Afforestation, Subsequent Forest Fires and Provision of Hydrological Services: A Model‐Based Analysis for a Mediterranean Mountainous Catchment

Abstract

Mediterranean landscapes have experienced extensive abandonment and reforestation in recent decades, which should have improved the provision of hydrological services such as flood mitigation, soil erosion protection and water quality regulation. However, these forests are fire‐prone, and the postfire increase in runoff, erosion and sediment exports could negatively affect service provision. This issue was assessed by using the Soil and Water Assessment Tool model for the Macieira de Alcôba mountain catchment in northwestern Portugal, covered by agriculture and forest plantations, monitored in 2010–2014 and partially burnt in 2011. The model was validated for streamflow, sediment yield and erosion in agricultural fields and burnt hillslopes, showing that it can be adapted for postfire simulation. Model results for a decadal assessment (2004–2014) show that the fire increased erosion in the burnt slopes from 0.1 to 46.5 Mg ha−1 y−1 and catchment sediment yield from 0.14 to 2.1 Mg ha−1 y−1. Erosion in the burnt forest during this decade was 1 order of magnitude above that in agricultural fields. Model results for different fire and land‐use scenarios indicate that postfire erosion and sediment yield were lower without postfire soil management for reforestation and when the fire occurred in pine forests (land use before the 1990s) or in shrublands (land‐use before afforestation in the 1930s). These impacts were robust to changes in postfire weather and to a longer (20 years) unburnt period. The results suggest that, in the long term, Mediterranean fire‐prone forests might not provide the anticipated soil protection and water quality regulation services. Copyright © 2017 John Wiley & Sons, Ltd.

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

“…At the watershed level, the increase in the total annual streamflow in the wildfire scenario was considerably lower than the expected increase of 10–20% reported in several studies (Loáiciga, Pedreros, & Roberts, ; Neary, Ryan, & DeBano, ). That limited increase (0.3% total annual flow) is similar to the findings of Nunes, Naranjo Quintanilla, et al () when using the SWAT model to assess the streamflow in a small catchment partially affected by a wildfire. The results also agree with Hawtree et al's () assessment of afforestation in the Caramulo Mountains during the 20th century.…”

“…Wildfires play a major role on land degradation in Mediterranean areas (Le Houérou, ). Notwithstanding, the number of wildfires is expected to increase due to climate change and new socioeconomic drivers (Nunes et al, ; Turco et al, ; Turco, Llasat, von Hardenberg, & Provenzale, ; Viedma, Moity, & Moreno, ). The reduction of vegetation and ground cover by wildfires increases soil susceptibility to raindrop impact.…”

“…Nutrient load increase due to the impact of wildfires can vary markedly among sites, ranging from a slight increase (0.3-2-times unburned) to a substantial increase (20-431-times unburned), depending on vegetation and soil type, burned area, and fire severity (Smith et al, 2011). Sharp decreases in dissolved oxygen concentrations and increased water turbidity are also commonly reported on reservoirs after wildfires (Nunes, Naranjo Quintanilla, et al, 2018), compromising the delivery of potable water.…”

Assessing water contamination risk from vegetation fires: Challenges, opportunities and a framework for progress

“…In contrast, Nyman, Sheridan, Smith, and Lane () showed that runoff‐generated debris flows dominated burned catchments in SE Australia; while in the Mediterranean, Shakesby () noted the absence of both mass failure and debris flows, implying the dominance of interill processes (Prats, Wagenbrenner, Santos Martins, Malvar, & Keizer, ). The dominant processes that govern contaminant mobilization are often controlled by higher‐order factors, for example, aridity controlling debris flows occurrence in Australia (Sheridan et al, ) or mobilization being affected by land use patterns (Nunes et al, ).…”

“…This exemplifies a situation where a large research focus on sediment mobilization (Tier 2 of the framework presented here) was not combined with a similar focus on transport (Tier 3 of the framework). This has recently been addressed by developing models which can account for the seasonal dynamics of hydrological connectivity in burnt areas (Nunes et al, , ; van Eck, Nunes, Vieira, Keesstra, & Keizer, ). These go some way in improving our ability to assess post‐fire contamination risk; a major uncertainty in adapting the Lisbon water supply to a changing environment (Groot, Rovisco, & Lourenço, ).…”