ALBERT

Description: Hydrological processes in small catchments are not quite understood yet, which is true in particular for catchments in data scarce, semi-arid regions. This is in contrast with the need for a better understanding of water fluxes and the interactions between surface- and groundwater in order to facilitate sustainable water resources management in such environments, where both floods and droughts can result in severe crop loss. In this study, event runoff coefficient analysis and limited tracer data of four small, nested sub-catchments (0.4–25.3 km2) in a data scarce, semi-arid region of Tanzania helped to characterize the distinct response of the study catchments and to gain insights into the dominant runoff processes. The estimated event runoff coefficients were very low and did not exceed 0.09. They were found to be significantly related to the 5-day antecedent precipitation totals as well as to base flow, indicating a close relation to changes in soil moisture and thus potential switches in runoff generation processes. The time scales of the "direct flow" reservoirs, used to compute the event runoff coefficients, were up to one order of magnitude reduced for extreme events, compared to "average" events, suggesting the activation of at least a third flow component, besides base- and direct flow, assumed to be infiltration overland flow. Analysis of multiple tracers highlighted the importance of pre-event water to total runoff, even during intense and high yield precipitation events. It further illustrated the distinct nature of the catchments, in particular with respect to the available water storage, which was suggested by different degrees of tracer damping in the individual streams. The use of multiple tracers subsequently allowed estimating uncertainties in hydrograph separations arising from the use of different tracers. The results highlight the presence of considerable uncertainties, emphasizing the need for multiple tracers in order to avoid misleading results. This study shows the value of hydrological data collection over one whole wet season using multi-tracers to improve the understanding of hydrological functioning and thus for water resources management in data scarce, semi-arid environments.

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.