Hydrological drivers of dryland riparian woodlands

Background/Question/Methods

Despite drylands occupying nearly 50% of the world’s land surface, the ecology of their river systems is poorly understood, and this is particularly true for ephemeral rivers. Ephemeral rivers have a dry riverbed for the majority of the year and only flood for short periods of time after large amounts of rainfall. The groundwater reserves below the riverbed can support abundant vegetation year round which is quite distinct from the sparse vegetation in the surrounding desert environment.

This study focuses on the Swakop River, a regulated river in Namibia, to investigate how the cessation of a river’s natural flooding regime impacts riparian woodlands. A large dam on the main channel of the river became operational in the late 1970s and because it does not have slew (water-release) gates the river provides a unique opportunity to study the role of withdrawal of flooding in riparian woodland dynamics.

We use a multi-method approach of field surveys, interviews and archival searches, and remote sensing to investigate how riparian woodland composition, health and cover have been impacted by dam creation.

Results/Conclusions

Preliminary analysis indicates that as a result of the dam construction and the subsequent reduction in main channel floods, the woody vegetation in the main channel has become more dependent on floods from tributaries rather than the main Swakop channel. Both woodland composition and health are determined by flood occurrence. Healthy Faidherbia albida tree populations are more closely associated with flood conditions than Prosopis species. Woodland health indicators –such as canopy dieback, and reproductivity are associated with flood occurrence. In order to preserve the riparian woodlands and their associated ecosystem services in this catchment, we suggest that future management should focus on maintaining tributary flow.