Hydro-Climatic Variability in Two Moroccan Basins: Comparative Analysis of Temperature, Rainfall and Runoff Regimes

Climate and Land Surface Changes in Hydrology 183 Proceedings of H01, IAHS-IAPSO-IASPEI Assembly, Gothenburg, Sweden, July 2013 (IAHS Publ. 359, 2013). Hydro-climatic variability in two Moroccan basins: comparative analysis of temperature, rainfall and runoff regimes KENZA KHOMSI1, GIL MAHE2, MOHAMED SINAN3 & MARIA SNOUSSI4 1 Direction de la Météorologie Nationale, cité de l’air, aéroport Casa Anfa, Maroc [email protected] 2 IRD et Université Mohamed V-Agdal, Rabat, Maroc 3 Ecole Hassania des Travaux Publics (EHTP) Km 7, Route d'El Jadida, BP. 8108, Casablanca, Maroc 4 Université Mohamed V-Agdal, Faculté des Sciences Département des Sciences de la Terre Rabat, Maroc Abstract The increase of temperature over Morocco, projected by climate models, should affect hydro- climatic regimes and ecological and socio-economic systems. In order to investigate the evolution of these regimes in the large basins of Tensift and Bouregreg, we compared the trends of observed temperature, rainfall and runoff variability. Annual temperature time series show significant increasing trends ranging between 0.07 and 0.25°C per decade in both basins. Shifts in annual and monthly temperature trends were recorded between the mid-1970s and mid-1980s and also in the early-1990s. In both basins, total annual rainfall decreased and changed its regime in the late-1970s, monthly rainfall regime changed only in the Tensift basin between early and mid-1970s. Runoff in both basins showed no significant trend for most of the stations, but a consistent decreasing trend since the early-1970s in the Tensift basin and the late-1970s and the early-1980s in the Bouregreg basin. Significant hydro-climatic changes occurred first in the southern basin (Tensift). Ruptures are most frequent in the spring and summer monthly time series. Key words temperature; rainfall; runoff; hydro-climatic regime; variability; trend; rupture; Tensift; Bouregreg; Morocco INTRODUCTION Climatic changes have now become a pressing issue on the world’s attention and concerns over their impacts are greatly increasing because they are likely to affect natural, social and economic systems at local and regional scales (Del Rio et al., 2011). One of the most important requirements of research about climate change is to analyse historical changes in the climatic system (Houghton, 1996). Several authors have carried out analyses of temperature trends at different spatial scales to quantify rates of climate warming, for instance: Moberg et al. (2006) for all Europe, Del Rio et al. (2011) in Spain, Hulme et al. (1995) for all Africa, Elagib & Mansell (2000) in Sudan, Rosell (2011) in Ethiopia and Speth &Fink (2010) in Benin. Temperature increase was also confirmed in Morocco and other North African countries (Schilling et al., 2012). Whereas the temporal evolution of temperature presents a clear warming pattern worldwide, precipitation does not show any generalized trend. Mosmann et al. (2004) studied summer precipitation in mainland Spain between 1961 and 1990; he found that July and August rainfall increased in 20% of the southern areas, while it declined in June and September in 10% of the territory. Significant decreasing precipitation trends were recognized by De Luis et al. (2000) in the region of Valencia (east Spain) while neither trends nor abrupt changes were found in a semi-arid area in the southeast of the country between 1967 and 1997 (Lazaro et al., 2001). Decreasing rainfall trends were detected in many countries in West and Central Africa, in particular after the 1970s (Mahé et al., 2001; Conway et al., 2009), such as in Nigeria (Oguntunde et al., 2011), Congo (Samba & Nganga, 2011), Botswana (Batisani et al., 2010) and Benin (Speth & Fink, 2010). Decreasing precipitation trends were also observed for North African regions, such as Algeria (Meddi et al., 2010) and Morocco (Schilling et al., 2012; Singla et al., 2010). Changes in precipitation regimes and increasing temperatures are likely to affect river runoff (Zhao et al., 2009). In West and Central Africa, the runoff of many rivers has decreased since 1970 (Paturel et al., 2007) while the runoff of others has increased (Oguntunde et al., 2006). The diverse studies cited above show that climate change will induce varied environmental responses depending on the areas involved. Thus, it is important to deal with the issue of global change at a local or regional scale. Hydroclimatic trends in Morocco have not been extensively investigated. Sebbar et al. (2011) studied recent evolution and ruptures in the annual rainfall regime in northern Morocco between 1935 and 2004. He showed that the region underwent a substantial rainfall reduction and that the Copyright 2013 IAHS Press 184 Kenza Khomsi et