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land degradation & development
Land Degrad. Develop. 21: 139–144 (2010)
Published online 16 April 2009 in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/ldr.918
IS RAINFALL EROSIVITY INCREASING IN THE MEDITERRANEANIBERIAN PENINSULA?
M. DE LUIS*, J. C. GONZALEZ-HIDALGO AND L. A. LONGARES
Department of Geography, University of Zaragoza, C/Pedro Cerbuna 12,50009 Zaragoza, Spain
Received 4 November 2008; Revised 19 January 2009; Accepted 30 January 2009
ABSTRACT
The risk of erosion and desertification is one of the main environmental concerns in the Mediterranean Iberian Peninsula. Changes inprecipitation are expected in Mediterranean areas because of climate change, but predictions are not certain. For this reason, denseprecipitation databases are required to explore observed changes in the amount, concentration and variability of precipitation, to gain a clearerunderstanding of the dynamics involved in the main climatological agent of erosion. For this study, we took the recently developedMOPREDAMES dataset, which includes 1113 complete and homogeneous monthly rainfall series from the Mediterranean fringe of the IberianPeninsula (IP) covering the period 1951–2000. These were used to calculate and analyse trends in Total Annual Precipitation (Pt), thePrecipitation Concentration Index (PCI) and Modified Fourier Index (MFI). Our results show that, although there were decreases in annualrainfall, increases in the concentration of precipitation also predominated in the Mediterranean Iberian Peninsula during the period 1951–2000. However, spatial variability of these trends is high, and changes in rainfall erosivity exhibit a complex spatial pattern. Thus, decreases inrainfall erosivity are detected under semiarid conditions (Central Ebro basin and South East IP), while increases mainly occur in dry and sub-humid areas. We present a detailed spatial description of the results and discuss their implication for the risk of erosion and desertification indifferent regions of the study area. Copyright # 2009 John Wiley & Sons, Ltd.
key words: rainfall erosivity; precipitation trends; Precipitation Concentration Index; Modified Fourier Index; Mediterranean Spain
INTRODUCTION
Soil erosion and desertification are one of the main
environmental problems in Mediterranean climate areas,
and Mediterranean regions of Spain, in particular, have been
described as the most threatened area in Europe (Vallejo
et al., 2005).
Most of the soil erosion in the Mediterranean areas is due
to rainfall (splash and wash), but soil erosion by water is a
complex phenomenon in which there is no exact relationship
between soil erosion and the total amount of rainfall. Neither
is there one with the intensity of rainfall and its distribution
in time (Kirkby and Neale, 1987).
Because of global warming, a higher risk of desertifica-
tion has been predicted, due to increased aridity around the
Mediterranean basin (Trenberth et al., 2007), but there is
also great uncertainty about the future development of soil
erosion by water, because the reliability of model outputs for
precipitation are less accurate than those for temperature
(Christensen et al., 2007).
* Correspondence to: M. De Luis, Department of Geography, University ofZaragoza, C/Pedro Cerbuna 12, 50009 Zaragoza, Spain.E-mail: mdla@unizar.es
Copyright # 2009 John Wiley & Sons, Ltd.
As far as possible, regional analysis of potential erosion
should take into account the variability of rainfall in space
and time, and this should be achieved by using dense spatial
information. At present, there are many datasets on a global
or continental scale (Klein-Tank et al., 2002; Wijngaard
et al., 2003), but these are not useful at sub-regional level
because of the low density of observations. This situation is
especially critical in areas with high variability in rainfall,
such as those in the Mediterranean climate.
Different indices of rainfall aggressivity (erosivity) have
been proposed for analysing soil erosion, and the most
appropriate ones seem to be those relating to soil erosion
with kinetic energy of rainfall, such as the well-known EI30
(Wischmeier, 1959). However, there is scarce availability of
high quality datasets on a regional scale, because EI30
requires rainfall data at intervals of one minute (Loureiro
and Coutinho, 2001). To avoid this problem, other indices
based on monthly data averages have been proposed, such as
Fourier Index (FI) and its modification by Arnoldus (1980)
(MFI). Agreement between FI or MFI with the USLE R
factor (rainfall aggressivity factor) has been described on
many occasions (Renard and Freimund, 1994; Gabriels,
2001; Loureiro and Coutinho, 2001; Diodato and Bellocchi,
140 M. DE LUIS ET AL.
2007), and, as consequence, they are commonly used as the
input aggressivity factor in the development of regional
models (Gregori et al., 2006). Basically, the Modified
Fournier Index (MFI) may be expressed as the product of
Total Annual Precipitation (Pt) and monthly precipitation
concentration (PCI) (MFI¼Pt� PCI) (Apaydin et al.,
2006). According to this relation, rainfall erosivity (MFI)
is more intense where there are high values of precipitation
concentration (high PCI) and Total Annual Precipitation
(Pt).
In this paper, we analyse the variability in space and time
of the MFI, the Precipitation Concentration Index (PCI) and
total precipitation (Pt) trends by using a dense precipitation
database recently developed for the Mediterranean fringe of
Iberian Peninsula (i.e., the runoff contribution area to the
Mediterranean Sea).
