Trends and Shifts in Time Series of Rainfall and Runoff in the Gambia River Watershed

Abstract

For several decades, climate change and climate variability issues and their impacts on the hydrological regime of rivers have constituted a major topic for hydroclimatological sciences research and water resources planning policies. Understanding of these issues needs enough long time series of rainfall and runoff data covering a large period, and a comprehensive diagnosis of the existing trends and shifts in these time series of data. This can be done by applying robust statistical tests to relevant rainfall and runoff time annual series. The aim of this paper is to highlight the effect of climate change in the Gambia River Basin and its impacts on the availability of the water resources of this basin. To reach this objective, we have selected runoff time series of the Gambia River Basin at Mako, Kedougou Diaguéri streamgauges and rainfall time series at Koulountou’s rain gauge. Statistical tests for shift detection presented in the Khronostat software, such as Pettit, Hubert and Buishand ellipse tests are first used, Mann Kendall test for annual trend are then applied to check whether trends exist or not in these times series. When the null hypothesis of no trend is rejected, the non parametric Sen’s test is then applied to validate the Mann Kendall trend test and to estimate the magnitude of the trend and its direction. Tests for homogeneity show an increasing shift for rainfall time series of Koulountou raingauge and for runoff time series of Mako and Diaguéri and a decreasing shift for Kedougou streamgauge. According to the Mann Kendall trend test, there is an upward trend for Koulountou rainfall time series, and Mako and Diaguéri runoff time series, and a downward trend for Kedougou annual runoff time series. The Buishand ellipse and the Hubert test indicate generally the same year of the beginning of the shift. Interesting perspectives for decision makers in evaluation and precise management of water resources and water projects in the Gambia River basin are offered as well.