Impact of climate change on the water resources of the small basin of Oti

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1 Impact of climate change on the water resources of the small basin of Oti Maléki H. BADJANA 1, Abidé ABALO-SAMA 1, Kodjovi S. EDJAME 1 Abidé ABALO-SAMA 2 1 University of Lomé, Lomé, Togo 2 University of Lomé, Lomé, Togo Abstract: This study highlights the impact of global warming on water resources of the small basin of Oti (North of Togo). The analysis of the interannual variability of Oti s river flows, the drying up coefficients and the volumes of water mobilized by the aquifers of Oti s sub-basin on the period shows a significant decrease in flow rates and water volumes mobilized by aquifers and increase in drying up coefficients. These changes lead to rapid emptying of the aquifers and thus lower the groundwater level which continues over the period according to projections. Climate change affects water resources of the Oti small basin by reducing the availability of water for the local people. Keywords: climate change, water resources, small basin of Oti, Togo. Introduction Climate change is today one of the most pressing environmental challenges which the world is facing. Statements of observation and climate projections have largely proven that fresh water sources are vulnerable and will suffer severely of climate change, with major impacts on human societies and ecosystems (Bates and al., 2008). Africa is at the forefront of the regions concerned by the question of the impact of climatic fluctuations on water resources (Kanohin and al., 2009). In Togo, especially in the plain of Oti where the rate of access to water is low (34%), the climate experienced a great variability since the last sixty years, marked by severe rainfall deficits and an intense warming over the two decades with a decline in overall rainfall and higher temperatures (Badjana, 2010). As a result, there is the early drying up of ponds, wells and rivers on which the local populations depend mainly for water supply. Face to the high dependence of the people of the basin of Oti to natural sources of fresh water and cope with climate extremes recorded during the last decades in the area, it is necessary to study the impact of climatic disturbances on the surface and groundwater that affect the availability of water for the people. This study is from that point of view and make hypothesis that the lower rainfall and higher temperatures result in lower groundwater volumes and the early draining of rivers and ponds would be linked to the local climate variability. The objectives consist to: - analyze the interannual variability in rainfalls, temperatures, flow rates, drying coefficients and volumes of groundwater; - highlight the impact of variability in Page 1 of 6

2 precipitation and temperature on the availability of water in the basin of Oti. Methods The main data used are: - rainfall, temperatures collected at the General Direction of National Meteorology; - the Oti River flows from the hydrology laboratory of ORSTOM, collected at the General Direction for Water and Sanitation. The methodological approach was to make a comparative study of interannual changes in rainfalls and temperatures on the one hand and the flows, the drying up coefficients and volume of water mobilized by aquifers on the other hand. The flow data exist only for the period while the rainfall and temperatures ranging up to 2008 are respectively available from 1960 and This led to the choice of study periods by not going beyond Drying up coefficient The calculation of the coefficients of depletion was done using the formula of Maillet (Castany, 1963; Forkasiewicz and Paloc, 1965; Savane et al., 2001) which admits that in a not influenced regime (in the absence of any precipitation), the drying is the exponential decay of the flow versus time. The mathematical expression of the depletion is written: (1) = flow at a given time t, = initial flow rate (flow rate at the beginning of drying); = coefficient of depletion of Maillet. Water volume mobilized by aquifers In a not influenced, the estimated annual depletion curve is the expression of draining the underground reservoir and therefore the amount of water collected at the gauging station corresponding only to dynamic volumes mobilized by all aquifers of the basin (Savané, 2001). The expression of the flowed volume at each time t is given by: (2) The integration of this formula from 0 to + gives the volume mobilized by all aquifers, either: (3) The Hanning low-pass filter of order 2 To reduce uncertainty from measures and gaps filling in order to better observe the interannual fluctuations, all data are filtered using the equations of the Hanning low-pass filter of order 2 (Kanohin and al., 2009, Bi Tié and al., 2006; Badjana, 2010). Index of Nicholson To better perspective the interannual variability of different parameters compared to the normal over the entire period, the index of Nicholson (Kanohin and al., 2009, Bi Tié and al., 2006; Badjana, 2010) is calculated. Correlation rank test of Kendall Finally, the correlation rank test of Kendall (Kendall & Stuart, 1943 ; WMO, 2000 ; Lubès-Niel and al., 1998) has been used to highlight trend in time series of different variables. Results Page 2 of 6

