The study and measurement of overland fl
FUEGORED 2010
Santiago de Compostela, 6-8 de octubre de 2010
Jornadas Internacionales
INVESTIGACIÓN Y GESTIÓN PARA LA
PROTECCIÓN DEL SUELO Y RESTAURACIÓN
DE LOS ECOSISTEMAS FORESTALES
AFECTADOS POR INCENDIOS FORESTALES
International Workshop
RESEARCH AND POST-FIRE MANAGEMENT: SOIL PROTECTION AND
REHABILITATION TECHNIQUES FOR BURNT FOREST ECOSYSTEMS
M. Díaz Raviña, E. Benito, T. Carballas, M. T. Fontúrbel, J. A. Vega (eds.)
The study and measurement of overland flow and soil erosion on slopes
affected by forest fires in Lousã Mountain – main results
A. Bento Gonçalvesa*, L. Lourençob**
a
Centro de Estudos em Geografia e Ordenamento do Território (CEGOT), Núcleo de Investigação em Geografia
e Planeamento (NIGP), Departamento de Geografia, Minho University, Campus de Azurém, 4800-058
Guimarães (Portugal); b Centro de Estudos em Geografia e Ordenamento do Território (CEGOT), Núcleo de
Investigação Científica de Incêndios Florestais (NICIF), Departamento de Geografia da Faculdade de Letras,
Coimbra University, 3004-530 Coimbra (Portugal)
*[email protected],**[email protected]
Introduction
In Portugal, forest fires are becoming increasingly frequent as a result of socio-economic
change and poor forest planning, with impacts on soil fertility and structure.
Wet Mediterranean regions are characterized by ecosystems with dense vegetation covers,
which allow the fast spread of fire.
The Portuguese mountain are, in general, characterized by poorly developed Humic
Cambisols where the only nutrient pool is located at the L, F and where present, H organic
layers, which are burned down by the forest fires.
The recurrent forest fires are degrading an important part of Portuguese patrimony, the soils,
which suffer important losses in sediments and nutrients after burn. It erodes the top soil
layers, where is located the only nutrient pool of the majority of Portuguese soils. The loss of
nutrients could affect soil fertility and have important implications for forest management.
Objectives
The aim of this poster is to present the main results relative to an old 2 years research in
Lousã mountain (central Portugal) (Figure 1), about the erosion produced as a direct and/or
indirect consequence of forest fires and its confrontation with other situations where the soils
are protected both by forest and by undergrowth.
Figure 1. Field study area (Serra da Lousã)
Methodology
The study is based upon thirteen plots (Figure 2) that include the situations supposed to be the
most representative of the Lousã mountain and with different declivities: burned areas, lands
prepared for reforestation through three different techniques (brush cutting, brush cutting and
raking, harrowing), fire-breaks, forest stands and undergrowth [1].
- 107 -
RESEARCH AND POST‐FIRE MANAGEMENT: SOIL PROTECTION AND REHABILITATION TECHNIQUES FOR BURNT FOREST ECOSYSTEMS
Figure 2. Plot scheme
Sums up, in general features, to the periodical visits to the experimental areas where the
values were registered, the material collected and processed in the laboratory and statistical
processing.
Results and conclusions
Rainfall. Throughout the studied period in the second year of analysis the occurred precipitation
(953.5 mm) reached 94.4% of the normal precipitation in Coimbra2 (1010.4 mm, between 195180) and that quantitative got divided on a heterogeneous way into space and into time.
Overland flow. In spite of the fact that the overland flow depends directly upon the
occurred precipitation, it also depends upon a series of other factors, such as the slope's
declivity, the constitutive material of the slope, density of that material, the proximity of the
rocky substratum, the existence of ravines, the existence and type of vegetation, the
infiltration, the kind of overland flow, the drainage speed, etc.
Furthermore, the precipitation, recorded during a week or fortnight had, very often, different
intensities thus originating, for the same precipitation quantities, different values of overland
flow and of erosion material.
Thus, it is easily understood that to identical precipitation values had corresponded overland
flow's values as different as the ones we found.
In this way, the largest water quantity of overland flow (l/m2) was observed in the burned areas
being followed by fire-breaks and soils made ready for reforestation, through the earth clearing
techniques: brush cutting and subsequent raking.
- 108 -
INVESTIGACIÓN Y GESTIÓN PARA LA PROTECCIÓN DEL SUELO Y RESTAURACIÓN DE LOS ECOSISTEMAS FORESTALES AFECTADOS POR INCENDIOS FORESTALES
The plots that presented a lower surface flow were the ones set under shrub or tree cover.
