Spatio-Temporal Analysis of the Impact of Rainfall Dynamics on the Water Resources of the N'zi Watershed in Côte D'ivoire

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

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].

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.

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. Rainfall, temperature and hydrometric data are mainly characterized by the difference in size of the samples under study. Indeed, if rainfall data have the advantage of being without gaps, long and consistent over time (optimal period of observation common to all stations: 1951-2000, or 50 years), such are not the case of temperature data from 1972 to 2000, 29 years, and hydrometric data from a 39-year observation period from 1959 to 1997. Moreover, the hydrometric data, except those of N'zianouan, have more or less important gaps, ranging from a few days to several years. An estimation of the missing flows over the period 1951-2000 of M'bahiakro, Fétékro, Bocanda and Dimbokro stations is performed by linear regressions. [39] have recognized that among a dozen methods for estimating missing flows in West Africa, the model of inter-station linear regressions provides acceptable data. This model is based on upstream / downstream links considered linear between the flow values of two stations. A large number of linear correlations could be determined between the concerned stations of N'zianouan, M'bahiakro, Fétékro, Bocanda and Dimbokro. For each station to be completed, the correlation is calculated with the surrounding stations and the conserved values correspond to the highest correlation coefficient. The N'zianouan station was considered as the reference station given the length and quality of the series (no data gaps), but mainly because of its situation downstream of all the other stations. It is therefore the main outlet for the entire watershed.

The correlation coefficients reflect the quality of the estimates made during the filling of the gaps. Overall, they range from 0.70 to 0.96. They are therefore acceptable [40]. The correlation coefficients used for the reconstitution of the data are those higher than 0.91. The good quality of the hydroclimatological data available facilitates the continuation of subsequent treatments.

2.3 Methods 2.3.1 Spatio-temporal rainfall analysis of the N'zi watershed Using the geographic coordinates of each rainfall station, the mean annual rainfall is spatialised by kriging in the Surfer software to construct the isohyets. Decadal rainfall isovalue maps (1951-1960, 1961-1970, 1971-1980, 1981-1990 and 1991-2000), the periods 1951 to 1970, 1970 to 2000, and 1951 to 2000 are developed to highlight the spatio-temporal dynamics of rainfall in the N'zi watershed.

2.3.2 Analysis of the rain-flow relationship The average monthly rainfall for all meteorological stations in or near the N'zi watershed is estimated and compared to the average monthly flow at the outlet of the watershed over an optimal period of common observation. Thus, the average monthly rainfall (1959-2000) on the N'zi watershed in Fétékro, M'bahiakro and N'zianouan are compared to monthly mean flows (1959-2000).

2.3.3 Estimate of recharge of underground water aquifers The assessment of the recharge of the underground water aquifers of the N’zi watershed is centered on the hydrological balance according to the method of [41]. This one incorporates few environmental parameters available (rain and temperature). One of the modules based on the Thornthwaite principle of the Climate Variability Assessment (CVA) program was used to undertake the calculations. In the N’zi watershed, average annual rainfall is derived from the average rainfall of the Tafiré, M’bahiakro and Tiassalé stations respectively representing the tropical transitional regime in the norh (upper N’zi), the equatorial transition regime attenuated in the central part (middle N’zi) and the equatorial regime of transition to the south (low N’zi). In addition, the average temperatures of synoptic stations of Korhogo, Bouaké and Abidjan were respectively assigned to upper, middle and low N’zi. The estimation of the terms (potential evapotranspiration, actual evapotranspiration, surplus, deficit, etc.) of the average water balance (1972-2000) of the N’zi watershed is based on the average of these different terms determined for the upper, the middle and the low N’zi. The quantity of water flowing over the N’zi watershed from 1972-2000 is estimated at N'zianouan (watershed outlet) according to the equation (1)°: Q×t R = A (1) where t = 31,536,000 s ; A : N’zi watershed area (355.10 8 m2). ; Q : measured annual flow at the N’zi outlet watershed in m 3.s -1 and R: runoff in m. Aquifer recharge in the N’zi watershed is assessed from the infiltration fluctuations deduced from the water balance equation (2). = + = + + P ETR S ETR (R I ) (2) = − − I (P ETR ) R (3) where I: Quantity of infiltrated water (mm); P: Rainfall (mm); ETR : actual evapotranspiration (mm); R: amount of water flowing (mm)°; S: Surplus (mm) The volume of infiltrated water is given by the formula (4). (4) with V: volume of infiltrated water in m 3; I: Quantity of infiltrated water in m.

