Assessing the adequacy of SWAT model to simulate postfire effects on the watershed hydrological regime and water quality

Abstract

Forest fires have intensified in the Mediterranean area over the last decades, becoming increasingly destructive. Catastrophic wildfires, such as the ones that occurred on the Portuguese territory during 2017, have emphasized the need for developing management tools capable of rapidly assessing their impact on downstream water bodies. This study focuses on the effects of the 2017 forest fires on the water quality of the Zêzere River (Portugal), one of the major sources of freshwater to Lisbon and the surrounding areas. During 2017, more than 100,000 ha (30% of the Zêzere watershed) were affected by wildfires, making it one of the largest burned areas in Portugal. Using the soil water assessment tool, the land use (curve number, crop vegetation management factor) and soil (soil erodibility factor) parameters were modified considering different magnitudes of the fire event (low, medium, and high severity). The impact of the different wildfires was then assessed at the subbasin level and at the entrance of Castelo de Bode reservoir. Simulations showed a significant increase in runoff and sediment concentration at the subbasin level in the years following the fire events, with high concentrations of nutrients occurring at the reservoir inlet. Nitrate concentration slightly overcame the threshold limits foreseen in legislation for drinking water, meaning that fires could have additional impact on human health. Phosphate concentration recurrently exceeded the legislation threshold, representing a severe risk to the ecosystem by potentially promoting the eutrophication of downstream water bodies, oxygen deficiency in the bottom substrate, and reduced biodiversity.

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

“…The wildfire scenario considered had the same characteristics of the 2017 season when almost 30% of the area that flows into the reservoir was hit by wildfires. The extension of the burned area, with 88% classified in the high fire severity class, 9% in the moderate, and 3% in the low, are presented in Figure 1B (Basso et al, 2020).…”

“…Flow and water quality data at the reservoir inlet were provided by the SWAT model. Input data were previously calibrated for a 12year period (1984)(1985)(1986)(1987)(1988)(1989)(1990)(1991)(1992)(1993)(1994)(1995)(1996) and then validated for a 4-year period (1996-2000) (Basso et al, 2020). Inlet data from the upstream basins flowing to each branch were inserted into the model.…”

“…The Soil and Water Assessment Tool (SWAT) is a watershedbased model widely used to simulate the impact of land use, land management practices, and climate changes on river flow and constituent transport (Arnold and Fohrer, 2005;Neitsch et al, 2011). The model was previously used to estimate hydrological and water quality changes in the upstream basin of the Castelo de Bode reservoir (Basso et al, 2020). Although SWAT can simulate water quality processes in lakes and reservoirs, the limitation of its steady-state 1-D empirical equations does not make it suitable to adequately describe those processes in large waterbodies.…”

“…The CE-QUAL-W2 hydrodynamic and water quality model can perform 2-D analysis in large waterbodies, simulating the transformations and interactions of several constituents within the studied volume (Cole and Wells, 2006). In this modeling exercise, the output data from the SWAT model implemented by Basso et al (2020) was used as inlet values for the CE-QUAL-W2 model (Cole and Wells, 2006) applied to the Castelo de Bode reservoir. The benefit of the combined use of these two models has been already established by few researchers (Flowers et al, 2000;White et al, 2004;Debele et al, 2008;White et al, 2010), confirming the competence of the integrated approach to develop land management and climate changes scenarios.…”

“…To do so, this work evaluates the response of a large reservoir to upstream disturbance, assessing the degree of contamination at the water supply tower. The specific objectives of this study are: i) to calibrate and validate the reservoir model to predict water level, nutrients, dissolved oxygen, chlorophyll-a, and sediment concentrations; ii) to model post-fire contaminated inflows impact on reservoir water quality and quantity; and iii) to assess whether the high concentrations observed at the reservoir inlets (Basso et al, 2020) are also maintained at the water intake tower, jeopardizing the proper operation of the treatment processes.…”