Spatio-temporal analysis of the impact of rainfall dynamics on the water resources of the N'zi watershed in Côte d'Ivoire ABSTRACT This study aims to analyze impacts of rainfall dynamic on the water resources (surface water and groundwater) of N'zi watershed in Côte d'Ivoire. It justifies use of monthly average climatological data (rainfall, temperature) and hydrometric data of the watershed. The methodology is to interpolate by kriging rainfall, to highlight relationship between the spatial variability of rainfall and the surface water flow, and to evaluate groundwater recharge of fractured aquifers in the watershed. At the end of the study, it appears that the ten-year average rainfall of the N'zi watershed has decreased significantly. From 1233 mm during the decade 1951-1960, it lowered to 1074 mm during the decade 1991-2000. During the decades 1961-1970, 1971-1980 and 1981-1990, average rainfall is estimated at 1213 mm, 1068 mm and 1050 mm, respectively. From a spatial point of view, decrease in rainfall intensity was strongly felt in the central and northern localities, as in those located in the south of the watershed. Evolution of stream flow and rainfall is similar in the upper and middle N'zi, with maximum flow period in september corresponding mainly to the period of high rainfall. However, two peaks of different amplitude and a slight shift are observed between the peaks of rain and those of flow mainly in low N'zi. Water quantity streamed in the N’zi watershed during the period of 1972 to 2000 is of 40.7 mm. Water quantity infiltrated to reload aquifers of the watershed is evaluated to 47.6 mm, that is to say a volume of water infiltrated of approximately 1.7 10 9 m3. Keywords: Dynamic, rainfall, flow, recharge, N’zi watershed 1. INTRODUCTION Study of climatic and hydrological phenomena is rather delicate because of the seasonal and interannual fluctuations that they can take. These phenomena indisputably induce impacts on natural resources; and indirectly on the environmental, social and economic dimensions of development activities [1 – 3]. In addition, they are undeniably among the most important factors to consider for sustainable management of water resources. Climatic and hydrological phenomena are complex and depend on the main parameters of precipitation and flow. However, precipitation, which contributes particularly to the constitution and renewal of water resources, is the most used to understand climate variability. Much research [4 – 13] have contributed to a better knowledge of climatic and hydrological phenomena. In Côte d'Ivoire, several global and local studies have better characterized the dynamic interactions between ecosystems and climate variability [14 – 25]. In the N'zi watershed, studies of the impacts of climate variability on water resources have justified the use of several methods. The rainfall index and the 2nd order Hanning low-pass filter showed the interannual rainfall fluctuation of the N'zi watershed. It is organized in an alternation of wet phase (1930-1968), and dry phase (1969-2000) [26] which was amplified during the 1980s [27]. In addition, the Hubert segmentation procedure and the Pettitt test proved a stationary rupture in 1968-1969 in the rainfall and flow series of all N'zi watershed stations [26]. The decrease in rainfall observed from 1971 to 2000 was determined from the patterns of rainfall (rainy days and precipitated water levels) [27]. This period is 250 to 240 days and 140 to 105 days in Tafiré and Dimbokro and, the rain height fluctuates between 10 and 50 mm. Also, the depletion coefficients of aquifers in the N'zi watershed after the 1968 rupture indicate a marked 1 decrease in the volumes of water mobilized by them. These variations in the volume of water mobilized by aquifers suggest a considerable regression of underground reserves [27]. This study is part of one of the major issues of research on climate variability, which is to quantify its impact on the natural environment, particularly on water resources. Its objective is to analyze impacts of the evolution of rainfall on the water resources of the N'zi watershed. On the one hand, maps show the spatio-temporal dynamics of precipitation from isovalue curves. On the other hand, the study aims to relate the spatial variability of precipitation with surface water (stream flows), and to evaluate the groundwater recharge on the basis of the quantity rains from the period 1972-2000. The hydrometric and rain gauges are unavailable after 2000 because of socio-political crisis, this work will be limited to a retrospective study from 1951 to 2000 for the sake of uniformity. 