3. RESULTS AND DISCUSSION 3.1. Results 3.1.1 Spatio-temporal dynamics of rainfall

Annual decadal mean rainfall plots 5

Annual rainfall has declined remarkably over the last five decades, from the decade 1951-1960 to the decade 1991-2000. This state of affairs is illustrated by the maps in Fig. 2.

The decade 1951-1960 characterizes a sufficiently rainy period. The N'zi watershed is bounded by isohyet curves 1360 mm at Niakaramandougou-Tafiré and 1160 mm at Dimbokro. Apart from the Tafiré and Niakaramandougou stations in the northern part of the watershed, and those of Tiassalé, Bongougouanou and Daoukro, which record the highest precipitation, above 1260 mm, all the other stations in the watershed have rainfall ranging from 1160 mm to 1240 mm. The average rainfall of the watershed is estimated at 1233 mm. The isohyet 1140 mm is non-existent in the watershed during this period.

The previously identified wet period continues during the 1961-1970 decade. However, a slight decrease in the annual rainfall amounts is observed at the Bouaké and Satama-sokoura stations and at the extreme north of the watershed, as evidenced by the appearance of the 1140 mm isohyet. Stations located east of the watershed, namely those of Bongouanou and Daoukro, recorded a slight decrease to reach 1200 mm compared to the previous decade. On the other hand, some stations such as Dimbokro and Bocanda record an increase, represented by the 1280 mm isohyet. The average rainfall of the watershed is estimated at 1213 mm.

The 1971-1980 decade is characterized by a significant drop in rainfall over the entire N'zi watershed. The average rainfall is 1068 mm. This decrease is recorded on almost all rainfall stations. The lowest rainfall (940 mm to 1040 mm) is recorded in the Tiébissou-Bouaké zone, then to the north-east of Dabakala where the rainfall is 1000 mm. Over the rest of the watershed, rainfall varies between 1060 mm and 1140 mm.

This drop in rainfall is strongly noticed during the 1981-1990 decade, especially in the northern half of the N'zi watershed. The area of Dabakala, with a rainfall of 1000 mm in the previous decade, has decreased to 900 mm and the isohyet has receded to the Katiola and Niakaramandougou stations. The exception comes from a slight rainfall increase which is observed in the extreme south-east of the N'zi watershed (Tiassalé, Bongouanou). It is illustrated by the reappearance of the isohyet 1160 mm, almost non-existent during the previous decade and which seems to mark a new dynamic beneficial in the watershed. Nevertheless, the average rainfall of the N'zi watershed is estimated at 1050 mm.

The 1991-2000 decade is still in the dry period despite a slight increase in annual rainfall especially in the northern zone of the watershed. The average rainfall of the watershed is estimated at 1074 mm. The stations of Bouaké, M'bahiakro, Satama-sokoura and Dabakala have the lowest annual rainfall compared to the other stations of the N'zi watershed. The slight increase in rainfall observed in Tiassalé and Bongouanou the previous decade, disappears to make way for the reference isohyet 1140 mm.

Fig 2 Average annual rainfall over the decades 1951-1960, 1961-1970, 1971-1980, 1981-1990 and 1991-2000

Mapping average annual rainfall from 1951 to 1970 and from 1970 to 2000 In the analysis of Fig 2, the year 1970 marks the transition from the wet period before 1970 to the dry period after 1970. This distinction is illustrated in Fig 3. Over the period 1951-1970 (Fig 3a), the annual precipitation levels are well above 1160 mm over the entire N'zi watershed, and reach 1360 mm at the Tiassalé station. The average rainfall is 1223 mm. This period contrasts remarkably with what is observed during the period 1970-2000 (Fig 3b), where rainfall is less than 1140 mm over the entire N'zi watershed, reaching 960 mm at the Dabakala station, with an average rainfall of 1064 mm.

