Effects of terrace construction on runoff and erosion in a recently burnt forest area in north-central Portugal

FLAMMA, 4 (2), 81-84, 2013 ISSN 2171 - 665X

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Effects of terrace construction on runoff and

erosion in a recently burnt forest area in

north-central Portugal

M.A.S. Martins (*), D. Serpa, A.I. Machado, R.F.H.de Lenne, A.G.v.d. Linden, S.R. Faria, R.S.V. Ferreira, I. Skulska, S.A. Prats, M.E.T. Varela, J.J. Keizer

Centre for Environmental and Marine Studies (CESAM), Dept.of Environment and Planning, University of Aveiro, 3810-193 Aveiro, Portugal

* Corresponding author: [email protected]

Keywords Abstract

Forestry Terraces Runoff Erosion Wildfires

In present-day Portugal, wildfires are a common phenomenon that, on average, affects some 100.000 ha of forest lands each year. Fires can markedly increase runoff generation and the associated sediment transport, nevertheless the magnitude of these impacts strongly depends on post-fire forestry management practices. This study evaluates the effect of terrace construction on runoff and erosion at micro-plot and catchment scale, six months after a wildfire in a forest area in north central Portugal. At the micro-plot scale, there was a clear trend for greater runoff volumes and sediment losses after the construction of terraces. In addition, pre-terracing sediment losses showed a visible relationship with rainfall amounts unlike post-terracing losses. At the catchment scale, an increase in sediment losses was observed after terracing consistently with micro-plot data.

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In Portugal, wildfires occur frequently and affect large areas, on average some 100.000 ha per year but in extreme years such as 2003 and 2005 over 300.000 ha (Pereira et al., 2005). As thoroughly reviewed by Shakesby and Doerr (2006) and Shakesby (2011), wildfires can lead to considerable changes in geomorphologic and hydrological processes. Previous studies in various parts of the world, including Portugal (Ferreira et al., 2005; Prats et al., 2012; Shakesby et al., 1996), have revealed strong and sometimes extreme responses in runoff generation and associated soil losses following wildfire, especially during the earlier stages of the so- alled i do

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Figure 1. Relationships of runoff (upper graphs) and sediment losses (lower graphs) with rainfall amounts before and after the construction of terraces for a recently burnt eucalypt plantation (left graphs) and a recently burnt Maritime Pine stand (right graphs).

one occasion, gully formation over the full hill slope length. Runoff and associated sediment losses, however, have hardly ever been quantified for recently terraced forest plantations.

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The main aim of this study was to assess the impact of the construction of terraces on overland flow generation and inter-rill erosion rates. This was done by comparing the results for micro-plots that were installed in a eucalypt a Maritime Pine plantation following a wildfire in July 2010, and then re-installed after the construction of terraces during February 2011. A second objective was to assess the terracing impact at catchment scale, by comparing bed loads before and after terracing.

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3.1 STUDY AREA AND EXPERIMENTAL SITES

This study was carried out in a forest area in the Sever do Vouga municipality, north-central Portugal, that was burnt by a wildfire during the summer of 2010. Within this

Er ida ur t area of roughl 250 ha, si hill slopes ere

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83 February 2011, the plots were re-installed on basically the same locations. This was not done on the terraces themselves but on newly-constructed dirt roads, first and foremost to maintain a similar slope angle to before the terracing.

Within the Ermida area, an entirely burnt catchment of roughly 25 ha was instrumented with two subsequent flumes with maximum discharge capacities of 120 and 1700 L s-1. The bed load that deposited in the smallest flume was removed and weighted in the field at regular intervals. Also, one or more samples were collected for laboratory analysis.

Rainfall was measured since the middle of August 2010, using several automatic and collecting gauges in the burnt area.

3.2 FIELD DATA AND SAMPLE COLLECTION

From August 2010 to August 2011, the runoff amounts

a u ulated i the ta ks o e ted to the plots outlets

were measured at 1- to 2-weekly intervals, depending on rainfall, and samples of 1.5 L taken for laboratory analyses. Rainfall accumulated in the collecting gauges was measured on the same occasions.

3.3 LABORATORY ANALYSIS

The runoff samples were filtered in the laboratory using filters with a 12-14 µm pore size. The filters were then dried in an oven for 24 hours at 105 oC to determine sediment concentrations. The bed load samples were also dried in an oven for 24 h at 105 oC to determine moisture content.

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At both study sites but especially at the eucalypt plantation on granite, there was a tendency for greater runoff volumes after the construction of the terraces then before it (Figure 1). Also, the differences in runoff volumes between the two study sites were clearer after than before terracing, with the eucalypt sites producing more overland flow than the pine site. The relationship of overland flow with rainfall, however, was similarly well-defined for both sites and both study periods.

Sediment losses revealed a more pronounced role of terracing than runoff volumes before with (almost) consistently higher values after than before terracing

(Figure 1). Also, the differences in sediment losses between the two sites were more marked after than before terracing. Pre-terracing sediment losses revealed a detectable relationship with rainfall amounts but post-terracing losses did not.

Thus, the present results showed that overland flow generation and especially mobilization of sediments on sloping terrain in terraced areas (such as dirt roads) can be substantial and can exceed immediate post-fire runoff and erosion rates.

The bed load data suggested that terracing also increased sediment losses at the catchment scale. Namely, 875 mm of rainfall that fell before terracing produced about 340 kg of bed load, whilst 800 mm that fell after terracing produced 620 kg. Bed load deposition rates per mm of rain were thus about twice as high after than before terracing, 0.39 and 0.74 kg mm-1 rain, respectively.

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