2. METHODOLOGY 2.1 Presentation of the study area The N'zi watershed is located between longitudes 3°49' and 5°22' west, and latitudes 6°00' and 9°26' north. It covers an area of 35500 km², or about 11% of the area of Côte d'Ivoire, on which are distributed homogeneously a set of eleven climatological stations and six hydrometric stations. It represents a sub-watershed of the Bandama river watershed. The N'zi watershed is characterized by the tropical transitional regime in the north, the equatorial transition regime attenuated in the central part and the equatorial transitional regime in the south. Due to its geographical configuration, the N'zi watershed is limited by that of Comoé to the north and east, and that of Bandama (upper Bandama to the west and lower Bandama to the south). Its main river, the N'zi, is the first major tributary of the left bank of Bandama with a length of 725 km. It rises at an altitude of 400 m east of Ferkéssédougou, in the north of Côte d'Ivoire, and merges with the Bandama at N'zianouan, located some 10 km upstream from Tiassalé in the region of Abidjan (Fig. 1). It reaches its peak in september with a maximum flow of 440 m3/s and a response time of two months (may and june) [28]. The watershed is dominated by a rather monotonous relief [29], whose altitude varies more than 400 m in the north and less than 100 m in the south. This geographical area is also the place where there is a contrast between plant cover (savannah in the north and forest in the south) [30] and between geological substrata (predominantly granitic in the north and schistous in the south) [31 – 35]. The main soil entities of unequal size covering the watershed are medium and/or poorly desaturated ferralitic soils and tropical ferruginous soils, plus a very small soil complex entity [36, 37]. 2 Fig 1. Location of the study area and hydroclimatological stations 2.2 Data used 2.2.1 Presentation of climatological and hydrometric data Climatological data refer to the monthly time series of precipitation (of 11 rainfall stations distributed homogeneously over the watershed) and temperatures (of 3 synoptic stations). These data are provided by the Airport Exploitation and Development Company, Aeronautics and Meteorology (SODEXAM), which is the main manager of the network and climate data in Côte d'Ivoire. With regard to the hydrometric data, they relate to the chronic daily flows of the stations of Fétékro, M'bahiakro, Bocanda, Dimbokro and N'zianouan. These data are provided by the Human Resources Directorate (DHH) of the Ministry of Economic Infrastructure. Fig. 1 illustrates the spatial distribution of hydroclimatological stations. Tables 1, 2 and 3 present the characteristics of climatological and hydrometric stations. 3 Table 1 Coordinates of rainfall stations, periods and duration of registration Rainfall stations Longitude Latitude Altitude (m) Years Duration Tafiré 5°7’44’’ 9°4’4’’ 405 1951-2000 50 Niakaramandougou 5°15’43’’ 8°41’24’’ 379 1951-2000 50 Dabakala 4°24’32’’ 8°21’46’’ 287 1923-2000 78 Katiola 5°6’39’’ 8°9’36’’ 329 1950-2000 51 Bouaké 5°2’24’’ 7°41’6’’ 351 1932-2000 69 M’bahiakro 4°27’43’’ 7°28’22’’ 212 1945-2000 56 Tiébissou 5°12’32’’ 7°9’54’’ 201 1951-2000 50 Bocanda 4°29’20’’ 7°3’32’’ 129 1951-2000 50 Dimbokro 4°41’34’’ 6°39’14’’ 85 1922-2000 79 Bongouanou 4°11’9’’ 6°39’14’’ 209 1948-2000 53 Tiassalé 4°50’6’’ 5°53’13’’ 21 1922-2000 79 Table 2 Coordinates of synoptic stations, periods and duration of registration Synoptic stations Longitud e Latitude Altitude (m) Years Duration Korhogo aéroport 5°38’ 9°26’ 314 1972-2000 29 Bouaké aéroport 5°2’24’’ 7°41’6’’ 300 1961-2000 40 Abidjan aéroport 4°01’ 5°19’ 9 1961-2000 40 Table 3 Coordinates of hydrometric stations, periods and duration of registration Hydrometric stations Longitude Latitude Altitude (m) Years Duration Fétékro 4°41’ 7°48’ 202 1959-1997 39 M’bahiakro 4°20’ 7°27’ 212 1954-1997 44 Bocanda 4°30’ 7°03’ 129 1955-1997 43 Dimbokro 4°42’ 6°38’ 85 1955-1997 43 N’zianouan 4°49’ 5°57’ 45 1953-2000 48 The choice of positions meets two criteria°: - geographical first, because the distribution of rainfall and flow dynamics result from factors such as latitude, longitude and altitude; - climatic then, especially as the extension of the watershed in latitude is reflected in the existence of three fairly clear areas (Guinean domain in the south, transition area or south Sudan in the center and pure tropical area or northern Sudan in the north), which determine the approximate rainfall limits [38]. 2.2.2 Reviews and analyzes of the data used For a reliable study, it is essential to first check the quality of the data of the selected stations and avoid the use of non-homogeneous series.