ALBERT

Description: A solid understanding of the hydrological processes in a catchment is important in order to guarantee appropriate management of the available surface and groundwater resources, both in terms of quality and quantity. In order to achieve this, insights in the behaviour of the water fluxes and the interaction between groundwater and surface water is of utmost importance. This paper discusses the applicability and constraints of using hydrochemical and isotope tracers in identifying the runoff contributing sources at different scales in a semi-arid catchment in Tanzania. The hydrograph separation techniques shows that at the smallest scale (0.3 km2), for all types of tracers, the pre-event contribution is between 74–82%. At the larger scale (26 km2), two sub-catchments contribute to the flow at the weir site in Bangalala. Using the hydrochemical tracers the calculated contributions for the sub-catchments are in agreement with the catchment size and rainfall contributions over these two catchments. This showed that at the weir site 20% of the total flow comes from event water (of which 2% from Vudee sub-catchment and 18% from Ndolwa sub-catchment). The large difference is mainly due to preceding wetness conditions. However, with the isotope tracers no unambiguous results could be obtained. Two end members have been investigated to account for the ambiguous nature of the isotopic concentrations. The rainfall analysis shows that during the season the isotopical concentration changes, with a clear distinction between the two seasons. In addition, within one event the isotopic concentrations vary substantially within the area. The spring analysis also shows substantial temporal and spatial variation. The research therefore shows that the assumption of stable isotopic end-members was not met in our study. At the smaller scale the spatial variability could be neglected and the hydrograph separation technique could be applied, although for each event, end member concentrations needed to be collected to account for the temporal variability.