Versão online: http://www.lneg.pt/iedt/unidades/16/paginas/26/30/185
Comunicações Geológicas (2014) 101, Especial II, 1051-1054
IX CNG/2º CoGePLiP, Porto 2014
ISSN: 0873-948X; e-ISSN: 1647-581X
Estimative of sediment yield in mountainous catchments: a
pilot study in a meso-scale catchment of the Douro Basin,
Northern Portugal
Estimativa de transporte de sedimentos em bacias de montanha:
um estudo piloto numa sub-bacia de dimensão média da Bacia
do Douro, NE de Portugal
A. R. Reis1,2*, J. M. M. Lourenço3, A. Parker4, A. Alencoão1,2
Artigo Curto
Short Article
© 2014 LNEG – Laboratório Nacional de Geologia e Energia IP
Abstract: This study describes the preliminary results of a GISbased mass balance model of overland sediment transport in the
River Corgo catchment, located in the River Douro basin. The
erosion, the first step of sediment transport, was estimated by an
empirical model - The Revised Universal Soil Loss Equation
(RUSLE). The objective is to construct a GIS based potential soil
loss spatial index model and posteriorly estimate the sediment yield
for different locations within the catchment. The preliminary results
show that about 2% of the study area is classified as highest erosion
risk potential, and 22% area is under low to moderate erosion risk;
these locate in the west, northwest and southern regions of the study
area. The estimated soil losses are related, essentially, with one of the
lithologies outcropping in the catchment area (48%).
Keywords: Soil erosion, USLE, GIS, Lithology, Sediment.
Resumo: O estudo apresentado descreve os resultados preliminares
de um modelo de balanço de massa, desenvolvido em ambiente SIG,
relativo ao transporte de sedimentos na bacia hidrográfica do rio
Corgo, a qual se integra na bacia hidrográfica do rio Douro. A
erosão, a primeira fase do transporte de sedimentos, foi estimada pelo
método empírico – A Equação Universal de Perda de Solo. O
objetivo é desenvolver um modelo do índice espacial de perda do
solo, em ambiente SIG, e posteriormente estimar a produção de
sedimentos em diversos locais da bacia de drenagem, em função da
litologia. Os resultados preliminares mostram que cerca de 2% da
área de estudo apresenta risco de erosão potencial elevado, e cerca de
22% da área é classificada com risco de erosão potencial baixo a
moderado, estando neste caso as zonas oeste, noroeste e sul da área
de estudo. As perdas de solo estimadas estão relacionadas,
essencialmente, com uma das litologias aflorantes na área da bacia
(48%).
Palavras-chave: Erosão do solo, USLE, SIG, Litologia, Sedimentos.
1
Department of Geology, University of Trás-os-Montes e Alto Douro
(UTAD), Quinta de Prados, 5000-801 Vila Real, Portugal.
2
Center for Geophysics from the University of Coimbra, Coimbra, Portugal.
3
Center for Geosciences, University of Coimbra, Coimbra, Portugal.
4
Soil Research Group, School of Human and Environmental Sciences,
University of Reading, Reading, UK.
*
Corresponding author / Autor correspondente: [email protected]
1. Introduction
Modelling the transport of sediment and adsorbed
contaminants within catchments can help to identify
possible contaminant sources, as well as to estimate the
delivered quantities of eroded material and associated
contaminants. The present study considers a meso-scale
mountainous rural catchment, in a temperate climate,
which integrates the transboundary River Douro Basin, in
the northeast of Portugal.
Previous studies performed in this catchment show
that, on the one hand, the intrinsic morphological features
of this mountainous catchment raise some issues
concerning the monitoring in situ of transported sediments:
(a) the low mass yield of suspended sediment from river
water, under low-flow conditions; (b) the maintenance of
the sediment sampler's devices in the streams, in periods of
high-flow or storm events. On the other hand, a
geochemical survey on oxic fluvial sediments of the river
network shows considerable contents of metals associated
to the finer particles (< 63 µm). The results on the study of
the sediment properties indicate that these are essentially
detrital in origin, derived from soils and weathering
products. Moreover, taking into account the hydrological
pattern of the catchment, the seasonal and spatial
variability of metal contents associated to the sediments
suggests that the control of metal in the sediments by their
mineralogical, geochemical and physical properties is
governed primarily at the level of the basin soils system,
especially in the Wet Period, when the sediments are
frequently remobilised (Reis, 2010). Although the soil
particles are a common pathway of transport and entrance
of metals in the fluvial network by runoff derived erosion,
this mechanism is naturally more marked in mountainous
catchments.
The aim of this study is to develop, in a first stage, a
GIS based potential soil loss spatial index model and
posteriorly estimate the sediment yield from different
locations within the catchment. The final objective is to
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A. R. Reis et al. / Comunicações Geológicas (2014) 101, Especial II, 1051-1054
combine data on rates of soil erosion, and inherent
sediment production, and sediment quality (related, in
particular, with lithology and land use) transported in the
drainage network. Hence, it is described the preliminary
results related with the assessment of the annual soil
erosion vulnerability of the studied catchment.
2. Study area
The study area is located in the transboundary River Douro
basin (northern Portugal); it comprises the major part of
the River Corgo fluvial network to the gauge station
Ermida, which drains a meso-scale rural catchment with an
area of 295 km2, in a temperate climate. The land use is
mainly forest and agriculture, with scattered urban
settlements. The industrial activity is scarce.
