Actualization of Climatic Parameters of the Empirical Methods of ORSTOM and CIEH: Case of Ivory Coast

Abstract

Climate change is a reality that affects several climate variables including precipitation. This change and the variability of the climate are considered to be the greatest challenges facing humanity in the decades to come, on all geographic scales and in all economic sectors. In this context of severe climate change, it will be judicious to review the sizing parameters of hydraulic structures. The determination of hydrological standards is necessary within the framework of development projects for the design of hydraulic structures and the storm water management. The objective of this study is to update the parameters of the ORSTOM and CIEH methods. These are annual rainfall normals, extreme rainfall norms, extreme rainfall quantiles as well as the extreme rainfall gradex. The study carried out was based on annual maximum daily rainfall data and annual rainfall data collected over the period 1931-2020 from twenty-six (26) rainfall stations. The methodological approach is based on the one hand on the evaluation of the normals (normals of annual rainfall, norms of extreme rainfall) and on the other hand on the determination of the quantiles of the daily maximum annuals. These quantiles were used to assess the extreme rainfall gradex. The annual rainfall norms evaluated over the period 1931-2020 vary between 1,180 and 1,457.4 mm. As for the normals of the maximum daily rains, they oscillate between 73.61 mm (Agnibilékro) and 136.59 mm (Tabou) with an average of 94.57 mm. The 10-year return period quantiles evaluated over the period 1931-2020 vary between 103 mm (Dimbokro) and 222 mm (Tiassalé) with an average of 143.6 mm. As for the centennial quantiles, they oscillate between 132 mm (Dimbokro) and 326 mm (Tiassalé) with an average of 211.5 mm. An analysis of the extreme rainfall gradex revealed values fluctuating between 0.28 (Dimbokro) and 0.71 (Guiglo) with an average of 0.47 on Ivorian territory. The gradex coefficients determined are all greater than the regional value of 0.38 defined by ORSTOM except for the Dimbokro station (0.28). Indeed, the calculated gradex coefficient biases are all positive (ranging from +2.63 to + 86.84%) except that of Dimbokro (-26.32%). The use of the regional value of 0.38 reflects an under designing of the values of the gradex coefficient for the whole country except the Dimbokro station.