**2. Study area**

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

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

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¸

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

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

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

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,

during the earlier stages of the so-called "window-of-disturbance" (Shakesby, 2011).

analysis and discussion is limited to the short-term (±1 yr) effects.

Coelho et al., 2004; Shakesby and Doerr, 2006).

Doerr, 2006; Shakesby et al., 2006; Shakesby, 2011).

of the aquatic habitat, and damage to human infrastructures.

may not be monitored.

66 Research on Soil Erosion Soil Erosion

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 20% and in the Piodão river more than a half of the watershed has slopes of over 50%. A comparative analysis shows the basin ruggedness and coefficient of torrentiality to be slight‐ ly higher in the Pomares basin.

jor fires occur in summer, essentially in July and August. At this time of year, several factors combine to create the right conditions for the onset and propagation of wildfires. It is the driest time of year as well as the season for tourism, which includes camping and picnick‐ ing, and it is also the time when agricultural refuse and slash are traditionally cleaned and

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

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

69

Consequently, as in other Mediterranean countries, Portugal's burnt area has increased sig‐ nificantly in recent decades. In the past three decades, the number of forest fires exceeded half a million ignitions and the total burnt area was approximately 3,236,890 ha, represent‐ ing more than a third of the surface area of mainland Portugal (Nunes, 2012). Within the last 30 years (1981-2010), 2003 and 2005 were the worst fire seasons in Portugal, resulting in the burning of almost 430,000 hectares and 325.000 hectares respectively of forest land, shrub

The Pomares and Piodão catchments have been severely affected by wildfires since the 1970s. Two large wildfires have affected the greater part of the area of both catchments: the first, be‐ tween 13th and 20th September 1987, burnt a total of 10,900 hectares, and the second, occurring eighteen years later between 19th and 24th July 2005, affected an area of 17,450 hectares (Louren‐

The post-wildfire hydrological and erosional responses are based on intensive post-event fieldwork to determine the geomorphological impacts and socio-economic implications by collating, collecting and analysing data from field studies that was essential to understand‐

The meteorological characteristics of the storms that affected the basins were determined us‐ ing data from a rain gauge installed in the Piodão basin. Daily and 30-minute rainfall inten‐ sity measures (I30) were chosen for each event, since rainfall frequency studies (Hershfield, 1961; Miller et al., 1973) indicate that in mountainous terrain 79% of the hourly rainfall oc‐ curs within 30 minutes and this type of storm has a short duration, lasting between 10 and

The fieldwork took place a few days after the events occurred and was based on identifying

Indicators of the peak discharge values, mainly cross-section surveys based on flood marks, in addition to signs of flow velocity (witness observations and water super-elevations in riv‐ er bends or in front of obstacles). High water marks on channel banks, mostly indicated by the deposition of vegetation fragments and silt, were visible in the sites. These marks are very important and provide approximate estimates for reconstructing peak discharges for

Sediment transfer processes (erosion and deposition on slopes and in river beds, hypercon‐ centrated mud or debris flow), which may give an indication of local runoff generation proc‐

ço, 2006a b, 2007). Figure 1 shows the burnt area associated with both wildfires.

ing the meteorology, hydrology and hydraulics of the event.

60 minutes (Moody and Martin, 2001).

esses and flow energy and velocity.

ungauged cross-sections of rivers affected by floods.

burned after crops have been harvested.

land and crops.

**3. Methodology**

certain variables:


1. maximum change in elevation within a basin, divided by the square root of the basin area (Melton, 1965);2. the total length of all channels within a basin, divided by the basin area, (Horton, 1945)

**Table 1.** Main characteristics of both basins.

Important demographic and socio-economic changes have affected the mountain areas of Portugal for at least the last five to six decades. The population of the mountain areas de‐ creased substantially during the second half of the 20th century, leading to the abandon‐ ment of agricultural land and a reduction in the size of herds and the amount of forest fuels consumed by grazing and the collection of firewood (Rego, 1992; Moreira et al., 2011; Lourenço, 1996, Nunes, 2012).

Consequently, the landscape has been drastically modified due to the sequential abandon‐ ment of traditional land use throughout the second half of the 20th century. The increase in uncultivated land has led to a secondary vegetation succession and modification of the veg‐ etation structure, favouring horizontal and vertical fuel continuity and a consequent in‐ crease in flammable biomass. The unmanaged accumulation of large quantities of fuel and the exclusion of fires from forest management has led to a dramatic increase in the magni‐ tude and frequency of forest fires (Carvalho et al., 2002; Moreira et al., 2011).

In addition, afforestation has focused primarily on highly inflammable species, mainly pines (predominantly *Pinus pinaster*) which also favours the proliferation of forest fires (Shakesby et al., 1996). Once fires break out under these highly dangerous conditions, they spread more easily and cannot be stopped. The low population density, delays in detecting fires, and diffi‐ culties in gaining access to the sites where fires tend to start, due to the rugged topography, are other factors that explain the large burnt areas in the central mountain area of Portugal.

The Mediterranean characteristics of the Portuguese climate (warm, dry summers and rela‐ tively wet winters) make it prone to wildfires and post-fire soil erosion. In Portugal, the ma‐ jor fires occur in summer, essentially in July and August. At this time of year, several factors combine to create the right conditions for the onset and propagation of wildfires. It is the driest time of year as well as the season for tourism, which includes camping and picnick‐ ing, and it is also the time when agricultural refuse and slash are traditionally cleaned and burned after crops have been harvested.

Consequently, as in other Mediterranean countries, Portugal's burnt area has increased sig‐ nificantly in recent decades. In the past three decades, the number of forest fires exceeded half a million ignitions and the total burnt area was approximately 3,236,890 ha, represent‐ ing more than a third of the surface area of mainland Portugal (Nunes, 2012). Within the last 30 years (1981-2010), 2003 and 2005 were the worst fire seasons in Portugal, resulting in the burning of almost 430,000 hectares and 325.000 hectares respectively of forest land, shrub land and crops.

The Pomares and Piodão catchments have been severely affected by wildfires since the 1970s. Two large wildfires have affected the greater part of the area of both catchments: the first, be‐ tween 13th and 20th September 1987, burnt a total of 10,900 hectares, and the second, occurring eighteen years later between 19th and 24th July 2005, affected an area of 17,450 hectares (Louren‐ ço, 2006a b, 2007). Figure 1 shows the burnt area associated with both wildfires.