al. main ruptures are recorded between 1968 and 1984. According to Singla et al. (2010), a diminution of rainfall was observed over the major part of the kingdom from 1976 to 1980; runoff rupture dates are identical to that of the rainfall. Schilling (2012) described the climate variability for the 21st century of five North African states, including Morocco, and reviewed the vulnerability of the country to the confirmed increasing temperature and decreasing precipitation. Very locally, Riad et al. (2006) studied hydroclimatic variability in the high Atlas basins; the study showed the important seasonal and inter-annual variability of mean rainfall and the general decreasing of mean runoff between 1980 and 1986. Most previous studies carried out in Morocco have focused on large-scale areas and were interested in only one hydro-climatic variable at a time. In this study, three climatic parameters are considered (temperature, rainfall and runoff) to provide a comparative overview of climatic variations and their impacts at the local and regional scales. The study deals with hydro-climatic trends and ruptures, during the last decades, in two large catchments of Morocco: Tensift and Bouregreg, using statistical approaches. The trends’ magnitudes and their significance were evaluated using Sen’s slope method and the Mann-Kendall test, respectively. Four methods were used for rupture detection: The tests of Pettitt, Buishand, Lee & Heghinian and the segmentation procedure of Hubert. The t-test was used for rupture significance. The main objective of this work is to: (a) examine the long-term annual trends and annual and monthly ruptures of mean air temperature, precipitation and runoff during the last decades, from the 1960s to the end of the 2000s for most of them, in two large basins of Morocco: Tensift and Bouregreg; (b) test if the trends and ruptures are statistically significant; and (c) compare the variables’ trends and ruptures, according to various methods (see Table 2), within each basin and between the basins. STUDY AREA AND DATASETS Study area Morocco is the extreme northwest African country, situated between 1.5 and 17°W longitude and 20.5 and 36°N latitude. The Bouregreg River basin (located between 5.4°–6.8°W and 32.8°– 34°N), occupies almost all of the central plateau of Morocco. It is a combination of monotonous plateaus, deep gorges and basins partitioned by steep ridges over an area of 9656 km2 (Marghich, 2004) thus 1.3% of the surface of the country. The elevation rises to 1627 m and 50% of the surface is located between 500 and 1000 m (SIGMED 1, 2009). The climate is semi-arid and average annual rainfall is about 400 mm in coastal regions to 760 mm in the western part of the basin. The number of rainy days per year is 75–100 in the mountainous regions and 60–75 else- where.The Tensift River basin (7.2°–9.4°W and 30.8°–32.2°N), covers an area of 19 400 km2, thus 2.7% of the surface of the country. The rainfall distribution is influenced by the relief of the High Atlas. Rainfall is low in the plains where the yearly total does not exceed 350 mm, whereas in the mountains it can reach more than 500 mm. The number of rainy days is 25–50 per year for coastal areas and the Haouz central plain, and 45–70 in the mountains (CID2, 2004). Temperature, rainfall and runoff data Temperature and rainfall data were obtained, respectively, from 5 and 10 meteorological stations of the synoptic network of the Moroccan Meteorological Office (Fig. 1). Runoff data were collected from 18 flow gauges of the Hydraulic Basin Agency of Bouregreg and Chaouia and the Hydraulic Basin Agency of Tensift,in Morocco. Each station provided at least 27 years of data. The hydrological year begins in September and ends in August. Data underwent a quality control before being made publiclyavailable andmissing values do not exceed 3% of the series. Figure 1 and Table 1 present the location of the meteorological stations and the periods analysed. 1The SIGMED project stands for “Spatial approach of the impact of agricultural activities in the Maghreb on sediment transport and water resources in large river basins” – http://armspark.msem.univ-montp2.fr/sigmed/ 2CID stands for the department research “Consulting, Engineering and Development” Hydro-climatic variability in two Moroccan basins 185 (a) (b) Fig. 1 Location of temperature, rainfall and runoff stations in Bouregreg (a) and Tensift (b) catchments. Table 1 Spatial location of used stations by basin. Station Basin Parameter Latitude N Longitude E Data period Rabat-Salé Bouregreg Temperature 34.05 –6.77 1951–2004 Rainfall 1951–2007 Kenitra Bouregreg Temperature 34.30 –6.60 1951–2005 Rainfall 1916–2007 Kasba Tadla Bouregreg Temperature 32.87 –6.27 1983–2009 Oued Zem Bouregreg Rainfall 32.87 –6.57 1956–1984 Ain Loudah

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