FLAMMA, 4 (2), 81-84, 2013
ISSN 2171 - 665X
CC Attribution-NonCommercial-ShareAlike 3.0 License
Effects of terrace construction on runoff and
erosion in a recently burnt forest area in northcentral 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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disturbance’’. Besides wildfire itself, however, also postfire forestry practices can strongly influence overland flow
and erosion in recently burnt areas (Fernández et al., 2007;
Walsh et al., 1995). For example, rip-ploughing during the
window-of-disturbance was far more damaging in terms of
soil loss than fire Shakesby et al., 1994). The construction
of terraces in preparation of a new eucalypt plantation has
become increasingly common in the mountain areas of
north-central Portugal, including in recently burnt areas.
Although terraces are traditionally viewed as a technique
for soil conservation (Novara et al., 2011), the authors
have measured high splash erosion rates on recent
terraces (Fernández-Raga et al., 2010), and have
frequently observed small-scale erosion features and, on
INTRODUCTION
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-called ‘‘window-of81
FLAMMA | Vol. 4 | 2 | 81-84
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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METHODOLOGY
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
“Ermida” burnt area of roughly 250 ha, six hill slopes were
selected as study sites for the FIRECNUTS project but only
two were considered for this study. One was covered by a
Maritime Pine plantation on schist-derived soils, and the
other by a eucalypt plantation on granite-derived soils.
Prior to any significant rainfall following the wildfire, both
slopes were instrumented with three micro-plots (0.25 m2)
located at the base of the slopes at distances of 2-3 m
from each other. After the construction of terraces in
OBJECTIVES
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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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.
(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 postterracing losses did not.
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.
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.
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
REFERENCES
From August 2010 to August 2011, the runoff amounts
accumulated in the tanks connected 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.
Fernández C, Vega JA, Fontúrbel T, Pérez-Gorostiaga P,
Jiménez E, Madrigal J. 2007. Effects of wildfire,
salvage logging and slash treatments on soil
degradation. Land Degradation and Development
18: 591-607.
Fernandéz-Raga M, Fraile R, Keizer JJ, Varela MET, Castro
A, Palencia C, Calvo AI, Koenders J, Marques RLC.
2010. The kinetic energy of rain measured with an
optical disdrometer: An application to splash
erosion. Atmospheric Research 96: 210-240
Ferreira AJD, Coelho COA, Boulet AK, Lopes FP. 2005.
Temporal patterns of solute loss following
wildfires in Central Portugal. International Journal
of Wildland Fire 14: 401-412.
Novara A, Gristina L, Bodí MB, Cerdà A. 2011. The impact
of fire in redistribition of soil organic matter on a
mediterranean hillslope under maquia vegetation
type. Land degradation & development 22: 530536.
Pereira J, Carreira J, Silva J, Vasconcelos M. 2005. Alguns
conceitos basicos sobre os fogos rurais em
Portugal. In: Pereira JS, Pereira JMC (Eds.),
Incêndios lorestais em Portugal - caracterização,
impactes e prevenção, ISAPress, Lisbon, pp. 133.
Prats SA, Macdonald LH, Monteiro MSV, Ferreira AJD,
Coelho COA, Keizer JJ. 2012. Effectiveness of
forest residue “mulching” in reducing post-fire
runoff and erosion in a pine and a eucalypt
plantation in north-central Portugal. Geoderma,
in press, DOI:10.1016/j.geoderma.2012.02.009.
Shakesby RA, Boakes D, Coelho COA, Gonçalves AJB, Walsh
RPD. 1996. Limiting the soil degradational impacts
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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RESULTS AND CONCLUSIONS
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
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of wildfire in pine and Eucalyptus forests in
Portugal. Applied Geography 16: 337-355.
Shakesby RA, Coelho COA, Ferreira AJD, Terry JP, Walsh
RPD. 1994. Fire, post-burn land management
practice and soil erosion response curves in
eucalypt and pine forests, north-central Portugal.
In: Sala M, Rubio JL (Eds.), Soil erosion and
degradation as a consequence of forest fires.
Geoforma Ediciones, Logroño, pp. 111-132.
Shakesby RA, Doerr SH. 2006. Wildfire as a hydrological
and geomorphological agent. Earth-Science
Reviews 74: 269-307.
Shakesby RA. 2011. Post-Wildfire soil erosion in the
Mediterranean: Review and future research
directions. Earth-Science Reviews 105: 71-100.
Walsh RPD, Boakes DJ, Coelho COA, Ferreira AJD, Shakesby
RA, Thomas AD. 1995. Postfire landmanagement
and runoff responses to rainstorms in Portugal. In:
McGregor D, Thompson D (Eds.), Geomorphology
and Land Management in a Changing
Environment. John Wiley & Sons Ltd, Chichester,
pp. 283-308.
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