3 The analysis of interannual changes in rainfalls and temperatures over the period shows generally a downward trend in rainfall and rising in temperatures. The period was very wet over the period , which is generally dry. Temperatures were also very high after 1970 compared to the previous period (figure 2). The Oti River flows have experienced strong interannual fluctuations between 1957 and 1989 with a downward trend (figure 3). The drying coefficients fluctuate widely from year to year but have seen a marked increase from 1957 to 1989 (figure 3). This increase drying coefficients reflects thus an acceleration of the drying up of river. The analysis of volumes mobilized by the aquifers shows that they are also experiencing strong fluctuations between years but have been trending downward in contrast to the depletion coefficients (figure 3). This drop in volumes mobilized by aquifers reflects the rapid emptying of aquifers. Projections over the period where temperatures has increased et rainfalls decreased show a similar evolution of flows, drying up coefficients and groundwater volumes. The correlation test of Kendall's rank confirms all the trends of the different variables studied to a confidence level of 99% (Table 1). when the drying coefficients increase, flow rates and volumes of water mobilized by the aquifers decrease. Figure 1: Presentation of the study area (small basin of Oti). Figure 2: Evolution of rainfalls and temperatures ( ). The comparative analysis of three curves of flows, drying up coefficients and the groundwater volumes show that the groundwater volumes vary according to the flows (figure 3). The correlation coefficient between the two variables is 0.79 showing a strong dependence one another. By cons, Page 3 of 6

4 mobilized volumes was demonstrated by Castany (1963c) for who between surface water and groundwater, the hydrologic relationships and interdependence is so narrow that both constitute a unique resource and each levy on one is made at the expense of other. Figure 3: Evolution of flows, drying coefficients and volumes of groundwater ( ). Table 1: Results of the statistical analusis by the correlation rank test of Kendall. Confidence Trend level Rainfalls downward 99% Temperatures upward 99% Flows downward 99% Dryind up coefficients upward 99% Volumes of mobilized water downward 99% Discussion and Conclusions The analysis of changes in flow rates, drying up coefficients and volumes mobilized by groundwater reserves shows that the variation of mobilized volumes is proportional to the flow but inversely proportional to the depletion coefficients. The proportional changes in flows and The evolution of the depletion coefficients inversely proportional to the flow or volumes mobilized by groundwater reserves is explained partly by the fact that the depletion is the decrease in speed over time and thus its coefficient increases more when the flow decreases and then secondly by Darcy's law (Darcy, 1856) which states that the coefficient of depletion is directly proportional to hydraulic conductivity (permeability) and inversely proportional to the size of the aquifer (Savané and al., 2001). The influence of rainfall on groundwater has been shown in numerous studies. Castany (1963b, 1963c) showed that groundwater is fed, in their catchment area, by rainfalls; this diet is based on the amount of water that reaches the tablecloth or effective infiltration. For the, the height and rate of rainfalls play a more direct or less role depending on the depth and type of the aquifer horizon (captive or free) on the groundwater regime and hence the renewal of reserves. Finally, he asserts that the potentiometric surface of the water undergoing free, under natural conditions, changes in levels, often large, in direct relation with the pace and intensity of rainfalls and the intensity of the evapotranspiration that increase according to temperature. More studies of the Intergovernmental Panel on Climate Change (IPCC) have shown significant relationships between the runoff, Page 4 of 6

