Nat. Hazards Earth Syst. Sci., 15, 617–625, 2015 www.nat-hazards-earth-syst-sci.net/15/617/2015/ doi:10.5194/nhess-15-617-2015 © Author(s) 2015. CC Attribution 3.0 License. Spatial distribution of the daily precipitation concentration index in Algeria A. Benhamrouche2, D. Boucherf1, R. Hamadache1, L. Bendahmane1, J. Martin-Vide2, and J. Teixeira Nery3 1National Climate Centre at the National Meteorological Office, Av. Khemisti B.P. 153, Dar el Beïda, Algiers, Algeria 2Climatology Group, University of Barcelona, C/Montalegre 6, 08001 Barcelona, Spain 3Department of Geography, UNESP Ourinhos, Av. Vitalina Marcusso, 1500 São Paulo, Brazil Correspondence to: A. Benhamrouche ([email protected]) Received: 10 February 2014 – Published in Nat. Hazards Earth Syst. Sci. Discuss.: 23 April 2014 Revised: 7 December 2014 – Accepted: 17 February 2015 – Published: 18 March 2015 Abstract. In this paper, the spatial and temporal distribution tation has not been sufficiently studied. Only a few papers on of the daily precipitation concentration index (CI) in Alge- the statistical structure of precipitation with daily resolution ria (south Mediterranean Sea) has been assessed. CI is an have been published. For example, studies of the Mediter- index related to the rainfall intensity and erosive capacity; ranean basin (De Luis et al., 1996, 1997; Martin-Vide, 2004; therefore, this index is of great interest for studies on torren- Sánchez Lorenzo and Martin-Vide, 2006; Benhamrouche tial rainfall and floods. Forty-two daily rainfall series based and Martin-Vide, 2011, 2012; Cortesi et al., 2012; Coscarelli on high-quality and fairly regular rainfall records for the pe- and Caloiero, 2012) concluded that 25 % of the rainiest days riod from 1970 to 2008 were used. The daily precipitation represent at least 75 % of the annual total, and the highest CI results allowed the identification of three climate zones: values of the concentration index (CI) (over 0.66) are found the northern country, characterized by coastal regions with on the eastern façade of peninsular Spain (from Barcelona CI values between 0.59 and 0.63; the highlands, with val- to Almería). The results of similar studies in Iran (Alijani et ues between 0.57 and 0.62, except for the region of Biskra al., 2008) and China (Zhang et al., 2009) indicated that the CI (CI D 0.70); and the southern region of the country, with values detected in China were higher than in Iran (0.74–0.80 high rainfall concentrations with values between 0.62 and and 0.58–0.73, respectively). 0.69. High CI values have been obtained in southeastern penin- sular Spain and the geographical proximity to our study area invites us to focus on the spatiotemporal daily rainfall distribution of 42 meteorological stations evenly distributed 1 Introduction over the entire Algerian territory. Statistical analysis was per- formed to determine the relative impact of different classes The rainfall analysis with daily resolution in Algeria is a sub- of daily precipitation and an assessment of accumulated rain- ject of great interest. This type of analysis is justified by the fall. existence of severe environmental hazards in the Mediter- The study of CI in Algeria is important due to its geo- ranean, such as floods and soil instability resulting from the graphical position between the Mediterranean zone and the high percentages of annual total precipitation falling in a lim- hot Sahara. The main objective of the article is to analyze the ited number of very rainy days. Thus, the area presents long spatial distribution of the concentration of the daily precipi- periods of drought (Martin-Vide, 1994) and irregular tempo- tation in Algeria. The specific objectives are (1) to calculate ral rainfall distribution in large areas of the western Mediter- the values of the CI, (2) map the CI values and (3) correlate ranean (Cortesi et al., 2012). the CI values with the values of other precipitation variables. The interest is not only focused on climate but also on the effect of heavy rainfalls on other areas of the environ- ment and society. However, the importance of daily precipi- Published by Copernicus Publications on behalf of the European Geosciences Union. 618 A. Benhamrouche et al.: Concentration index in Algeria Figure 1. Geographical location of the study area (Algeria). 