Mechanisms affecting stormflow generation and solute behaviour in a Sahelian headwater catchment

The aim of this study was to analyse stormflow processes and the behaviour of solutes therein (Ca2+, Mg2+, Na+, K+, alkalinity, NO3-, SO42-, Cl−, Si), during flood events in tropical semi-arid conditions. The study site was a small Sahelian catchment (1.4 ha) located in northern Burkina Faso. Runoff and rain water was sampled over a 2-year period (1999 and 2000). In addition to dissolved load, suspended load was measured in the stream water collected at the outlet of the catchment. Isotopic tracing using δ18O was also conducted during two very different flood events. The results indicated that: (i) event water was by far the major contributor to the stream stormflow, with Hortonian overland flow being the main stormflow process at work; (ii) a small fraction of pre-event soil water may have contributed during the recession of heavy flood with wet antecedent conditions; (iii) solute concentrations were higher in runoff compared to rainwater. With the exception of NO3- and Cl−, the highest concentrations were measured at the onset of floods, and almost always decreased during the rising stage of the hydrographs; (iv) a good correlation was found between suspended load and the concentrations of Ca2+, Mg2+, alkalinity and Si. It was concluded that fast physico-chemical interactions between event water and reactive suspended phases may explain most of the chemical changes between rainwater and floodwater.