Storage Mixing Variability Across Seasons and Scales in Tanzania

Abstract:

Our ability to accurately assess water residence times and storage volumes hinges on data availability. However, hydrological data is often limited or non-existing in most regions of the world. This study synthesizes hydrological tracer data with hydroclimatic information in order to disentangle storage volume dynamics and variability across data-limited African catchments. Specifically, we focus on the Kilombero Valley in Tanzania where there is a large potential to develop and expand the agricultural sector and thus increasing food security nationally. The lack hydrological data and subsequent limited process understanding hinders our capacity to assess the sustainability of such an increased and intensified agriculture landscape.

We demonstrate how hydrological tracers constitute an exceptionally valuable piece of information for constraining model parameterizations, improving process understanding and representing storage volumes in data limited regions. Geochemical (e.g., Ca²⁺, Na⁺, K⁺, Mg²⁺, SO₄^2-, Cl^-) and stable water isotope (d¹⁸O and d²H) tracers were used to estimate recharge rates and seasonal shifts in hydrologic flow pathways. End member mixing analysis (EMMA) applied within the GLUE uncertainty framework was used to assess relative source contributions to streamflow and storage volume connectivity across scales.

Strong variations in stable water isotopes between rainfall seasons in Tanzania and geological partitioning of storages allowed for clear characterization of seasonal variations in hydrologic flow pathway development. Wetlands dominated the wet season flows while variability in the connectivity of water storages could be seen during the dry season. Differences in the wetting up versus drying down storage mixing across the landscape highlights process shifts. This characterization improves our ability to utilize the limited data available in Kilombero Valley as it provides the basis for modelling surface-groundwater interactions regionally and their implicit controls on water residence times and, in turn, resource vulnerability.