The objectives of this paper are to analyse the relationship
between trends of Pt, MFI and PCI to describe the evolution
of rainfall aggressivity during 1951–2000 in environments
subject to a high erosion risk, and to look for spatial
distribution patterns.
MATERIAL AND METHODS
We used the monthly precipitation database (MOPRE-
DAMES) recently presented by Gonzalez-Hidalgo et al.
(2009) and De Luis et al. (2009). MOPREDAMES was
produced after analysing the archives of National Meteor-
ological Agency of Spain, where large amounts of monthly
precipitation data are stored. After being exhaustively
processed for quality control and homogenisation, 1113
monthly precipitation stations were reconstructed, covering
the period 1951–2000, with an overall density of 1
observatory per 150–200 km2 (Figure 1).
Figure 1. Study area. Precipitation stations and hydrological divisions ofEastern Spain. EBR, Ebro; EPY, Eastern Pyrenees; JUC, Jucar, SEG, Segura
and EAN, Eastern Andalusia.
Copyright # 2009 John Wiley & Sons, Ltd.
This database is, at present, probably the most dense and
the highest quality monthly precipitation database available
for the western Mediterranean area and comprises five
hydrological divisions which are drained into the Medi-
terranean Sea: Eastern Pyrenees, Ebro, Jucar, Segura and
Eastern Andalusia.
The Modified Precipitation Index (MPI) (Fournier, 1960;
Arnoldus, 1980) and Precipitation Concentration Index
(PCI) (Oliver, 1980) were calculated on an annual basis for
each station, according to the following equations:
MFI ¼X12
i¼1
p2i
Pt
PCI ¼
P12
i¼1
p2i
P12
i¼1
pi
� �2
with pi being the monthly precipitation at month i, and Pt the
annual precipitation.
The annual time series of MFI, PCI and Pt were used to
test for trends and to detect changes during the second half of
the 20th century. The intensity of observed changes was
estimated by using linear regression techniques (Suppiah
and Hennessy, 1998; De Luis et al., 2000; Gonzalez-Hidalgo
et al., 2001).
The spatial distribution of results (rate of change
measured as the slope of linear models) is shown by spatial
interpolation (Inverse Distance Weighted Methods) of
observed trends at each of the 1113 stations (Ninyerola
et al., 2007). Finally, we studied surfaces affected by
different trends by using raster maps and area statistics
obtained for each variable, using ArcGis 9�11 software.
RESULTS
The mean annual precipitation for the whole study area is
562 mm; however, values higher than 2000 mm y�1 (the
Pyrenees and most SW mountain areas) and lower than
200 mm y�1 (extreme South East) have been observed
(Figure 2). The trend analysis of Total Annual Precipitation
shows an overall decrease in annual precipitation (Figure 3).
The exception is the extreme SE, where a very complex spatial
pattern emerges, and includes patches of both positive and
negative trends. Thus, on an annual scale, precipitation
diminished over 90�1 per cent of the territory, with a decrease
in the value of the global mean annual precipitation of�12�4per cent during the period 1951–2000.
Precipitation concentration (PCI) exhibits a different
pattern from those observed for Total Annual Precipitation.
Higher values are observed in the south and south-eastern
part of the study area, while lower values are found in the
Central Pyrenees (Figure 4). The trend analysis also shows a
different pattern, and a general increase in precipitation
concentration (PCI) is observed (Figure 5). Thus, precipi-
LAND DEGRADATION & DEVELOPMENT, 21: 139–144 (2010)
Figure 2. Mean annual rainfall during the period 1951–2000 in the studyarea.
Figure 4. Mean value of the Precipitation Concentration Index during theperiod 1951–2000 in the study area.
RAINFALL EROSIVITY IN MEDITERRANEAN IP 141
tation concentration has increased over 80�5 per cent of the
territory, with a mean increase of 8�8 per cent during the
period 1951–2000.
Rainfall aggressivity (MFI) is relatively high in most parts
of study area, with higher values observed in the Pyrenees,
inner coastland of the Valencia region and Southern
Andalusia (Figure 6). However, because of opposite results
Figure 3. Trends in rainfall amounts. Percentage of change related to themean value for 1951–2000.
Copyright # 2009 John Wiley & Sons, Ltd.
from annual precipitation and precipitation concentration,
complex patterns are observed when analysing trends in
rainfall aggressivity (MFI) (Figure 7). Thus, we found
increases in rainfall aggressivity mainly in the Central
Pyrenees and extensive areas of the regions of Valencia and
Andalusia, while decreases are mainly located in the Central
Ebro basin.
Figure 5. Trends in the Precipitation Concentration Index (PCI). Percen-tage of change related to the mean value for 1951–2000.
LAND DEGRADATION & DEVELOPMENT, 21: 139–144 (2010)
Figure 6. Mean value of the Modified Fournier Index during the period1951–2000 in the study area.
