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Effects of forest fires and post-fire land management practice on soil erosion and stream dynamics, Águeda basin, Portugal

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Soil and groundwater research report V
Environmental degradation
Environmental research

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ISSN 1018-5593
European Commission
SOIL AND GROUNDWATER RESEARCH REPORT V
Effects of forest fires and post­
ij fire land management practice
' on soil erosion and stream
dynamics, Águeda basin, Portugal European Commission
environment
and quality of life
Effects of forest fires and post-fire land
management practice on soil erosion and
stream dynamics, Águeda basin, Portugal
Soil and groundwater research report V
C. de O. A. Coelho,1 R. A. Shakesby,2 R. P. D. Walsh2
1 Departamento de Ambiente e Ordenamento
Universidade de Aveiro
P-3800 Aveiro
department of Geography
University College of Swansea
Singleton Park
Swansea SA2 8PP
United Kingdom
Contract No EV4V-0106-C(TT)
Final report
Directorate-General
Science, Research and Development
Environment research programme
1995 EUR 15689 EN Published by the
EUROPEAN COMMISSION
Directorate-General XIII
Telecommunications, Information Market and Exploitation of Research
L-2920 Luxembourg
LEGAL NOTICE
Neither the European Commission nor any person acting on behalf of
the Commission is responsible for the use which might be made of the
following information
This document has been reproduced from the best original available
Cataloguing data can be found at the end of this publication
Luxembourg: Office for Official Publications of the European Communities, 1995
ISBN 92-826-9687-1
© ECSC-EC-EAEC, Brussels · Luxembourg, 1995
Printed in Spain CONTENTS
Page
1. INTRODUCTION 1
2. BACKGROUND TO THE STUDY AREA
(a) Topography, geology and soils
(b) Climate, hydrology and drainage density 4
(c) Land use 5
(d) Fire history
3. EXPERIMENTAL DESIGN 7
(a) The catchment experimental design
(b) Experimental design of slope- and plot-scale studies 8
(c) Ancillary approaches 9
4. THE PROJECT CATCHMENTS
5. INSTRUMENTATION AND TECHNIQUES
(a) Rainfall and climate
(b) Streamflow measurement 26
(c) Water quality monitoring and analysis7
(d) Monitoring soil loss with the soil erosion bridge 2
(e)g soil loss with wash traps 34
(f) Monitoring soil loss with bounded plots5
6. RESULTS 3
(a) Rainfall during the project
(b) Soil loss on slopes 4
(i) Soil erosion bridge
(ii) Wash traps 52
(iii) Bounded plots3
(c) Hydrology 61
(i) Streamflow and catchment water balances 6
(ii) Flood peak of streams
(iii) Slope hydrology4
(iv) Suspended sediment transport 7
(v) Dissolved load transport and streamwater quality 7
III 7. DISCUSSION 75
(a) Post-fire soil erosion behaviour: the character of soil
erosion response curves
(b) Soil erosion in the Águeda Basin compared with other fire-prone areas 79
(c) Implications for soil conservation 82
(d)s for medium-term environmental change 83
(e) Stream hydrology, suspended sediment and hydrochemistry4
8. CONCLUSIONS 8
9. ACKNOWLEDGEMENTS8
10. REFERENCES9
- IV 1. INTRODUCTION
Forest fire is a growing hazard in Portugal and is increasingly influencing decisions on land
use and land management practices. Very little, however, is known about the erosional and
hydrological consequences in the Atlantic Mediterranean environment of northern Portugal of
the forest fires and the associated changes in land use and land management practices that
follow in their wake.
This project arose out of a concern for the effects on soil erosion and stream dynamics of a
major forest fire that destroyed some 6000 hectares of Pinus pinaster and Eucalyptus globulus
forest in June 1986 in the Águeda River Basin (lat. 40° 35' N, long. 8° 26' E), north-central
Portugal. It has involved geographers from Universidade de Aveiro and University College of
Swansea and was financed for a 3 year period beginning in January 1988. While the original
focus of the project concerned the effects of fire, it was apparent by the time the project
started that post-fire land management practice was an important additional influence on soil
erosion and water quality that needed to be considered. Additional fires in 1988 and 1989
have allowed the early effects of fire to be investigated. The specific aims of the project
were:
(1) to monitor the effects of fire and post-fire land management practice on soil erosion and
surface runoff by means of various techniques (wash traps, erosion plots, and repeat-survey
with a specially-designed soil micro-profiling device). In addition to sites established in areas
affected by fire and land use change, monitoring was also carried out in unburnt areas which
acted as control sites.
