Abstract
Forest fires are an increasing problem over recent decades. The fires, among other consequences, lead to an increase in the soil vulnerability to water erosion and a consequent increase in sedimentation rates. When barriers are present, such as dams or weirs, there is an amplified risk of sediment and ash deposition in their reservoirs, causing siltation. Thus, there is an interest in studying in more detail the risk of siltation of barriers and reservoirs in the Douro River watershed following wildfires. A detailed barrier inventory was lacking for the Douro River, hampering the identification of siltation-prone areas. In order to fill in this gap, an extensive inventory of barriers in the Douro river basin was carried out for the present study. The result was an abundant and reliable dataset on the Douro River barriers, which allowed a prognosis on the watershed siltation risk. The method for calculating the siltation risk relied on the relationship between the frequency of forest fires, the erosion risk and the frequency of reservoirs. The sub-basins with the greater siltation risk are the Tâmega, Corgo, Sousa and Paiva river basins. Most reservoirs with the highest siltation risk were from small dams. The modelling results were compared with stream connectivity and concentrations of stream water phosphorus (associated with the sediments that flow into the rivers due to the fires). With regard to connectivity, only two reservoirs were at high risk of sedimentation due to fires, so the categories of connectivity risk and fire-based sedimentation risk are probably not related. With regard to risk of high phosphorus loadings, in 8 basins the upper classes for fire-based erosion risk coincided with the upper class for phosphorus loadings suggesting that high phosphorus loading could be associated with fire-based erosion. This study works as a simple but reliable example on the assessment and mapping of siltation risk in stream networks intersected by abundant barriers. It allowed for identifying barriers that can accumulate a large quantity of fine sediments and ashes, interfering with water quality and soil erosion as well as with the storage capacity of the respective barriers.
Highlights
In Portugal, as well as in other Mediterranean countries, forest fires and burned areas have increased significantly over the last few decades, and often these fires occur again in areas that have been burned before [1,2].Water 2020, 12, 2959; doi:10.3390/w12112959 www.mdpi.com/journal/waterThe Mediterranean region climate generally is characterized by summers with a combination of high temperatures and low air humidity [3,4]
There is a wide variation in the recurrence of fires: from virtually non-burned areas to a recurrence less than 3 years; (b) illustrates the distribution of soil loss in the Douro river basin, where we can observe an intensification of erosion closer to the main watercourses, related to the topography and drainage density
In order to obtain the risk of incoming sediment load, it was necessary to calculate the reservoir frequency by sub-basin
Summary
In Portugal, as well as in other Mediterranean countries, forest fires and burned areas have increased significantly over the last few decades, and often these fires occur again in areas that have been burned before [1,2].Water 2020, 12, 2959; doi:10.3390/w12112959 www.mdpi.com/journal/waterThe Mediterranean region climate generally is characterized by summers with a combination of high temperatures and low air humidity [3,4]. The fire hazard is constantly amplified [7,8], driven by socio-economic changes, including rural depopulation and abandonment of cultivated land, afforestation with flammable species [6,9], coupled with the ongoing climate change [10,11]. The severity of this increase in Portugal is even greater since it differs from other countries in southern Europe in that it has the highest number of ignitions and the highest proportion of areas burned, especially in the central and northern regions [12]. There are several causes for soil erosion, the evolving decrease of vegetation cover and infiltration rates has led, in large part, to its exponential increase [16,17]
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