Do sediments matter? Assessing sedimentation effects on surface water storage in a semi-arid ephemeral river system

Ephemeral river systems are key hydrological components of dryland catchments, particularly in semi-arid regions where surface water availability is highly episodic and closely linked to short-lived rainfall events. Shallow channel depressions temporarily store water and extend its availability beyond individual flow events. However, the extent to which sedimentation affects their long-term storage capacity remains poorly constrained, especially in data-scarce regions of southern Africa. This knowledge gap is increasingly relevant under rising water demand, pronounced climate variability, and more frequent droughts.

This study focuses on the Iishana system, a transboundary network of ephemeral channels and shallow depressions in northern Namibia and southern Angola. The system constitutes the primary rural water resource for a densely populated semi-arid region, with surface water availability largely restricted to short periods following rainfall events. Consequently, the capacity of channel depressions to retain water over extended periods is critical for domestic use, livestock, and small-scale agriculture. Understanding sediment accumulation processes in these depressions is therefore essential for improving water resource management in dryland catchments. The Iishana system is characterized by very low gradients, episodic runoff, and highly variable hydrological connectivity, making it representative of semi-arid environments where event-driven processes, storage, and transmission losses dominate.

The objective of this study is to characterize sediment properties and quantify sedimentation rates in selected depressions in order to assess their influence on surface water storage. Sediment cores were collected from multiple depressions and analyzed using physical and geochemical methods. Chronologies were established using ²¹⁰Pb and ¹³⁷Cs radionuclides, supported by radiocarbon dating. Sedimentation rates were calculated using Constant Flux-Constant Sedimentation (CFCS) and Constant Rate of Supply (CRS) models.

Sediments are predominantly fine-grained sand and silt, with weak pedogenic development, indicating limited and discontinuous deposition. The CFCS model results show low accumulation rates ranging from 0.017 to 0.12 cm yr^-1, while CRS-derived mass accumulation rates range between 0.05 and 0.07 g cm^-2yr^-1. Below approximately 10 cm depth, sediment ages commonly exceed 150 years. In contrast, ¹³⁷Cs activities were very low and lacked identifiable peaks, rendering this radionuclide unsuitable for chronologies in the study area.

The consistently low sedimentation rates indicate that natural sediment infill currently plays a negligible role in reducing the surface water storage capacity of ephemeral depressions within the Iishana system. From a hydrological perspective, this suggests that storage limitations are primarily controlled by hydrological connectivity, event-driven runoff generation, and infiltration rather than by progressive sediment accumulation. The results provide an empirical basis for evaluating the potential of selected depressions for targeted deepening or lateral expansion to enhance short-term surface water storage during episodic flow events.

Furthermore, spatial variability in sediment properties and accumulation rates highlights the importance of site-specific characteristics such as channel connectivity, flow velocity, and local catchment conditions, which are key controls on hydrological processes in dryland systems. By linking sediment dynamics with surface water storage, this study contributes to hydrological modeling, scenario-based planning, and sustainable water management in semi-arid catchments under increasing climatic stress.