THE CLIMATE SYMPOSIUM 2014 13-17 october 2014, Darmstadt, Germany/Conference Session 2 : The changing water cycle Variability on Daily Rainfall Characteristics Over Senegal River Basin Before and After Rainfall Shift Ansoumana BODIAN* and Ousmane NDIAYE** * Laboratoire Leïdi « Dynamique des territoires et développement » Université Gaston Berger ; ** Agence Nationale de l'Aviation Civile et de la Météorologie (ANACIM) Contact : [email protected]/[email protected] MOTIVATION : Several studies have attempted to characterize the variability of rainfall in sub-Saharan Africa. These studies showed a clear decrease in annual and monthly totals since the sixties and seventies, but didn’t look what rainfall characteristics undergo significant changes. This could be justified by difficulty in acquiring reliable daily data with few gaps (Kouassi et al., 2010). However, in recent decades, concern for sustainable management of water resources in a context of climatic variability has led to creation of basin organizations such as Organization for the Development of the Senegal River (OMVS), Organization for the Development of the Gambia River (OMVG), the Niger River Basin Authority (NBA ) etc. To fulfill their mandates, these basin organizations need reliable data. For this purpose, they collaborate with technical services (meteorological and hydrological) of their member states allowing development of hydrological and climate databases for different scales (annual, monthly, daily and even hourly with rain intensities in some cases). These databases allow today to conduct analysis of climate variability at finer time scales. Fig. 1 : Location of rainfall stations DATA : The data used in this study came from the National Meteorological Services from Guinea, Mali, Senegal and Mauritania and have been extracted from the database of the OMVS. Data covers four to twenty rainfall stations (Fig. 1). They are of different quality and length (Fig. 2), depending on the country. A reference period, from 1950 to 1998, was chosen based on the data quality. We then calculated the percentage of missing (by missing, we mean same day without data available even during the dry season) in the data series during the period 1950-1998. On the basis of this criterion (Fig. 3) twenty three rainfall stations with less than 5% have been identified as reference stations in this study. The spatial distribution of these stations is shown in Fig. 3. Fig. 2 : Percentage of missing stations per year METHODS : The objective of this study is to analyze, across the Senegal River Basin, changing classes of daily precipitation, number of rainy days and the duration of the rainy season. The first method is to apply statistical tests of homogeneity to the series of annual rainfall during the reference period 1950-1998 for detecting the break. Then we calculate classes of precipitation frequency : P1 represents daily precipitation between 1 and 10 mm; P2 daily rainfall between 10 and 30 mm ; P3 daily precipitation between 30 mm and 50 mm and P4 daily précipitation greater than 50 mm and duration of the rainy season and the number of rainy days per year, and finally we characterize evolution of these six variables on each side of the break. Fig. 3 : Percentage of missing data in the reference period 1950-1998 (left); Spatial distribution of stations selected for this study (right). Fig. 4 : Year of the shift in the annual rainfall total during the reference period (left); Percentage of rainfall deficit between period after the break and that before the break (right). Fig. 5 : Change in the number of rainy days on each side of the break in the annual rainfall series (top left). Percentage of deficit in rainy days during the period after break (bottom left). Variation of the deficit in rainy days by latitude (top right). Fig. 7 : Rainfall frequencies differences around the break (left). Evolution of highest frequency class (P4) by latitude (right). RESULTS : With twenty three stations selected, analysis of annual rainfall data shows that the time series are not stationary and they experience a break between 1966 and 1970 (Fig. 4). Deficits in rainy days range between 6.2% in Kédougou and 38.6% in Saint Louis (Fig. 5). Analysis of the duration of the rainy season on both sides of the break shows two cases (Fig. 6) : (i) one characterized by a tendency of shortening of the rainy season and (ii) another one with mixture of both positive and negative treends. So stations in the Sahelian zone are more affected by drought over the course of the season than southern stations. A look of P4 class deficits reveals that the most important deficit is recorded in the Sahelian zone (Fig. 7). PERSPECTIVES : (i) Assess the impacts of climate variability and change (using global and regional climate models) on water resources in the watershed of Senegal River. (ii) Model rainfall-runoff relationship in order to extend the series of hydrological data, which are often incomplete, using available climate data (ground data and satellite estimates) for better understanding of water availability. Fig. 6 : Variation in the duration of the rainy season on both sides of the break (top left). differences between the period before and after break (bottom left) and the change by latitude (top right). Référence : Bodian A. Ndiaye O. and Dacosta H : Evolution des caractéristiques des pluies journalières dans le bassin versant du fleuve Sénégal: avant et après rupture (Accepted in Hydrological Sciences Journal : DOI:10.1080/02626667.2014.950584).