Temperatures at soil level. As might be expected, due to a greater insulation, in a general way,
the temperatures were higher in the slopes exposed to South and the minimum temperatures
were lower in the slopes exposed to North.
The lowest temperatures, in certain conditions, might have helped the phenomenon’s of rocks
disintegration at soil level, such as the crioclasty, or help the phenomena of individual
dislocation of the particles in the slope, such as the creeping namely through the pipkrakes
formation.
During the studied period, the minimum temperatures, though sometimes have reached negative
values, situated themselves, in the coldest season, between zero and one positive. For this
reason, the activity of the referred processes was minimum throughout the considered time.
The slopes exposed to South helped mainly the processes connected to high temperatures,
which, eventually during summer, can originate thermoclasty processes that help to prepare the
material to be afterwards mobilized by the first rains. For that reason, the slopes with this
exposure can present, in the beginning of the rainy season, a largest concentration of erosion
material, than the slopes turned to North. In the same manner, they can present higher
concentrations than the ones registered, with equal precipitation, at the end of the winter.
Among these two cases, there are more complex situations, where the fundamental seems to
have been the alternating temperatures. The daily thermal amplitudes, with the consequent
materials contraction and distension, led to the breaking of the rocks, namely when the
temperatures were lower than 0° C and when there was ice and thaw formation. The phenomena
above mentioned have a particular importance in dry periods, mainly very cold or very hot.
Eroded material. When the eroded material was analyzed, collected in the boxes and in
suspension on the water of overland flow, in each plot of the different experimental areas, it
was verified that, inside the same area, the differences were often significant, for similar
conditions of rainfall and temperature.
Such as the overland flow, also the eroded material depended directly on the rainfall, though
each plot, representing a very well defined situation, has answered with different amounts of
material.
It was verified that the most serious situations, on erosion terms, were the ones of soil
preparation for reforestation based upon the processes of brush cutting and of brush cutting
associated with raking. It was also observed that the harrowing did not reveal itself dangerous,
on erosive terms, and for this reason it should be more and more utilized.
Equally serious, though having less fine loose material was the situation noted in the firebreaks, but, since the occupied area is more restricted, their consequences were lesser.
The less serious situation have been registered in the plots that had a vegetal cover, which not
only reduces the effect of "splash", but also helps to settle down the land through its radicular
systems.
The burned area presented a preoccupying situation, reaching identical values to the ones
verified in the most fire-breaks, but with the aggravation of having suffered an accentuated
increase relatively to the first year.
Calculation of the erosion normal rates. When this present calculation was performed, it was
expected to give an idea, the most approached that can be possible, of the annual average rates of
erosion, which allows among other analysis, a more accurate comparison between the different
plots.
The normal precipitation (1951 - 1980) referring to the period from September to June was, on
average, 97.3% (1010.4 mm) of the annual totality (1038.3 mm). In the 2 year period it was
91.83%, not differing too much from the normal one.
On the basis of these quantities we tried to calculated erosion rates in the different plots, for a
- 109 -
RESEARCH AND POST‐FIRE MANAGEMENT: SOIL PROTECTION AND REHABILITATION TECHNIQUES FOR BURNT FOREST ECOSYSTEMS
year with normal precipitation, believing that there was a linear relation between precipitation
and eroded material, what can contain some error due to the ignorance of the variables
behavior to such high values because the erosiveness of the summer rains, generally intense,
seems to be superior than the calculated annual average.
The risk of erosion is considered serious, according to [2], when values are superior to 100
Ton / ha / year, moderate, when they are situated between 30 and 100 Ton / ha / year, light,
when they varied between 11 and 30 Ton / ha / year and fairly good when they are inferior to 11
Ton / ha / year.
According to the referred limits, it was verified that there are three situations requiring
preservation measures.
We dealt, as might be expected, with the plots set up in lands made ready for reforestation
through the brush cutting clearing technique and the brush cutting associated with the raking.
The same happened in the plot set up in fire-break in the East side, which, contrarily to the two
previously referred, saw its erosion rate for a normal year increase, relatively to the first year
[3], near 5.5 Ton/ha/year, what have revealed to be very preoccupying.
Though not reaching the 30 Ton/ha/year, there were situations which, either by approaching
this limit (fire-break in the South side), or by having suffered an important addition in a year's
time (burnt West side - 7.17 Ton/ha/year and burnt East side - 10.21 Ton/ha/year), revealed
themselves equally preoccupying.
