in ArcGIS 10.3, software from ESRI, using a national elevation model from Lantmäteriet (2015a) that had a resolution of 2x2 m and accuracy of 0.5 m. When rain hits the surface it will run in the steepest slope direction which is determined in the elevation model. By grouping the surfaces of the steepest slopes with the same direction watersheds were delineated. Catchment delineation was visaully quality checked to ensure high accuracy of the delineation process. Peatland cover for each catchment was estimated from the Swedish soil layer raster (The Geological Survey of Sweden, SGU).