Fig 3 Average Annual Rainfall Rates for 1951-1970 and 1970-2000

Mapping of the average annual rainfall over the period 1951-2000 Mean annual rainfall (1951-2000) varies from 1220 mm in the southern part (Tiassalé, Bongouanou) of the watershed to 1100 mm in the north (Tafiré). They reach 1060 mm in Dabakala, in the north- eastern part of the watershed. The analysis of annual mean values shows an inequality in the spatial distribution of rainfall in the N'zi watershed (Fig 4). Annual rainfall amounts are decreasing from south to north and from west to northeast of the N'zi watershed. The average rainfall of the watershed is 1128 mm.

Fig 4 Average annual rainfall from 1951 to 2000

3.1.2 Evolution of flow under the influence of rainfall in the N'zi watershed The reaction of the monthly mean flows (1959-2000) of N'zi respectively to Fétékro, M'bahiakro and N'zianouan towards the average monthly precipitation (1959-2000) on the respective outlet watershed Fétékro, M'bahiakro and N'zianouan are shown in Fig 5.

Fig 5 Evolution of flows under the influence of rainfall in the N’zi watershed 10

The evolution of the flow and the precipitation is similar in the middle and upper N'zi. However, in lower N'zi mainly, two peaks of different amplitude and a slight shift are observed between the peaks of rain and those of flows. Indeed, in upper and middle N'zi, the river N'zi has a unique flood in september. Rain reaches its maximum of 175 mm and 168 mm respectively in upper and middle N'zi, at the same time that the flood of N'zi (115 m3/s and 152 m3/s) reaches its own; that is, in september (Fig 5-a and 5-b). The flood is followed by a rapid drying up in november and december, then a long period of low water from january to may. This period coincides with the dry season from november to march. However, in lower N'zi (Fig. 5-c), the seasonal distribution of rainfall causes the doubling of the annual flood. The first high water period is in june-july, and the second is in september-october. A period of low water occurs in august and another more marked, from december to march. The beginning of the first flood, which peaks in july, is consecutive to the maximum rainfall in june; that is, one month after the first peak of precipitation. The same phenomenon is observed for the second rainy season. The highlight is that for a smaller amplitude of precipitation (september to november with a maximum of 140 mm reached in september) than the first, it causes a flood in october whose amplitude exceeds the first of july.

3.1.3 Quantity of water infiltrated on the N'zi watershed The estimated water balance terms of the N'zi watershed are presented in Table 4. Table 4 Comparison of the terms of the hydrological balance of the N'zi watershed (1972-2000) Water balance terms Upper N’zi Middle N’zi Lower N’zi N’zi watershed Rainfall (mm) 1060.3 1017.3 1144.8 1074.13 ETP (mm) 1710 1722 1516 1649.33 ETR (mm) 987.6 1104.9 1238.9 1110.47 Surplus (mm) 172.7 33.42 59 88.37 Deficit (mm) 722.4 618.1 277.1 539.2 ETP : potential evapotranspiration ; ETR : Real evapotranspiration The rainfall rises from upper N'zi to lower N'zi where it is especially marked. The real evapotranspiration is quite intense throughout the watershed. It grows from upper N'zi to lower N'zi. Moreover, it is especially pronounced in the southern half of the watershed. The water quantity used for surface flow and infiltration (surplus) is 172.7 mm in upper N'zi and only 59 mm in lower N'zi. On average, the water quantity precipitated over the entire N'zi watershed area from 1972 to 2000 is estimated at 1074 mm, while the portion used for potential evapotranspiration is 1649 mm. The amount of water used for surface flow and infiltration is estimated at 88.4 mm. The knowledge of the surplus makes it possible to estimate the specific quantity of water to runoff (Table 5) and then that relating to infiltration on the N'zi watershed (Table 6).