2 Area of study ders on the desert. Though generally low, the Saharan Atlas reaches a height of over 2320 m. It presents a very dry climate Regardless of the variable or index analyzed, there is a need that is varied as a result of its elevation. Finally, much of the to study spatial precipitation patterns in Algeria to provide a country is occupied by the Sahara desert, one of the world’s better understanding of this phenomenon. These studies are most desolate areas, which has a tropical hot climate. Av- ◦ relatively complex because of (1) the latitudinal position of erage summer temperatures are above 35 C and frequently ◦ Algeria between the Tropic of Cancer and the mid latitudes, rise as high as 43 C or over. Temperatures vary radically which produces several climate zones, and (2) the consider- from day to night. Total precipitation is very low or almost able variety of altitudes, orientations and sloped reliefs. zero; for example, in the extreme southeast of the country (Il- Algeria, situated in northern Africa (between latitudes 19◦ lizi, Bordj Badji Moukhtar, and Adrar), amounts lower than and somewhat over 37◦ N and longitudes 9◦ W and 12◦ E), 20 mm are recorded. In the Hoggar mountains to the south is the largest country on the continent, occupying an area of lies Mount Tahat, Algeria’s highest point, rising to 3000 m. 2 381 740 km2 (Fig. 1). Algeria is bordered by the Mediter- ranean Sea to the north, Tunisia and Libya the east, Mauri- 3 Data and method tania, Niger and Mali to the south, and Morocco and West- ern Sahara toward the west. The Saharan region (center and We obtained daily rainfall data for 42 Algerian weather sta- south of the country), accounting for 80 % of the territory, is tions from the ONM. The stations’ recording periods varied almost completely uninhabited. in duration, but many data are available from 1970 to 2008. Algeria’s climatic regions and landscapes can be divided We therefore selected stations that provide daily rainfall data into three zones distributed parallel to each other along the presenting relatively uniform coverage throughout the coun- country’s N–S axis. Coastal Algeria has a Mediterranean cli- try. The quality of the data series has been verified by the mate with mild, rainy winters and hot, dry summers. Mean ONM. The station with less recorded data over the whole pe- temperatures in January are above 10 ◦C and slightly higher riod has over 93 % of the total data. Station characteristics than 27 ◦C in July. Average annual rainfall for Algeria is are shown in Table 2 and their spatial distribution is mapped 83 mm, but most of the northern area receives over 400 mm in Fig. 4. total annual precipitation, with some areas receiving more The methodology applied in this study, proposed by than 1270 mm according to the National Meteorological Of- Martin-Vide (2004) and including the definition of daily pre- fice (ONM). The highest ranges of the Tell Atlas, which runs cipitation concentration and index, will be illustrated by an parallel to the coast, are 2290 m above sea level. The uneven example (Table 1). Data from the Chlef station was avail- terrain is interrupted by small plains, particularly along the able for the period from 1970 to 2008. These meteorologi- coast. To the south of the Tell Atlas lie High Plateaus, char- cal stations do not show high daily precipitation values and, acterized by a dry steppe climate with cooler winters and therefore, their frequency distribution can be presented in a slightly hotter summers. Rainfall averages 250 to 500 mm an- moderate-length table. nually, concentrated in winter and spring. The southernmost range of the Atlas mountains is the Saharan Atlas, which bor- Nat. Hazards Earth Syst. Sci., 15, 617–625, 2015 www.nat-hazards-earth-syst-sci.net/15/617/2015/ A. Benhamrouche et al.: Concentration index in Algeria 619 adding up the values of the previous column; the value of the last class is the total precipitation of 13 958.5 mm recorded in the study period (1970–2008). The last two columns can be interpreted as follows: almost one-third of rainy days (29.75 %), with 0.9 mm or less, represent only 2.93 % of the total accumulated rainfall. These results give the graphic representation shown in Fig. 2: the cumulative percentage of rainy days (next-to- last column of Table 1), P Ni (%) or X is plotted against the cumulative percentage of rainfall amounts (last column), P Pi (%) or Y . Note that the resulting polygonal line is markedly exponential in the statistics and is called concen- tration curve or Lorenz. However, we can refine Lorenz or concentration curves of the Chlef meteorological station by replacing polygons by exponential curves (Jolliffe and Hope, 1996). The work Figure 2. Concentration curve of Chlef meteorological station of Riehl (1949), Olascoaga (1950) and Martin-Vide (2004) (1970–2008). showed that such functions are of the kind Y D aXebx; (1) with a and b as constants. Parameters a and b of Eq. (1) were determined by the least squares method, given by Eqs. (2) and (3): P 2 P P P P 2 P P P x lny1 C x1 x1 lnx1 − x lnx1 − x1 x1 lny1 lna D 1 1 (2) P 2 − P 2 N x1 . x1/ N Px Plny C Px Plnx − N Px lnx − Px Plny b D 1 1 1 1 1 1 1 1 ; P 2 − P 2 N x1 .