142 M. DE LUIS ET AL.
DISCUSSION
Erosion and desertification are key elements to under-
standing the implications of climate change in Mediterra-
nean climate environments. In this regard, not only changes
in total precipitation, but also in its distribution and
Figure 7. Trends in the Modified Fournier Index (MFI). Percentage ofchange related to the mean value for 1951–2000.
Copyright # 2009 John Wiley & Sons, Ltd.
concentration, are the key to understanding and quantifying
these processes on a regional scale.
The decrease in rainfall trends around the Mediterranean
basin has been related to global warming and changes in
pressure gradients between the islands of the Azores and
Iceland associated with the North Atlantic Oscillation
(Houghton et al., 2001). However, in the aforementioned
study, the average spatial density of observations was low,
and therefore, spatial variability and transient areas might
not be correctly identified. Furthermore, the most recent
global revision covering the Mediterranean basin high-
lighted large spatial variations and did not detect any clear
spatial pattern of precipitation trends during the period
1951–2000 (Norrant and Douguedroit, 2006). Our results,
based on a high-density data-set (1113 time series) and
covering the five hydrological divisions of the eastern IP,
describe high variability in precipitation regimes and trends,
and constitute a new tool for detailed spatial analysis.
Furthermore, they allow us to identify some general patterns
not previously described. Thus, annual trends are charac-
terised globally by a general decrease in precipitation, but an
increase in precipitation concentration during the period
1951–2000.
Indices based on monthly data average such as the Fourier
Index and its modification by Arnoldus (1980) (MFI) are
often used to quantify the nature of rainfall variability and its
effects on the distribution of soil erosion. As has been
recently described by Apaydin et al. (2006), MFI can be
expressed as the product of Total Annual Precipitation and
the Precipitation Concentration Index described by Oliver
(1980). Consequently, opposite trends observed in total
rainfall and precipitation concentration draw a complex
pattern for the trends of rainfall aggressivity in the
Mediterranean IP.
The importance of precipitation seasonality has been
highlighted previously. Langbein and Schumm (1958)
suggested that the available energy for erosion and transport
increases positively with the amount of annual rainfall up to
about 300 mm, at which perennial and annual vegetation
cover increases surface protection and limits soil erosion. On
the other hand, Kirkby and Neale (1987) proposed a seasonal
erosion model in which peak maximum precipitation, plant
cover and soil erosion are out of phase. Thus, the vegetation
dynamic emerges as a key factor in quantifying and
interpreting the risk of erosion and desertification.
It is widely accepted that precipitation concentration and
seasonality play a leading role in vegetation dynamics.
Moreover, rainfall seasonality is directly related to
disturbances like forest fires, which have a critical effect
on erosion. Thus, on one hand, the frequency and extent of
forest fires in Mediterranean areas are directly related to
summer water stress (Moreno, 1998; Prosper-Laget et al.,
1998; Rambal and Hoff, 1998). On the other hand, winter,
LAND DEGRADATION & DEVELOPMENT, 21: 139–144 (2010)
RAINFALL EROSIVITY IN MEDITERRANEAN IP 143
and mainly spring, rainfall is linked to fine fuel growth, this
fact being critical in determining the occurrence (Bessie and
Johnson, 1995) and spread (Viegas and Viegas, 1994) of
forest fires. Last, but not least, the regeneration capacity of
ecosystems and restoration activities after summer fires are
highly dependent on seasonal rainfall, particularly in autumn
(Bautista et al., 1996; Vallejo, 1996; De Luis et al., 2001).
In short, the observed decrease in total precipitation
should not be interpreted as a decrease in the risk of erosion,
because the process of soil erosion is closely linked to the
vegetation dynamic and to the probability of disturbances,
such as forest fires, occurring, and these are mainly linked to
rainfall seasonality and concentration.
CONCLUSIONS
Trend analysis of a dense monthly precipitation database in
the Mediterranean fringe of Spain (western Mediterranean
basin) indicates opposite behaviour between annual pre-
cipitation (P) and seasonal precipitation concentration
(PCI). Thus, during the period 1951–2000, decreases in
annual rainfall, but increases in precipitation concentration,
predominated in the Mediterranean Iberian Peninsula. As a
consequence, and due to the opposing influence of
precipitation components, trends in rainfall aggressivity
(MFI) exhibit a complex spatial pattern.
Is spite of this general behaviour, there is high spatial
variability among these trends, and we attach detailed spatial
maps of these precipitation components. Our results may
contribute to a better understanding of the local dynamic of
the main climatological agents of erosion in Mediterranean
areas, and may also be valid for identifying areas subject to
different erosion and desertification risks in Mediterranean
ecosystems in Spain when creating suitable mitigation
strategies in the assessment of global warming.
ACKNOWLEDGEMENTS
This study was supported by the Spanish Government,
CGL2005-04270 and CGL2007-65315-CO3-01/CLI and
by Gobierno Regional de Aragon, Grupo de Investigacion
Consolidado ‘‘Clima. Agua, Cambio Global y Sistemas
Naturales’’ (BOA 69, 11-06-2007). We thank National
Meteorological Agency of Spain for providing the original
data set for the study area. We also thank Cindy Verkijk for
her work in improving the English of this manuscript.
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