The micro‐topography of the wetlands of the Okavango Delta, Botswana

Abstract

The surface of the 40 000 km 2 Okavango alluvial fan is remarkably smooth, and almost everywhere lies within two to three metres of a perfectly smooth theoretical surface. Deviations from this perfect surface give rise to islands in the Okavango wetlands. This microtopography was mapped by assigning empirical elevations to remotely sensed vegetation community classes, based on the observation that vegetation is very sensitive to small, local differences in elevation. Even though empirical, the method produces fairly accurate results. The technique allows estimation of depths of inundation and therefore will be applicable even when high resolution radar altimetry becomes available. The micro-topography has arisen as a result of clastic sedimentation in distributary channels, which produces local relief of less than two metres, and more importantly as a result of chemical precipitation in island soils, which produces similar local relief. The micro-topography is, therefore, an expression of the non-random sedimentation taking place on the fan. Volume calculations of islands extracted from the micro-topography, combined with estimates of current sediment influx, suggest that the land surface of the wetland may only be a few tens of thousands of years old. Constant switching of water distribution, driven by local aggradation, has distributed sediment widely. Mass balance calculations suggest that over a period of c. 150 000 years all of the fan would at one time or other have been inundated, and thus subject to sedimentation. Coalescing of islands over time results in net aggradation of the fan surface. The amount of vertical aggradation on islands and in channels is restricted by the water depth. Restricted vertical relief, in turn, maximizes the distribution of water, limiting its average depth. Aggradation in the permanent swamps occurs predominantly by clastic sedimentation. Rates of aggradation here are very similar to those in the seasonal swamps, maintaining the overall gradient, possibly because of the operation of a feedback loop between the two. The limited amount of local aggradation arising from both clastic and chemical sedimentation, combined with constant changes in water distribution, has resulted in a near-perfect conical surface over the fan. In addition to providing information on sedimentary processes, the micro-topography has several useful hydrological applications. Copyright

HESS Opinions "The art of hydrology"*

“…Gumbricht et al (2004) showed that the islands have a range of sizes that completely fit fractal behaviour, suggesting some self-organising principle for island formation. Gumbricht et al (2005) came up with an interesting explanation, which comes from yet another discipline: chemistry. The Okavango river carries very small amounts of salts, but considerable amounts of calcite and silica in solution, stemming from the Kalahari sands that it drains.…”

Reducing uncertainty based on model fitness: Application to a reservoir model

“…The system is large. The entire geologic alluvial fan encompasses 40 000 km 2 (Gumbricht et al, 2005) and flooding extents range from 6 000-12 000 km 2 (Wolski et al, 2006). No roads traverse the area and the field collection of data is extremely cumbersome.…”

Mapping global tropical wetlands from earth observing satellite imagery

“…Hence, the peat in these extensive wetlands is not as deep as, for instance, in the peat domes of South East Asia. Gumbricht et al (2005) developed a simple, but successful method for estimating the microtopography of Okavango wetlands on an alluvial fan in Botswana. From empirical relationships between the wetland vegetation communities and the depth of flooding, they were able to accurately estimate the surface topography of the Okavango peatlands indirectly based on the levels of the underlying minerogenic sediments.…”

“…From empirical relationships between the wetland vegetation communities and the depth of flooding, they were able to accurately estimate the surface topography of the Okavango peatlands indirectly based on the levels of the underlying minerogenic sediments. Thus the approach used by Gumbricht et al (2005) could be used to estimate the volumes of peat of the Okavango and similar wetlands.…”