Impacts of Climate Variability and Agricultural Intensification on the Origin of Runoff: The Case Study of the Watershed Kolondieba in the South of Mali

As part of the international research program RIPIECSA (Interdisciplinary and Participatory Research on Interactions between Ecosystems, Climate and Society in Africa), watershed Kolondieba (under Sudanese climate) was selected to understand the mechanism of runoff process in order to improve hydrological model in a context of strong climate variability and agricultural intensification (cotton culture is the main economic activity in the basin). The method used is based on rainfall, hydrometric, geochemical and piezometric data monitoring over the period 2009-2011. The results showed that 2009 and 2010 were normal rainfall years (1300 mm, in average), compared to the average of chronic 1960-1969 (wet period ), while 2011 has emerged as a dry year compared to the chronic 1970-1992 (dry period). During the last contrasted two years, the runoff coefficient has decreased by half from 2010 to 2011 occasioned groundwater discharge deficit estimated at 33%. Monitoring the mineralization of targeted water compartments: rainfall, surface water, and groundwater with the integrator chemical parameter (Electrical Conductivity), showed a very little mineralization of rainfall with an average of 16.99 ± 8.53 μScm-1. Mineralization of surface water is closer to the rainfall's, but it's far from the groundwater's consist of shallow aquifers and deep ones with respectively 120.58 ± 90.07μScm-1 and 133.57 ± 85.68 μScm-1 in average. This chemical relationship between water compartments showed that deep aquifers don't contribute enough to the runoff. This allowed to deduct a double origin of the runoff on the watershed consists of stormflow and subsurface flow. The separation of the hydrograph in a normal year (2010) gave a contribution of stormflow about 77%. This contribution has increased by 3% in dry year (2011). In these conditions runoff doesn't depend only on rainfall variability, it can be assigned to the land use because cotton culture area is increasing on the basin since 1960.