Table 5 Quantity of water streamed on the N’zi river (1972-2000) N’zi Streamed monthly Time (s) Area (km 2) Volume of Quantity of medium flow (m 3s-1) water streamed streamed (m 3) water (mm) N’Zianouan 45.87 31536000 35500 1446556320 40.75 From 1972 to 2000, the quantity of streamed water on the N'zi river is 40.75 mm, that is to say a volume of water streamed estimated at 1.4 10 9 m3. The effective infiltration of rain to recharge aquifers in the N'zi watershed is shown in Table 6.

Table 6 Parameters of hydrological balance sheets of N’zi watershed aquifers S (mm) R (mm) I (mm) A (km 2) V (m 3) Upper N’zi 172.7 66.26 106.44 10000 1064400000 Middle N’zi 33.42 91.93 -58.51 15700 -918607000 Lower N’zi 59 41.33 17.67 35000 618450000

N’zi watershed 88.37 40.75 47.62 35500 1690510000 S = surplus ; R = runoff ; I = infiltrated water ; A = area°; V = volume of infiltrated water

The quantity of infiltrated water to recharge the aquifers is higher in upper N'zi (106.44 mm) than in lower N'zi (17.67 mm). The aquifer of middle N'zi seems to have been the object of no infiltration. The quantity of infiltrated water to recharge the aquifers of the N'zi watershed from 1972 to 2000 is estimated at 47.6 mm, i.e. a volume of infiltrated water of about 1.7 10 9 m3.

3.2 Discussion The realization of isohyets in the N'zi watershed is considered acceptable because of the flatness of the relief. Their layout is consistent with the spacing of the stations as well as the quality and variability of the data [42]. The kriging method used for spatial interpolation of rainfall data remains a very flexible and accurate method compared to other methods [43].

The results of the spatio-temporal dynamics of rainfall in the N'zi watershed are in perfect harmony with those of [21] throughout the ivorian territory.

The values of the Thornthwaite method's hydrological balance terms do not really reflect the reality, given that their estimate is the average of the data from three stations representing the different climatic regions of the N'zi watershed. The estimate of the hydrological balance is therefore approximate. The value of the real evapotranspiration, which is especially pronounced in the southern half of the watershed, is due to the influence of forest cover and numerous dams and streams [44]. The difference in water infiltration depending on where one is located on the N'zi watershed is justified by the combined action of the state of fracturing of the aquifer, drainage and vegetation of the watershed. Moreover, all these factors must be added to the deficit period after the 1970s. This period would justify the drop in water infiltration on the entire watershed, and more felt in middle N'zi. Hence the negative value, which expresses a zero infiltration during the period 1972-2000 in Middle N'zi. Indeed, [27] showed a decrease in the volumes of water mobilized by aquifers after 1968. The Thornthwaite method remains an effective method since it allows to give a range of quantities of the various terms involved in the process of estimation of soil reserves, and with few means [19]. Also, according to [45], the results obtained are satisfactory when the work is carried out on a large scale (watershed) and in cumulative values (case of hydrological balance).

The annual flood in the middle and upper N'zi always results from either a succession of rains, or exceptional rainfall concentrated in a few days. According to [28], the annual flood reaches a certain magnitude and often floods large areas. In lower N'zi, the fact that the first flood peaks in july is due to the fact that the first rains reconstitute firstly the water reserves of the soil which, due to the previous dry season, are for the most part dried up, and in turn, result in lower surface flow (infiltration outweighs surface flow). The second flood caused in october with an amplitude greater than that of july is justified by the fact that the flows of the N'zi are, at this moment, influenced by two contributions of precipitation: that of the only rainy season of the tropical transitional climate located in the upper and middle N'zi and then, by the fact that the short dry season (july to august) does not act too much on the soil [28].

