Potential Post-Fire Impacts on a Water Supply Reservoir: An Integrated Watershed-Reservoir Approach

Abstract

Wildfires are an increasing threat in the Mediterranean region, causing frequent losses of goods and human lives. Not only are wildfires a concern due to their immediate effects on vegetation and soil, but they can also have substantial impacts on surface water quality. Approximately one-third of the world’s largest cities obtain their drinking water from forest catchments. The removal of vegetation and consequent runoff increase with a high concentration of ash and sediment often leads to increased nutrient and contaminant loads to downstream reservoirs, damaging the aquatic ecosystem and threatening human health. This study focused on the post-fire degradation of surface water in Castelo de Bode reservoir, a strategic freshwater supply for Lisbon’s metropolitan area (2,000,000 inhabitants), Portugal. Output data from the catchment model Soil and Water Assessment Tool were used as inputs to the CE-QUAL-W2 reservoir model. CE-QUAL-W2 was then calibrated for water level, temperature, nutrients, total suspended solids, chlorophyll-a, and dissolved oxygen. The post-fire impacts were assessed by adjusting land use features (curve number, crop vegetation management factor), and soil properties (soil erodibility) in the catchment model, considering the different impacts of fire (low, medium, and high severity). The reservoir model was able to perform temperature seasonality and stratification while a weak performance was found for chlorophyll-a probably for having considered only a group of algae. Simulations showed a deterioration of water quality at the dam wall during the first year after the forest fire. Nevertheless, contamination did not appear worrisome with regards to water quality standards likely due to the capability of the reservoir to attenuate inflow concentrations.

A call for strategic water-quality monitoring to advance assessment and prediction of wildfire impacts on water supplies

“…The next generation of strategic post-wildfire water-quality monitoring can extend beyond the priorities outlined here. We focus on in-stream processes, but understanding wildfire effects on reservoirs is an important next step in post-wildfire monitoring (Nunes et al, 2018;Basso et al, 2021;Paul et al, 2022). Groundwater, which supplies water to over 2 billion people and accounts for up to a third of global water withdrawals (Famiglietti, 2014), can be detrimentally affected by wildfire (Mansilha et al, 2020), yet there is almost no data or guidance on potential wildfire impacts on public or private supply wells.…”

Advances on water quality modeling in burned areas: A review

“…Another reason for the lack of post-fire water quality modelling studies can be attributed to the scarcity of data available for model calibration and validation. Such parameters are rarely measured or monitored at too low frequency by typical water quality monitoring protocols, the exception being for water reservoirs for human supply [37].…”

“…The aquatic models would allow complementing previous predictions with hydrodynamic, geochemical, and biological models, to account, not only for fire-induced changes in water quality, but also in aquatic habitats and biota. So far, Basso et al [37] combined a watershed with a reservoir model to study the post-fire impacts on drinking water supply. However, this framework presented limitations since the adapted models were not developed for post-fire conditions.…”

“…Within the post-fire context, empirical models such as the Universal Soil Loss Equation (USLE) and its revised version (RUSLE) have often been used for soil erosion predictions, while more complex models have been generally used to predict post-fire runoff and erosion [30][31][32], as well as the effects of post-fire erosion mitigation treatments at slope and catchment scale [32,33]. However, only a limited number of models have been used to predict the impacts of fires on water quality [34][35][36][37][38][39]. It should also be highlighted that only a few models were designed for post-fire conditions [26].…”

Long-term erosion and the impact of wildfires: two different approaches

“…Fire disturbance is usually regarded as a key agent of soil erosion and land degradation (Shakesby, 2011). Fire is pointed as responsible for: (i) overland flow and reduction of the capacity of infiltration (Basso et al, 2021;Fernández et al, 2010); (ii) increase of the availability of ash and debris and disruption of the soil nutrient cycle (Basso et al, 2021); and (iv) increase of connectivity across the watershed (van der Grift, 2021). The potential fire effects on soils and aquatic resources have created a strong demand for a post-fire sediment loss prediction tool (Fernández et al, 2010;Larsen & MacDonald, 2007;Vieira et al, 2014).…”