modified fournier index

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: [email protected]

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.


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.


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.


Figure 6. Mean value of the Modified Fournier Index during the period1951–2000 in the study area.


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.


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,


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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