The work of [46] showed a change in the rainfall-discharge relationship in the N'zi watershed. Their results show that significant hydrological changes may have affected the N'zi watershed (Bandama) in general, related to hydroclimatic variability and changes in land use / land cover. Nevertheless, the hydrological functioning of West African rivers in particular is often modeled on rainfall patterns [47]. The main limitation of this study remains the extent of the data. The disproportion between the different types of data required finding coherent and concomitant variations between the data so that an acceptable analysis could be performed.

4. CONCLUSION At the end of the study, which aimed to analyze the impact of rainfall dynamics on the water resources of the N'zi watershed, several results were obtained. 12

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. Spatially, rainfall intensity has not been the same everywhere in the N'zi watershed. Localities in the center and the north felt it more strongly than those in the south. From 1951 to 1970, the average rainfall estimated at 1223 mm contrasts remarkably with that of the period 1970-2000 during which it is 1064 mm. The increase in rainfall in the N'zi watershed is along a north / south gradient. Indeed, the relative abundance of annual rainfall increases from upper N'zi to lower N'zi. The evolution in space and time of rainfall has a direct impact on the flow and infiltration of rain, and therefore on the water resources of the N'zi watershed. Remarkably, there is a good rain-flow correlation in the N'zi watershed. The period of maximum flow in september corresponds mainly to the period of high rainfall mainly in the middle and upper N'zi. The evolution of the flow and that of rainfall is similar in the N'zi watershed. The drop in rainfall has however had the effect of reducing the flow of water that passes through the rivers of the watershed. The quantity of streamed water in the N'zi watershed during the period 1972 to 2000 is 40.7 mm. The quantity of water infiltrated to recharge the aquifers of the watershed is estimated at 47.6 mm, ie a volume of infiltrated water of about 1.7 10 9 m3. The hypothesis that hydrological regimes closely resemble precipitation regimes is therefore verified. The dynamics of rainfall require the adoption of a sustainable water resources management policy.

REFERENCES 1. Ardoin-Bardin S. Hydroclimatic variability and impacts on the water resources of large watersheds in the Sudano-Sahelian zone. PhD Thesis, Univ. Montpellier II, France; 2004. 2. Vissin EW. Impact of climatic variability and dynamics of surface states on the flows of the Benin basin of the Niger River. PhD Thesis, University of Burgundy (France); 2007. 3. MDAEP (Ministry of Development, Economic Analysis and Foresight). Modeling the economic impacts of climate change by development sector: water resources sector. SAP-Benin Project; 2014. 4. Nicholson SE. The nature of rainfall fluctuations in subtropical West Africa. My. Wea. Rev. 1980; 108: 473-487. 5. Leroux M. Rainfall Variability in West Africa: Aerological Components. VCS. 22, 1988: 26-45. 6. Paturel JE, Servat E, Kouame B, Lubes-Niel H, Fritsch JM, Masson JM. Demonstration of hydrological variability in West and Central Africa. IAHS Pub. 1997 ; 240: 21-30. 7. Paturel JE, Servat E, Delattre MO. Analysis of long-term rainfall series in non-Sahelian West and Central Africa in a context of climatic variability. J. Sci. Hydrol. 1998; 43: 937-945. 8. Ouedraogo M, Servat E, Paturel JE, Lubes-Niel H, Masson JM. Characterization of a possible modification of the rain-flow relationship around the 1970s in West and Central Non-Sahelian Africa. IAHS Pub. 1998; 252: 315-321. 9. Servat E, Paturel JE, Kouamé B, Travaglio M, Ouedraogo M, Boyer JF et al. Identification, characterization and consequences of hydrological variability in West and Central Africa. IAHS Pub. 1998; 252: 323-337. 10. Servat E, Paturel JE, Lubes-Niel H, Kouamé B, Masson JM, Travaglio M et al. Different aspects of the variability of rainfall in West and Central Africa, Rev. Sci. Water. 1999; 12: 363-387. 11. Ouédraogo M. Contribution to the study of the impact of climate variability on water resources in West Africa. Analysis of the consequences of persistent drought: hydrological standards and regional modeling. PhD Thesis, Univ. from Montpellier II, France; 2001. 12. The Beard L, Lebel T, Tapsoba D. Rainfall Variability in West Africa during the 1950-90 Years. J. Climate. 2002; 15: 187-202. 13. André M. Spatio-temporal variability of rainfall in Réunion and impact on vegetation. Master's thesis 2, Univ. Joseph Fourier (Grenoble), France; 2014.