Insignificant were the values of the plots set up both on undergrowth and on forest.
Conclusion. The forest fires and the consequent mechanical preparation of the lands
for reforestation can cause serious consequences due to the acceleration of soils destruction.
The existence of a generalized increase of the erosion values relatively to the first year was
proved, as a consequence of the precipitation's increase, the same was not verified regarding
the erosion rates for a normal year, when only three situations saw an aggravation of their
values.
The few situations where it was noted a diminishing of the values relatively to the first year,
were due to the vegetation growth inside and outside some plots or to the progressive soil
exhaustion, remaining over only the bare rock.
References
1. Lourenço, L., Monteiro, R. 1989. Quantificação da erosão produzida na serra da Lousã na
sequência de incêndios florestais. Resultados preliminares. Grupo Mecânica de Fluidos,
Coimbra, 45 p.
2. Díaz-Fierros, F. et al. 1982. Efectos erosivos de los incendios forestales en suelos de
Galicia. Anales de Edafología y Agrobiología, 41: 627-639.
3. Lourenço, L., Bento Gonçalves, A. 1990. The study and measurement of surface flow and
soil erosion on slopes affected by forest fires in the serra da Lousã. Proceedings, International
Conference on Forest Fire Research, Coimbra p. C.05-1-13.
4. Lourenço, L., Bento Gonçalves, A., Monteiro, R. 1990. Avaliação da erosão dos solos
produzida na sequência de incêndios florestais. II Congresso Florestal Nacional, Porto, 1990,
II vol, p. 834-844.
5. Lourenço, L. 1993. Fenómenos de erosión/acumulación como consecuencia de incêndios
forestales. El Cuaternario en España y Portugal. Actas de la II Reunión del Cuaternario
Ibérico, Madrid, 1993, vol. II, p. 783-789.
6. Lourenço, L. 2004. Manifestações do Risco Dendrocaustológico. Colectâneas Cindínicas
IV, Colecção Estudos nº 50. Núcleo de Investigação Científica de Incêndios Florestais e
Faculdade de Letras da Universidade de Coimbra, Coimbra, 174 p.
http://www.nicif.pt/estudos%20cindinicos%204.htm.
- 110 -
Santiago de Compostela, 6-8 de octubre de 2010
Jornadas Internacionales
INVESTIGACIÓN Y GESTIÓN PARA LA
PROTECCIÓN DEL SUELO Y RESTAURACIÓN
DE LOS ECOSISTEMAS FORESTALES
AFECTADOS POR INCENDIOS FORESTALES
International Workshop
RESEARCH AND POST-FIRE MANAGEMENT: SOIL PROTECTION AND
REHABILITATION TECHNIQUES FOR BURNT FOREST ECOSYSTEMS
M. Díaz Raviña, E. Benito, T. Carballas, M. T. Fontúrbel, J. A. Vega (eds.)
The study and measurement of overland flow and soil erosion on slopes
affected by forest fires in Lousã Mountain – main results
A. Bento Gonçalvesa*, L. Lourençob**
a
Centro de Estudos em Geografia e Ordenamento do Território (CEGOT), Núcleo de Investigação em Geografia
e Planeamento (NIGP), Departamento de Geografia, Minho University, Campus de Azurém, 4800-058
Guimarães (Portugal); b Centro de Estudos em Geografia e Ordenamento do Território (CEGOT), Núcleo de
Investigação Científica de Incêndios Florestais (NICIF), Departamento de Geografia da Faculdade de Letras,
Coimbra University, 3004-530 Coimbra (Portugal)
*[email protected],**[email protected]
Introduction
In Portugal, forest fires are becoming increasingly frequent as a result of socio-economic
change and poor forest planning, with impacts on soil fertility and structure.
Wet Mediterranean regions are characterized by ecosystems with dense vegetation covers,
which allow the fast spread of fire.
The Portuguese mountain are, in general, characterized by poorly developed Humic
Cambisols where the only nutrient pool is located at the L, F and where present, H organic
layers, which are burned down by the forest fires.
The recurrent forest fires are degrading an important part of Portuguese patrimony, the soils,
which suffer important losses in sediments and nutrients after burn. It erodes the top soil
layers, where is located the only nutrient pool of the majority of Portuguese soils. The loss of
nutrients could affect soil fertility and have important implications for forest management.
