Prediction, validation, and uncertainties of a nation-wide post-fire soil erosion risk assessment in Portugal

Assessing post-fire water quality changes in reservoirs: Insights from a large dataset in Portugal

Wildfires in the Mediterranean basin, especially in Portugal, have increased in extent and frequency over the last few years. One of the impacts of wildfires on humans and ecosystems is on the water quality of surface waters. Ashes and increased erosion rates might, for example, change oxygen levels and elevate the influx of sediments, nutrients, or other water quality-related components like metals and polycyclic aromatic hydrocarbons (PAHs), possibly affecting water supply. In this study, time series of eight water quality parameters: biological- and chemical oxygen demand (BOD and COD), electrical conductivity (EC), total phosphorous (TP), nitrate (NO3-), total suspended sediments (TSS), dissolved oxygen (DO), and pH, were assessed via changepoint analysis to identify events of post-fire water contamination in over 60 Portuguese reservoirs. Further, possible fire, watershed, reservoir, and climate-related drivers were linked with the occurrence of these contamination events through logistic regression using generalized additive models. All measured parameters exhibited post-fire changes, with some being more frequently affected than others. The concentrations of TP, NO3-, and TSS showed a noticeable increase following 9.6 %, 12.6 %, and 13.6 % of all wildfires, respectively. Most changes fell into the unusually large fire seasons of 2003-2005 and 2017. The most significant impacts could be seen in southern reservoirs after the fire seasons of 2003-2005. The burned area ratio of the watershed was identified as the main driver of post-fire water contamination, while reservoir and climate-related characteristics like water levels also played a significant role in some parameters. Increased levels of suspended sediments were identified as a potential threat to water supply, especially when large wildfires coincide with drought-induced low reservoir water levels. The identification of post-fire water contamination events and their drivers from large datasets can inform water managers about potential threats to water supply.

How much does it cost to mitigate soil erosion after wildfires?

Wildfires usually increase the hydrological and erosive response of forest areas, carrying high environmental, human, cultural, and financial on- and off-site effects. Post-fire soil erosion control measures have been proven effective at mitigating such responses, especially at the slope scale, but there is a knowledge gap as to how cost-effective these treatments are. In this work, we review the effectiveness of post-fire soil erosion mitigation treatments at reducing erosion rates over the first post-fire year and provide their application costs. This allowed assessing the treatments' cost-effectiveness (CE), expressed as the cost of preventing 1 Mg of soil loss. This assessment involved a total of 63 field study cases, extracted from 26 publications from the USA, Spain, Portugal, and Canada, and focused on the role of treatment types and materials, and countries. Treatments providing a protective ground cover showed the best median CE (895 $ Mg^-1), especially agricultural straw mulch (309 $ Mg^-1), followed by wood-residue mulch (940 $ Mg^-1) and hydromulch (2332 $ Mg^-1). Barriers showed a relatively low CE (1386 $ Mg^-1), due to their reduced effectiveness and elevated implementation costs. Seeding showed a good CE (260 $ Mg^-1), but this reflected its low costs rather than its effectiveness to reduce soil erosion. The present results confirmed that post-fire soil erosion mitigation treatments are cost-effective as long as they are applied in areas where the post-fire erosion rates exceed the tolerable erosion rate thresholds (>1 Mg^-1 ha^-1 y^-1) and are less costly than the loss of on- and off-site values that they are targeted to protect. For this reason, the proper assessment of post-fire soil erosion risk is vital to ensure that the available financial, human and material resources are applied appropriately.

Wildfires in Europe: Burned soils require attention

Annually, millions of hectares of land are affected by wildfires worldwide, disrupting ecosystems functioning by affecting on-site vegetation, soil, and above- and belowground biodiversity, but also triggering erosive off-site impacts such as water-bodies contamination or mudflows. Here, we present a soil erosion assessment following the 2017's wildfires at the European scale, including an analysis of vegetation recovery and soil erosion mitigation potential. Results indicate a sharp increase in soil losses with 19.4 million Mg additional erosion in the first post-fire year when compared to unburned conditions. Over five years, 44 million Mg additional soil losses were estimated, and 46% of the burned area presented no signs of full recovery. Post-fire mitigation could attenuate these impacts by 63-77%, reducing soil erosion to background levels by the 4th post-fire year. Our insights may help identifying target policies to reduce land degradation, as identified in the European Union Soil, Forest, and Biodiversity strategies.