**1. Introduction**

In Portugal, as well as in other Mediterranean countries, wildfires and burnt areas have in‐ creased significantly since 1970. This rising trend, although encompassing some periods of lower burnt areas, distinguishes Portugal from other southern European States with the highest number of ignitions and the greatest proportion of burnt areas, particularly in the central and northern regions (Nunes, 2012). Forest fires therefore constitute one of the most significant environmental problems (Moreno, 1989; Vallejo, 1997) and are frequently consid‐ ered the major cause of soil degradation and desertification (Rubio, 1987).

Wildfires can considerably change hydrological processes and the landscape's vulnerability to major flooding and erosion events (Shakesby and Doerr, 2006; Stoof et al., 2012). Post-fire mud‐ flows and flash floods represent a particularly acute problem in mountainous regions (Try‐ horn et al., 2007). In fact, vegetation cover is an important factor in determining runoff and erosion risk (Nunes, 2011). Its removal by fire increases the raindrop impact on the bare soil and reduces the storage of rainfall in the canopy, thus increasing the amount of effective rain‐ fall. Burned catchments are therefore at increased hydrological risk and respond faster to rain‐ fall than unburned catchments (Meyer et al. 1995; Cannon et al. 1998; Wilson, 1999; Stoof et al., 2012). Wildfires also affect the hydrogeological response of catchments by altering certain physical and chemical characteristics of the soils, including their water repellent conditions (Conedera et al. 1998; DeBano et al. 1998; Letey 2001; Martin and Moody 2001; Shakesby and Doerr 2006). Increased runoff can lower the intensity threshold and the amount of precipita‐ tion needed to cause a flood event and also exacerbate the impact of precipitation. Combined with steep slopes, this can create the potential for flash floods.

© 2012 Lourenço et al.; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 Lourenço et al.; licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Various studies in different parts of the world, including Portugal, have shown strong and sometimes extreme responses in runoff generation and soil loss following fires, especially during the earlier stages of the so-called "window-of-disturbance" (Shakesby, 2011).

often localised, precipitation can cause severe post-fire erosion and increase the risk of flash

Soil Erosion After Wildfires in Portugal: What Happens When Heavy Rainfall Events Occur?

http://dx.doi.org/10.5772/50447

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The main objective of this work was to evaluate the impact of fire at catchment level, with particular reference to the implications of the off-site hydrological response and erosional processes after severe rainstorms (involving one occurrence in June 2006 and another in July 2006). In fact, the growing probability of catastrophic wildfires in Portugal and elsewhere in the world has increased the need to understand the flood risk and the erosion and depositio‐

Two catchments (the Pomares and Piodão basins), both located in the mountains of central Portugal, were studied (Figure 1). The study area has a high annual precipitation rate, with an average of 1600/1700 mm yr-1. The rainfall is generally concentrated during the period from October to May, whereas July and August are dry months. According to the Köppen

climate classification, it has a Mediterranean Csb type climate.

**Figure 1.** Location of the study basins and the areas affected by forest fire of 2005.

Both catchments lie on Precambrian schist and have shallow, stony, umbric leptosol soils. Both rivers are tributaries of River Alva and, according to the Strahler classification, are fiveorder streams. Some of the characteristics of both basins are presented in Table I. The Piodão and Pomares basins have areas of 34.3 and 44.7 km2 respectively and both have a high eleva‐ tion gradient of over 1,000 metres. In general, both are surrounded by steep slopes with a top convexity and no basal concavity. More than 90% of the basin areas have slopes of over

flooding and debris flow.

**2. Study area**

nal responses of burned watersheds.

In general, the first 4–6 months after a fire is often the period of greatest vulnerability to ero‐ sion because of the maximum fire potential in summer (July–August) and the likelihood of intense post-wildfire rainfall the following autumn–winter (November–January) (Sala et al., 1994; Andreu et al., 2001). However, soil erosion may reach its peak during the first year af‐ ter a wildfire and subsequently decline, or in some situations be delayed until later, (much later in some cases) during the window of disturbance, in the third or even the fifth year after a fire (Mayor et al., 2007; Llovet et al., 2009). As noted by Ferreira et al. (2009), since the greatest effects of fire on hydrology and erosion generally occur shortly after a fire, data analysis and discussion is limited to the short-term (±1 yr) effects.

Post wildfire hydrological and erosional responses have been assessed at plot and hill slope level in various parts of the world, especially in the Mediterranean region, under natural rainfall conditions (Lourenço, 1989; Sala et al., 1994; Ferreira et al., 1997; Andreu et al., 2001¸ Coelho et al., 2004; Shakesby and Doerr, 2006).

The hydrogeomorphic responses to wildfire at catchment level have received much less at‐ tention than those on smaller scales in locations worldwide, mainly because of the greater practical difficulties and expense involved in monitoring on this scale, and the large chance factor involved in the wildfire burning even a small catchment completely (Shakesby and Doerr, 2006; Shakesby et al., 2006; Shakesby, 2011).

Despite the high rate of occurrences of fires in the European Mediterranean area (Moreira et al., 2001; Pausas, 2004), catchment-scale wildfire studies have mostly been carried out in the USA (Moody et, 2008; Moody and Martin, 2001; Gottfried et al., 2003; Meixner and Wohlge‐ muth, 2003; Nasseri, 1989; Seibert et al., 2010), South Africa (Scott and Van Wyk, 1990; Scott, 1993, 1997) and Australia (Brown, 1972; Langford, 1976; Prosser and Williams, 1998), and in only a few locations in the European Mediterranean area (Lavabre et al., 1993; Mayor et al., 2007; Ferreira et al, 2008; Stoof et al., 2012). In addition, post-fire monitoring is generally comparatively brief (usually 2–3 years) due to logistical and financial constraints, meaning that infrequent severe storms may be missed and the full recovery to pre-fire conditions may not be monitored.

Therefore, the impact of burned areas on peak flow and sediment transport in large river catchments has not been fully studied, although it is of the utmost importance to understand the off-site impacts of forest fires (Ferreira et al., 2008). A better understanding of the hydro‐ geomorphic impacts of fire at catchment level can improve our ability to understand, and therefore possibly predict, the risk of flooding and erosion in burned areas. In fact, when a precipitation event follows a large, high-severity fire, the impacts can cause various kinds of damage on- and off-site including high sediment inputs, downstream flooding, destruction of the aquatic habitat, and damage to human infrastructures.

Moreover, in the Mediterranean region precipitation patterns are highly variable in terms of time, space, amount and duration of events (Durão et al., 2010). The occurrence of heavy, often localised, precipitation can cause severe post-fire erosion and increase the risk of flash flooding and debris flow.

The main objective of this work was to evaluate the impact of fire at catchment level, with particular reference to the implications of the off-site hydrological response and erosional processes after severe rainstorms (involving one occurrence in June 2006 and another in July 2006). In fact, the growing probability of catastrophic wildfires in Portugal and elsewhere in the world has increased the need to understand the flood risk and the erosion and depositio‐ nal responses of burned watersheds.