(2) to monitor the effects of fire and post-fire land management practice on discharge and
water quality. This was achieved by establishing five small instrumented catchments of
similar size and relief characteristics in the Águeda Basin. These comprised two unburnt
control catchments and three catchments reflecting different land management practices
involving pine and eucalyptus.
(3) to construct composite post-fire response curves for stream discharge, water quality and
soil erosion and to investigate in the 'falling limb' of these factors for the different post-fire
land management practices.
(4) to make recommendations concerning appropriate action that might be taken to minimise
soil erosion losses and detrimental effects on water quantity and quality.
(5) to assess the likely long-term implications of the present-day scene of frequent fires and
frequent cropping of eucalyptus on catchment hydrology and geomorphology.
2. BACKGROUND TO THE STUDY AREA
(a) Topography, geology and soils
The research area (Fig. 1) lies some 40 km from the coast in a well-dissected upland part of
the Águeda River Basin, which is a tributary basin of the Vouga. The underlying geology of
the area, including the five selected catchments (Fig. 1), is schist, though granite crops out to Figure 1 - Location map of project area (a) Rainfall
Oliveira do Bairro Campia
1931-85 1931-85
1077mm 1 926mm
300
200-
mm
100
J F MAMJJ ASOND J F MAMJJ ASOND
(b) Runoff
Alfusqueiro at Ribeiro Águeda at Pt. Redonda
1977-84 1977-84
Annual Runoff - 1 1 38mm Annual Runoff- 1243mm
300-1
200-
mm
100-
J F MAMJJ ASOND J F MAMJJ ASOND
(c) Streamflow
Mean Flow - 7.4m3/s Mean Flow - 6.0m /s
15-
10 -
m3/s
JFMAMJJASOND JFMAMJJASOND
Figure 2 Rainfall and runoff in the project area
-3-the north-east. The topography is hilly and deeply dissected by the Águeda river and its
tributaries. Altitude ranges from less than 20 m at the confluence of the Águeda and
Alfusqueiro rivers to over 500 m on ridges and hill summits in the east. Valley side slopes
are generally over 20° throughout the region and in some locations exceed 40°. Slope profiles
are y convexo-rectilinear with little or no basal concavity.
As part of the project research programme, a reconnaissance survey of soils in the region was
carried out in March 1988; this is reported in full in a separate report (Bridges, 1990) and
only key findings are summarised here. Soils were studied in transect fashion down slope
profiles in each of the five selected study catchments. The same broad pattern was noted in
each catchment.
The steep valley side slopes are characterised by Humic Cambisols: these are shallow stony
soils with a dark brown, silt loam surface horizon and no Β horizon. The more
gently-sloping upper convexity and slope hollows, however, are covered by Dystric
Cambisols. These are deeper soils developed on greater depths of weathered and partially
weathered schist; these soils, though still stony, typically comprise both a 0-20 cm deep dark
brown silt loam surface horizon and an underlying horizon of 10-40 cm depth and made up of
yellowish brown silt loam mottled with yellowish brown weathering schist.
(b) Climate, hydrology and drainage density
The climate is Mediterranean with a dry summer season. Rather heavy depressional rains,
enhanced by relief and proximity to the Atlantic coast, occur in autumn, winter and spring.
Prior to the study, there were no rainfall stations in the study area, but records for Oliveira do
Bairro (annual rainfall 1077 mm), a lowland station 12 km to the south-west, and Campia
(annual rainfall 1925 mm), in the upper reaches of the Águeda Basin some 15 km to the
northeast, suggest that annual rainfall in the research area is around 1600 mm. Rainfall
regimes for these two stations (Fig. 2) confirm the wetness of the winter months, particularly
November to March, and the shortness (by Mediterranean climate standards) of the dry
summer season, with only July and August having mean rainfalls of less that 30 mm.
Rainfalls in the winter months are often large and intense. At Campia, the highest daily fall
exceeds 90 mm in most years and a fall of 165 mm was recorded on 14th January 1977.
No streamflow records existed for the study catchments prior to the project, but there are
river flow records for the Águeda at Ponte Redonda and for its major north bank tributary the
Alfusqueiro at Ribeiro (for locations see Fig. 1). Monthly runoff (mm) and monthly mean
flow (m3/s) data for the two rivers are given in Fig. 2. Annual runoff is high, exceeding 1100
mm in both catchments, reflecting the high annual rainfall of the area. The runoff regimes,
because of the shortness of the flow records (just 5-6 years of data), not surprisingly do not
conform exactly to the rainfall regimes, which are based on 54 years of record, but the
seasonality of the flows is nevertheless quite clear. Mean flows for the Alfusqueiro range
from 15.34 m3/s in December to 1.04 m3/s in August and for the Águeda range from 14.45