14. Aka AA, Lubes H, Masson M, Servat E, Paturel JE, Kouamé B. Analysis of the temporal evolution of flows in Ivory Coast. Statistical approach and characterization of phenomena. IHP-V. 1997; 16: 49- 63. 15. Brou YT, Serat E, Paturel JE. Human activities and climatic variability: case of the Ivorian southern forest, IAHS Pub. 1998; 252: 365-373. 16. Kouamé KF, Hydrogeology of discontinuous aquifers in the semi-mountainous region of Man- Danané (western Côte d'Ivoire). Contribution of satellite imagery data and statistical and fractal methods to the development of a hydrogeological information system with spatial reference. Postgraduate doctoral thesis, Univ. Cocody, Ivory Coast; 1999. 17. Savané I, Coulibaly KM, Gioan P. Climatic variability and groundwater resources in the semi- mountainous region of Man. Séch. 2001; 12: 231-237. 18. Bigot S, TY Brou, Bonnardot V, Servat E. Inter-annual rainfall stability in Ivory Coast from 1950 to 1996. IAHS Pub. 2002; 274: 507-514. 19. Saley MB. Hydrogeological Information System with Spatial Reference, discontinuities pseudo- image and Thematic cartographies of the water resources of the semi-mountainous region of Man (West of Ivory Coast). Unique doctoral thesis, Univ. Cocody, Ivory Coast; 2003. 20. Ardoin-Bardin S, Lubes-Niel H, Servat E, Dezetter A, Boyer JF. Analysis of the persistence of drought in West Africa: Characterization of the situation of the 1990s. IAHS Pub. 2003; 278: 223-228; 21. YT. Climate, socio-economic changes and landscapes in Côte d'Ivoire. Synthesis dissertation, Univ. Science and Technology of Lille, France; 2005. 22. Soro N, Lasm T, Kouadio BH, Soro G, Ahoussi KE. Variability of the rainfall regime of southern Côte d'Ivoire and its impact on the feeding of the Abidjan aquifer. Rev. South Sci. and Technol. 2006; 14: 30-40. 23. Kouassi AM, Kouamé KF, Goula BTA., Lasm T, Paturel JE, Biemi J. Influence of climate variability and land-use change on the rainfall-discharge relationship from a global modeling of the watershed of N'zi (Bandama) in Côte d'Ivoire. Rev. Isee. Sci. Technol. 2008; 11: 207-229. 24. Ahoussi KE, Koffi YB, Kouassi AM, Soro G, Soro N, Biemi J. Study of the hydroclimatic variability and its consequences on the water resources of the southern forest and agriculture of the Ivory Coast: Case of the region Abidjan, Agboville. Inter. Mult. J. Contemp. Res. 2013; 1: 38-61. 25. Kouamé YM, Soro GE, Kouakou KE, Kouadio ZA, Meledje NEH, Goula BTA and ai. Scenarios of climate change for rainfall and temperature in tropical sub-Saharan Africa: case of the Davo watershed, Côte d'Ivoire. Larhyss J. 2014; 18: 197-213. 26. Goula BTA, Savane I, Konan B, Fadika V, Kouadio GB. Impact of climate variability on the water resources of the N'zo and N'zi basins in Côte d'Ivoire (humid tropical Africa). VertigO, Rev. Sci. Approx. 2006; 7: 1-12. 27. Kouassi AM. Characterization of a possible modification of the rainfall-discharge relationship and its impacts on water resources in West Africa: case of the N'zi (Bandama) watershed in Côte d'Ivoire. PhD Thesis, Univ. Cocody, Ivory Coast; 2007. 28. Girard G, Sircoulotj J, Touchebeue P. Overview on hydrological regimes, In: The natural environment of Côte d'Ivoire. Same. ORSTOM. 1971; 50: 109-155. 29. Avenard JM, Eldin M, Girard G, Sircoulon J, Toucheboeuf P, Guillaumet JL, et al. The natural environment of Côte d'Ivoire. Same. ORSTOM; 1971. 30. Monnier Y. Vegetation in "Young Atlas Africa Ivory Coast". J.A. 1983; 16-18. 31. Arnould M. Geological section of N'zi between parallels 8 ° and 8 ° 52 '(Ivory Coast). Serv. Geol. Prosp. Min; 1958. 32. Tagini B. Structural sketch of Côte d'Ivoire. Regional geotectonic test. Thesis Univ. Lausanne (Switzerland). Bull. SODEMI; 1971. 14

