The volcanic soils of Canary Islands are especially susceptible to events of runoff and erosion of great magnitude after the fires forestry like the one suffered last August in the mountains of Tenerife that burned almost 14,000 hectares, very close to 8% of the island’s surface, in a perimeter of 90 kilometers of the forest crown. This is revealed by various studies developed since 2015 by the ÁArea of Soil Science and Agricultural Chemistry of the University of La Laguna (ULL)in collaboration with the councils of Tenerife and Gran Canaria, the regional government and other research institutions in Swansea (United Kingdom), the US Forest Service or the University of Melbourne (Australia). This is the reason why the Tenerife Island Corporation coordinates with the 12 municipalities affected by the fire -of 31- the actions to be carried out to avoid the effects of winter rain (when it arrives) Both administrations will apply preventive measures to avoid erosion in the event of episodes of intense rain.
The Cabildo of Tenerife held a meeting with the municipalities to coordinate actions and avoid the effects of rain in the areas most affected by the fire. The meeting was attended by the Minister of the Natural Environment, Blanca Pérez, and the insular director of Security and Emergencies, Iván Martín, as well as technicians from the area, Civil Protection and Highways, as well as members of the Insular Water Council and the Consortium. firemen’s.
During the meeting, the consequences that may arise from the interaction of the first rains were discussed and are fundamentally related to the runoff, solids transport and ash accumulation (sludge). In this regard, the Natural Environment Minister, Blanca Pérez, indicated: “We want to maintain contact with all the municipalities affected by the fire to identify the main threats and risks derived from the possible appearance of episodes of intense rain and the preventive measures to be undertaken. “.
Pérez recalled that “the fire has caused damage straight in the vegetation, but we must be prepared for the appearance of rain that also “can cause erosion problems.”. Thus, the aim is to preliminarily identify the areas most affected by the forest fire (loss of soil vegetation cover and erosion) and the establishment of areas identified with the greatest hydrological risk, fundamentally in the areas close to the fire. In the coming days, the actions to be carried out in the different areas affected by the forest fire will be established.
The academic vision
The series of studies by ULL and its associated centers has addressed – according to an article published on October 20 by the press office of the university institution – the effect of forest fires on runoff, erosion and the occurrence of floods, as well as earthworks in all types of terrain and, in particular, in volcanic soils. The field work, funded by the European Union (EU) and the United Kingdom Ministry of the Environment, has been carried out in the US, the United Kingdom, Australia and the Canary Islands after the 2015 and 2018 fires in Tenerife (Altos de Arafo and de Vilaflor and Madre del Agua) and in Gran Canaria (Artenara 2019).
This proclivity for runoff and erosion is especially true in northern areas of the islands where weather conditions allow deep soil development which, in many cases, are found in areas of high slope. The impact of fire in these areas is usually severe, which increases the risk of floods and drifts. To give an example, after the Artenara fire in 2019, up to 6 tons of haulage (soil and stones) were quantified in the wetter areas affected by the fire. Regarding material costs, in the case of the Tenerife fire, which affected 14,000 hectares, it is estimated that it will amount to 30 million euros in three years, including forestry work in the affected area.
Effect for years
One of the researchers at the University of La Laguna responsible for these studies, Jonay Neris Tomé, explains that, although the most notable effect of fires is their direct impact on the vegetation, there are others that can be seen even years later and, in some cases, in areas that have not been affected by the fire. This is the case of the increase in the aforementioned processes of runoff and erosion and the subsequent floods and drifts in which a large amount of soil is transported. Recent events in areas affected by forest fires have shown that these events, in conjunction with extreme rainfall, can have severe consequences on the infrastructure and the population located downstream of an area affected by forest fires in the years following the fire.
In volcanic areas
This is especially true in volcanic areas, where the characteristics of the geology (volcanic rocks) together with the climatic conditions and vegetation of the area allow the development of very deep soils even in areas of high slope, such as the soils of the Canarian mountains. These soils, although they are very fertile and stable under natural conditions, are very unstable after being affected by fire both due to their depth conditions and their location in areas of high inclination.
The expert cites, for example, that after the great fire of La Palma in 2009 and during the torrential rains in December of that year, there were large movements of earth from the slopes affected by the fire that produced cuts and even Collapse of roads and affected several homes and crop areas on the coast when the fire broke out at about 800 meters above sea level. Similar events had already been described on the Island in 1973, 1979 and 1990. In 2020, after a small fire in Tenerife, landslides occurred on the TF-5 highway that forced the closure of a road with a traffic intensity of 40,000 vehicles a day.
Direct and indirect costs
The direct and indirect costs derived from these floods and hauls, both for the restoration of infrastructure and the application of measures to reduce risks of runoff and erosion, are usually large: the budget for infrastructure restoration after the 2009 fire in La Palma and the subsequent flood amounted to 8 million euros. After the great fire in Tenerife in 2007, the budget for the application of risk reduction measures amounted to 7 million. And in the case of this year’s Tenerife fire, it is estimated that it will amount to those already mentioned 30 million euros in three years. Post-fire management includes the application of measures to control runoff and erosion on fire-affected slopes to reduce the risk of floods and drifts. Along these lines, the University of La Laguna, together with the councils of Tenerife and Gran Canaria and the Government of the Canary Islandshas carried out studies on the effectiveness of different methods for control after fires.
These studies have provided information on the effectiveness of the control methods currently used (mainly fascines) and have allowed propose the use of alternative methods of high effectiveness in reducing runoff and erosion, such as the use of chips obtained from burned plant material, or pine needles as soil cover to improve infiltration and reduce runoff and soil transport, recommended in certain scenarios. The results obtained have also allowed the adaptation of the WEPPcloud runoff and erosion prediction model, developed by the US Forest Service, for Tenerife and, soon, the rest of the islands. This model, freely accessible and designed for forest managers, allows predicting the probability of occurrence and magnitude of runoff and erosion phenomena in basins affected by forest fires. Currently, the Soil Science and Agricultural Chemistry Area of the academic institution collaborates with technicians from the Cabildo of Tenerife to design measures to control runoff and erosion in the areas affected by the recent Arafo fire in 2023, using the results of this model.
