Active earth-surface processes following rainfall events in the southern Namib Desert – Insights from the Kaukausib catchment by means of remote sensing

Active earth-surface processes in desert environments can be well studied by utilizing recent spaceborne remote sensing imagery e.g. from the European Sentinel Missions. Insights on these processes are important to serve as modern analogues for the long-term landscape evolution of drylands and the preservation of paleoenvironmental archives. The multi-sensor fusion of latest earth-observation data, e.g. multispectral optical imagery, Synthetic Aperture Radar (SAR) data and/or digital elevation models (DEM´s), allows to distinguish landforms between very young and active to stable – presumably older – geomorphological units, the co-existence of which is a striking phenomenon in arid environments. Based upon this methodological approach, the current landscape dynamics in the hyper-arid southern Namib Desert are studied in a key area for past, current and future environmental changes in desert environments of the Southern Hemisphere: the Kaukausib catchment.

The Kaukausib catchment, located in-between two major atmospheric circulation patterns with tropical (summer rainfall) and extratropical (winter rainfall) influence, is highly sensitive to changes and interactions of both climate systems. The catchment is bounded by the Great Escarpment and receives no discharge from the higher hinterland. As such, fluvial activity and resulting landforms are related to local precipitation only. Consequently, the landform inventory of this distinct catchment is a unique recorder of recent and past climate dynamics of the Southern African drylands.

Preliminary investigations identified the high sensitivity of the Kaukausib catchment to recent short-term environmental changes. Rare extraordinary rainfall events, exceeding the average annual amount of less than 50 mm, lead to temporary changes in vegetation cover and density. These events seem to occur in a frequency of 6–11 years, at least during the last 30 years. They are mostly associated with atmospheric interaction of the tropical and extratropical circulation patterns in spring and autumn, e.g. in April 2006 with an unusual northward position of a cut-off-low from the temperate climate system in phase with a Temperate Tropical Through from the tropics unusually far south. The spatio-temporal changes of vegetation cover subsequent to these extraordinary rainfalls are studied by analyzing time series of Landsat 5. Vegetation vitality has its maximum three months after the rainfalls, where in some regions a rather dense cover of annual and ephemeral grass occurred (Henselowsky et al. 2019 Z. f. Geomorph https://doi.org/10.1127/zfg_suppl/2019/0552).

In addition, fluvial events following rainfalls in 2020 and 2018 are studied using Sentinel-1 data to identify short-term surface changes, but also to detect presumable stable sediment surfaces. Sporadic fluvial activity in turn is revealed by investigating signal differences in SAR intensity and InSAR coherence before and after fluvial activity. Information on channel activity is interpreted in the context of the morphometric characteristics and first field-investigations in the Anib and Arasab Pan. These pans limit the current runoff of the upper Kaukausib and represent the largest sediment basins of the southern Namib Desert. Therefore, the identification of current surface processes and sediment provenances, identified by spectral indices of optical satellite data, sets the baseline for future in-depth investigation of its sedimentary record and paleoenvironmental changes in the Kaukausib catchment.