Rainfall Variability and Water Supplies in the Diarha Watershed (Tributary of Gambia River)

The present study consisted of analyzing the spatial and temporal variability of the rainfall series in the Diarha watershed between 1921 and 2014, using the Nicholson indices, the stationarity tests and the Thiessen polygons method for spatial analysis, to quantify surface water supplies and to determine the impact of this rainfall variability on the hydrology of the basin. The results show that the rainfall series are not stationary. They show breaks between 1965 and 1979 (with 1967 as the year of rupture for 6 of the 10 stations in the basin). From this break-up period (1967) to the present day, average annual rainfall decreased by 10.5%. On a monthly basis, it appears that precipitation decreased significantly for almost every month between the two periods. On a daily basis, the analysis of the daily rainfall fractions reveals a tendency to decrease the average annual rainfall over 40 mm (which are considered to be the heavy rains) from years of rupture. At the spatial scale, the variability of annual precipitation is marked by a translation of isohyets towards the South from the decades 1971-1980 and 1981-1990. During this period, the decline in rainfall resulted in a shift of the isohyets towards the south in the less rainy zone in the north (in the South Sudanian domain); while the latter are tightening in the "strong" precipitation zones which are located in the South (in the Guinean climate domain) and in the Center-east of the basin. On the other hand, a slight trend of rainfall recovery is noted during the decades 1991-2000 and 2001-2010. At the same time, the flows of the Diarha vary from one year to another according to the precipitated water slides on the basin and the ETP. To this must be added the influence of pedology and geology (the river flowing over the Proterozoic and Paleozoic formations of the Birrimian basement), but also of the high topography of the basin, in this case the relief and its slope system. All these factors combine in space and time to allow for high values of basin runoff.