Objectives
The aim of this poster is to present the main results relative to an old 2 years research in
Lousã mountain (central Portugal) (Figure 1), about the erosion produced as a direct and/or
indirect consequence of forest fires and its confrontation with other situations where the soils
are protected both by forest and by undergrowth.
Figure 1. Field study area (Serra da Lousã)
Methodology
The study is based upon thirteen plots (Figure 2) that include the situations supposed to be the
most representative of the Lousã mountain and with different declivities: burned areas, lands
prepared for reforestation through three different techniques (brush cutting, brush cutting and
raking, harrowing), fire-breaks, forest stands and undergrowth [1].
- 107 -
RESEARCH AND POST‐FIRE MANAGEMENT: SOIL PROTECTION AND REHABILITATION TECHNIQUES FOR BURNT FOREST ECOSYSTEMS
Figure 2. Plot scheme
Sums up, in general features, to the periodical visits to the experimental areas where the
values were registered, the material collected and processed in the laboratory and statistical
processing.
Results and conclusions
Rainfall. Throughout the studied period in the second year of analysis the occurred precipitation
(953.5 mm) reached 94.4% of the normal precipitation in Coimbra2 (1010.4 mm, between 195180) and that quantitative got divided on a heterogeneous way into space and into time.
Overland flow. In spite of the fact that the overland flow depends directly upon the
occurred precipitation, it also depends upon a series of other factors, such as the slope's
declivity, the constitutive material of the slope, density of that material, the proximity of the
rocky substratum, the existence of ravines, the existence and type of vegetation, the
infiltration, the kind of overland flow, the drainage speed, etc.
Furthermore, the precipitation, recorded during a week or fortnight had, very often, different
intensities thus originating, for the same precipitation quantities, different values of overland
flow and of erosion material.
Thus, it is easily understood that to identical precipitation values had corresponded overland
flow's values as different as the ones we found.
In this way, the largest water quantity of overland flow (l/m2) was observed in the burned areas
being followed by fire-breaks and soils made ready for reforestation, through the earth clearing
techniques: brush cutting and subsequent raking.
- 108 -
INVESTIGACIÓN Y GESTIÓN PARA LA PROTECCIÓN DEL SUELO Y RESTAURACIÓN DE LOS ECOSISTEMAS FORESTALES AFECTADOS POR INCENDIOS FORESTALES
The plots that presented a lower surface flow were the ones set under shrub or tree cover.
Temperatures at soil level. As might be expected, due to a greater insulation, in a general way,
the temperatures were higher in the slopes exposed to South and the minimum temperatures
were lower in the slopes exposed to North.
The lowest temperatures, in certain conditions, might have helped the phenomenon’s of rocks
disintegration at soil level, such as the crioclasty, or help the phenomena of individual
dislocation of the particles in the slope, such as the creeping namely through the pipkrakes
formation.
During the studied period, the minimum temperatures, though sometimes have reached negative
values, situated themselves, in the coldest season, between zero and one positive. For this
reason, the activity of the referred processes was minimum throughout the considered time.
The slopes exposed to South helped mainly the processes connected to high temperatures,
which, eventually during summer, can originate thermoclasty processes that help to prepare the
material to be afterwards mobilized by the first rains. For that reason, the slopes with this
exposure can present, in the beginning of the rainy season, a largest concentration of erosion
material, than the slopes turned to North. In the same manner, they can present higher
concentrations than the ones registered, with equal precipitation, at the end of the winter.
Among these two cases, there are more complex situations, where the fundamental seems to
have been the alternating temperatures. The daily thermal amplitudes, with the consequent
materials contraction and distension, led to the breaking of the rocks, namely when the
temperatures were lower than 0° C and when there was ice and thaw formation. The phenomena
above mentioned have a particular importance in dry periods, mainly very cold or very hot.
Eroded material. When the eroded material was analyzed, collected in the boxes and in
suspension on the water of overland flow, in each plot of the different experimental areas, it
was verified that, inside the same area, the differences were often significant, for similar
conditions of rainfall and temperature.
Such as the overland flow, also the eroded material depended directly on the rainfall, though
each plot, representing a very well defined situation, has answered with different amounts of
material.
It was verified that the most serious situations, on erosion terms, were the ones of soil
preparation for reforestation based upon the processes of brush cutting and of brush cutting
associated with raking. It was also observed that the harrowing did not reveal itself dangerous,
on erosive terms, and for this reason it should be more and more utilized.
Equally serious, though having less fine loose material was the situation noted in the firebreaks, but, since the occupied area is more restricted, their consequences were lesser.
