An experimental fire that was deliberately set in Portugal last Friday has provided Wageningen University researchers in the Netherlands with the first information about the soil erosion that occurs after forest fires. A research team set fire to 10 ha of heathland under controlled conditions. The experiment is not only important for areas in the Mediterranean that have burned or are threatened with fire, but also for Australia, where it is expected that the drinking water supply will be threatened following the recent forest fires. The first measurements in Portugal have shown that the flames reached searing temperatures of up to 900 °C.
PhD student Cathelijne Stoof of Wageningen University is studying what happens to the soil following a forest fire. This knowledge should provide techniques to prevent the ensuing erosion and floods. Part of the study concerns an experimental fire, where an area of 10 ha (equivalent to 15 football fields) in central Portugal was deliberately burned; this is the first experimental fire at such a large scale. The Valtorto research terrain is located near Goís in central Portugal.
Every year in the Mediterranean region, fierce forest fires totally destroy the vegetation in large areas. The first rains following a fire often lead to severe floods and soil erosion, which threaten the livestock and possessions of the population. Prevention of erosion can lead to more rapid recovery of the burned forest and agricultural areas.
The research team installed dozens of sensors and instruments to map out the effects of the fire in the steep experimental terrain. During the fire – which was monitored by three teams of firemen – the temperatures in the soil and the flames were registered. Flame temperatures frequently reached about 550 degrees, with flames rising more than 2 metres. Because the area was burned from both sides at the same time, the fire reached its climax when both fire fronts joined. The temperature rose to 900°C and the flames rose to more than 10 meters. At this point, the amount of heat generated was so great that the smoke and air from the surroundings was sucked into the fire.
The first measurements at Valtorto were made in 2007. From that point, instruments have measured the rainfall, the quantity of precipitation taken up by the vegetation, the soil moisture content and the degree of erosion on the hillsides. An important indicator is the resistance that water encounters as it flows down a slope. As this quantity becomes smaller, the rate of flow increases, and a hillside erodes more quickly. The measurements will be continued for at least two more years.
The recent uncontrolled, extremely hot forest fires and Australia emphasise the relevance of the research of the Wageningen PhD student Cathelijne Stoof: "Following the severe fires near Sydney a few years ago, there were major problems with the supply of drinking water which continued for a long time, because the reservoirs were flooded with eroded sediment. Melbourne is also going to have this problem in the future. This city relies entirely on surface water for its drinking water supply, and there is no water purification system."
The research in Portugal is part of the EU project Desire, in which researchers from around the world are searching for methods to counteract desertification. The participants in the experimental fire research – Escola Superior Agrária de Coimbra in Portugal, Swansea University in the UK and Wageningen University – are working together to prevent land degradation and to restore degraded areas.