The geology comprises crystalline rocks: in the
southern part Paleozoic metasediments outcrop, which are
composed essentially of schists, greywackes and
quartzites; in the northern part of the basin, granites
intruded the older metamorphic rocks. Locally the
crystalline rocks are covered by Quaternary deposits, with
major expression in two agricultural valleys: Campeã
valley in the southwest area, and Vila Pouca de Aguiar
valley, in the northern area. The altitudes in the basin vary
between 300-1400m.
3. Methodology
The erosion (the first step of sediment transport), was
estimated by an empirical model - The Universal Soil Loss
Equation (USLE) - developed by Wischmeier & Smith
(1978). A comprehensive methodology that integrates this
Equation and Geographic Information Systems (GIS)
techniques was adopted.
The R factor (rainfall – erosivity index) was obtained
applying the model proposed by Ferro et al. (1991),
adapted to the portuguese continental territory by Brandão
et al. (2001), using the values of 24 udometric stations.
The K factor (soil erodibility index) was derived from the
Soil Map of Trás-os-Montes (Agroconsultores, 1991),
applying the values proposed by Pimenta (1998). The
combined determination of LS factor (slope length and
stepness factor) was calculated from the digital terrain
model (DEM) of the area under study, by applying the
methodology proposed by Moore & Burche (1986a, b); to
avoid the influence of the areas corresponding to the
bottom of the valleys, where the accumulated flows are
higher and do not translate any relationship with the
lengths of slopes, there was attributed the value
corresponding to the maximum length of the slopes in the
DEM (750 m/25 m). The C factor (land cover management
index) was calculated from the Corine Land Cover 2006
coverage and land use map published for continental
Portugal, at scale 1:100.000 (Caetano et al., 2009),
assigning, to the different classes, the values recommended
by Pimenta (1998). The P (conservation practice index)
values resulted from the adaptation of the values proposed
by Wischmeier & Smith (1978) and Tomás (1993) to the
agricultural areas of the Corine Land Cover 2006 map
(Caetano et al., 2009), classified by slopes (from the
DEM) and according to the configuration of agricultural
crops, obtained by analysis of aerial photographs from
2004 to 2006.
4. Results and discussion
The preliminary results indicate that the model is in
accordance with the knowledge of the study area, and can
be used as an initial indicator of areas of potential
sediment source. So, the results show that potential loss is
typically higher along the areas where the tributaries are
deeply incised and bordered by steeper slopes, with locally
extreme values.
Taking into account the field conditions, the assessed
annual soil loss rates of River Corgo catchment were
grouped into 5 classes, according to their natural breaks,
and analysed against lithologies in terms of areal
distribution. The results are presented in table 1 and the
correspondent map in figure 1; about 76% of the study
area is classified as very low potential erosion risk, while
the rest of the area is under low to moderate erosion risk
(22%); the areas showing the highest erosion risk
potential, corresponding to 2%, are located in the west,
northwest and southern regions of the study area, while
the areas with low erosion risk are in the eastern, central
part and in the major valleys occurring in the study area.
Table 1. Areal distribution of estimated soil loss classes by lithology (%).
Tabela 1. Distribuição areal das classes de perda de soloestiadas por
litologia (%).
The contribution of each type of lithology to the
estimated soil losses (= estimated soil loss per lithology x
area occupied per lithology, expressed in ton/year) was
Sediment yield in mountainous catchments
also analysed (Table 2 and Fig. 2). The results suggest that
almost half of the estimated soil losses are related to one
type of outcropping lithologies. The lithology ϒI4.3
(medium-to-coarse-grained, with scarce phenocrysts, twomica granite – patch 123) contributes 48% of the estimated
soil loss, although the respective occupied area is only
12% of the study area. Furthermore, the assemblage of the
lithological units ϒI4.3 (medium-to-coarse-grained, with
scarce phenocrysts, two-mica granite – patch 123), DE
(Desejosa Formation – patch 119) and ϒIIIb (coarse-tomedium-grained porphyritic biotite granite – patch 152),
which occur spatially in contiguous areas in the southwestern area of the catchment, contribute 69.7% of the
estimated soil loss. An interconnected drainage network
drains these lithologies.
1053
in development. This includes field work with the aim of
assess sediment volumes transported in specific sites of the
drainage network, and its relation with different drained
lithologies, to provide quantitative predictions of river
sediment yield. The analysis of potential/possible
relationships between the mineralogical and chemical
characteristics of the oxic sediments, and transport of
associated contaminants will also be considered.
Table 2. Estimated soil losses per lithological patch.
Tabela 2. Perdas de solo estimadas por mancha litológica.
Fig 1. Estimated soil loss map of River Corgo catchment.
Fig 1. Mapa de perda de solo estimada para a bacia hidrográfica do Rio
Corgo.
In the central and north areas of the Corgo basin, the
contribution of each type of lithology to the estimated soil
losses is more uniformly distributed by lithology, though
the lithology ϒIIIb (coarse-to-medium-grained porphyritic
biotite granite – patch 251), which also outcrops in the
north limit area of the basin, shows slightly higher values,
in relation to the surrounding lithologies.
The results of this study, obtained by the application of
USLE model in a GIS environment, allowed to identify the
spatial patterns of the soil erosion risk in a mountainous
rural catchment, developed in crystalline rocks. The model
outcomes need to be validated and further investigation is
Fig 2. Estimated soil loss per lithological patch for the Corgo River
catchment.
Fig 2. Perda de solo estimada por mancha litológica para a bacia
hidrográfica do Rio Corgo.
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A. R. Reis et al. / Comunicações Geológicas (2014) 101, Especial II, 1051-1054
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