The less serious situation have been registered in the plots that had a vegetal cover, which not
only reduces the effect of "splash", but also helps to settle down the land through its radicular
systems.
The burned area presented a preoccupying situation, reaching identical values to the ones
verified in the most fire-breaks, but with the aggravation of having suffered an accentuated
increase relatively to the first year.
Calculation of the erosion normal rates. When this present calculation was performed, it was
expected to give an idea, the most approached that can be possible, of the annual average rates of
erosion, which allows among other analysis, a more accurate comparison between the different
plots.
The normal precipitation (1951 - 1980) referring to the period from September to June was, on
average, 97.3% (1010.4 mm) of the annual totality (1038.3 mm). In the 2 year period it was
91.83%, not differing too much from the normal one.
On the basis of these quantities we tried to calculated erosion rates in the different plots, for a
- 109 -
RESEARCH AND POST‐FIRE MANAGEMENT: SOIL PROTECTION AND REHABILITATION TECHNIQUES FOR BURNT FOREST ECOSYSTEMS
year with normal precipitation, believing that there was a linear relation between precipitation
and eroded material, what can contain some error due to the ignorance of the variables
behavior to such high values because the erosiveness of the summer rains, generally intense,
seems to be superior than the calculated annual average.
The risk of erosion is considered serious, according to [2], when values are superior to 100
Ton / ha / year, moderate, when they are situated between 30 and 100 Ton / ha / year, light,
when they varied between 11 and 30 Ton / ha / year and fairly good when they are inferior to 11
Ton / ha / year.
According to the referred limits, it was verified that there are three situations requiring
preservation measures.
We dealt, as might be expected, with the plots set up in lands made ready for reforestation
through the brush cutting clearing technique and the brush cutting associated with the raking.
The same happened in the plot set up in fire-break in the East side, which, contrarily to the two
previously referred, saw its erosion rate for a normal year increase, relatively to the first year
[3], near 5.5 Ton/ha/year, what have revealed to be very preoccupying.
Though not reaching the 30 Ton/ha/year, there were situations which, either by approaching
this limit (fire-break in the South side), or by having suffered an important addition in a year's
time (burnt West side - 7.17 Ton/ha/year and burnt East side - 10.21 Ton/ha/year), revealed
themselves equally preoccupying.
Insignificant were the values of the plots set up both on undergrowth and on forest.
Conclusion. The forest fires and the consequent mechanical preparation of the lands
for reforestation can cause serious consequences due to the acceleration of soils destruction.
The existence of a generalized increase of the erosion values relatively to the first year was
proved, as a consequence of the precipitation's increase, the same was not verified regarding
the erosion rates for a normal year, when only three situations saw an aggravation of their
values.
The few situations where it was noted a diminishing of the values relatively to the first year,
were due to the vegetation growth inside and outside some plots or to the progressive soil
exhaustion, remaining over only the bare rock.
References
1. Lourenço, L., Monteiro, R. 1989. Quantificação da erosão produzida na serra da Lousã na
sequência de incêndios florestais. Resultados preliminares. Grupo Mecânica de Fluidos,
Coimbra, 45 p.
2. Díaz-Fierros, F. et al. 1982. Efectos erosivos de los incendios forestales en suelos de
Galicia. Anales de Edafología y Agrobiología, 41: 627-639.
3. Lourenço, L., Bento Gonçalves, A. 1990. The study and measurement of surface flow and
soil erosion on slopes affected by forest fires in the serra da Lousã. Proceedings, International
Conference on Forest Fire Research, Coimbra p. C.05-1-13.
4. Lourenço, L., Bento Gonçalves, A., Monteiro, R. 1990. Avaliação da erosão dos solos
produzida na sequência de incêndios florestais. II Congresso Florestal Nacional, Porto, 1990,
II vol, p. 834-844.
5. Lourenço, L. 1993. Fenómenos de erosión/acumulación como consecuencia de incêndios
forestales. El Cuaternario en España y Portugal. Actas de la II Reunión del Cuaternario
Ibérico, Madrid, 1993, vol. II, p. 783-789.
6. Lourenço, L. 2004. Manifestações do Risco Dendrocaustológico. Colectâneas Cindínicas
IV, Colecção Estudos nº 50. Núcleo de Investigação Científica de Incêndios Florestais e
Faculdade de Letras da Universidade de Coimbra, Coimbra, 174 p.
http://www.nicif.pt/estudos%20cindinicos%204.htm.
- 110 -