5 rainfall and temperatures (Bates and al., 2008) The averages of the different studied variables before and after 1970 show that the drying coefficient rose of 29.57% after 1970 while the flows and volumes mobilized by aquifers respectively decreased of 36.92% and 27.63%. These results are consistent with the work of Mahe and Olivry (1995) which concluded that West Africa has experienced persistent rainfall deficit from 1970 which has accelerated the depletion leading to a rapid emptying of the tablecloth. The draining and the early drying up of the rivers, the pools and the wells thus can related to the fall of the volumes mobilized by the aquifers which is reflected on the level of the aquifers. It should be noted that the recharge of aquifers also depends on other factors such as the vertical permeability of the land (Castany, 1963b). In view of the foregoing, it appears clear that climate change affects water resources of the Oti small basin by reducing the availability of water for the people. Thus urgent measures deserve to be taken to protect durably the water resources of that basin in order to reduce the vulnerability of the populations of this zone which suffer already from the problems of water to the harmful effects of climate change. References Badjana H. M., Perceptions locales des changements climatiques et mesures d adaptation dans la plaine de l Oti. Mémoire de DEA de Biologie de développement, option gestion de l Environnement. Faculté des Sciences, Université de Lomé, Togo. Bates B. C., Kundzewicz Z. W., Wu S. et Palutikof J. P., Le changement climatique et l eau, document technique IV publié par le Groupe d experts intergouvernemental sur l évolution du climat, Secrétariat du GIEC, Genève, ISBN: , 236 p. Bi Tié A. G., Savané I., Konan B., Fadika V. et Kouadio G. B., Impact de la variabilité climatique sur les ressources hydriques des bassins de N Zo et N Zi en Côte d Ivoire (Afrique tropicale humide). Vertigo - la revue électronique en sciences de l'environnement, 7(1), 12p. Castany G. & Margat J. (1977) Dictionnaire français d'hydrogéologie, BRGM Service géologique national, Orléans, 249 p. Castany G., 1963a. Courbes de tarissement et calcul des réserves régulatrices, pp Castany G., 1963b. Réserves en eaux souterraines : classification et évaluation. Bulletin de l AIHS, II. Partie scientifique (II. Scientific part), pp5-15. Castany G., 1963c. Unité des eaux de surfaces et des eaux souterraines, principe fondamentale de la mise en valeur des ressources hydrauliques Darcy H., Les fontaines publiques de la ville de Dijon, V. Dalmont, Paris. Kanohin F., Saley M. B., Savané I., Impacts de la variabilité climatique sur les ressources en eau et les activités humaines en zones tropicale humide: cas de la région de Daoukro en Côte d Ivoire. European Journal of Page 5 of 6

6 Scientific Research, ISSN X, 26(2), Kendall S.M., Stuart A., The Advanced Theory of statistics. Charles Griffin, London, UK. Vol. 1, 2 & 3, p 472, 723, 585. Kendall S.M., Stuart A., The Advanced Theory of statistics. Charles Griffin, London, UK. Vol. 1, 2 & 3, p 472, 723, 585. Lubès-NieL H., Masson J.M., Paturel J.E., Servat E., Variabilité climatique et statistiques. Etude par simulation de la puissance et de la robustesse de quelques tests utilisés pour vérifier l homogénéité de chroniques. Révue des Sciences de l Eau, Rev. Sci. Eau 3(1998), pp Mahé G., Olivry J-C., Variations des précipitations et des écoulements en Afrique de l ouest et centrale de 1951 à Sécheresse 1995, 6 : Savané I., Coulibaly M. K., Gioan P., Variabilité climatique et ressources en eau souterraines dans la région semimontagneuse de Man. Sciences et changements planétaires/sécheresse, 12(4), World Meteorological Organisation (WMO), Detecting trend and other changes in hydrological data (ed. by Z. W. Kundzewicz & A. Robson). World Climate Programme Data and Monotoring. WCDMP 45, WMO/TD no. 1013, World Meteorological Organization, Geneva, Switzerland. Acknowledgments Our acknowledgments to the professors Kpérkouma WALA, Komlan BATAWILA and Koffi AKPAGANA. Page 6 of 6