33. Casanova R. Geochemistry and mineralogy of the Eburnian granitoids of Ivory Coast. PhD Thesis, Univ. Nice and Univ. Abidjan. Publ. SODEMI; 1973. 34. Yacé I. Eburnal volcanism in the Central and Precambrian parts of Fetekro in Côte d'Ivoire. PhD Thesis, Univ. Abidjan, Ivory Coast; 1976. 35. Lemoine S. Geological evolution from the Dabakala region (north-eastern Côte d'Ivoire) to the early Paleozoic. Possibility of extension to the rest of Côte d'Ivoire and Burkina Faso: similarities and differences; the lineaments of Greenville-Ferkéssedougou and Grand Cess-Niakaramandougou. State PhD Thesis, Univ. Blaise Pascal ; 1988. 36. Perraud A. Soils. In the natural environment of Côte d'Ivoire. Same. O.R.S.T.O.M., Paris. 1971; 50: 269-390. 37. Monnet C. Contribution to the study of the dynamics and nature of the suspensions of an intertropical river, Bandama, Ivory Coast. Evolution of the chemical elements of the waters of its estuary. PhD Thesis, Univ. from Nice ; 1972. 38. Koudou A, Kouamé KA, Niamke KH, Kouamé KF, Saley MB, Adja MG. Contribution of principal components analysis to the regionalization of rainfall in the N'zi watershed, Central Côte d'Ivoire. Rev. Isee. Sci. Technol. 2015; 26: 156-172. 39. Gyau-Boake P, Schultz GA. Fillings gaps in runoff times in West Africa. J. Hydrol. 1994; 91: 621- 636. 40. Laborde JP. Elements of surface hydrology. Fascicule, CNRS, Univ. from Nice, Sophia Antipolis; 1998. 41. Thornthwaite CW. The measurement of potential evapotranspiration. J.M. Mather, sea-book, New Jersey; 1944. 42. WMO, Guide to Hydrological Practices. Data acquisition and processing, analysis, forecasting and other applications. Fifth edition; 1994; 168. 43. Baillargeon S. Kriging: review of the theory and application to spatial interpolation of precipitation data. Master's thesis in statistics, Univ. Laval, Quebec; 2005. 44. DCGTx, Inventory and diagnosis of dams. Major Works Control Department, Rural Development Sector Program (PSAR); 1996. 45. Savané I. Contribution to the geological and hydrogeological study of discontinuous aquifers of the crystalline basement of Odienné (North-West of Côte d'Ivoire). Contribution of remote sensing and a spatially referenced hydrogeological information system. PhD Thesis, Univ. Abidjan, Ivory Coast; 1997. 46. Kouassi AM, Kouamé KF, Saley MB, Yao BK. Identification of trends in the rainfall-discharge relationship and recharge of aquifers in a context of hydroclimatic variability: case of the N'zi (Bandama) watershed in Côte d'Ivoire. European Journal of Scientific Research. 2007; 16: 412-427. 47. Goula BTA, Savane I, Konan B, Fadika V, Kouadio GB. Comparative study of the impact of climate variability on the water resources of the N'zo and N'zi basins in Côte d'Ivoire. Poster AOC, International Seminar. 2005: 5-36.