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Earth Surface Processes and Landforms 27, 11 - During the dry season, the wetland is exposed to oxidising conditions in an aerobic environment. Over a prolonged period of time, repeated cycles of wetting and drying result in a loss of volume in the centre of the depression, with concomitant aggradation on the margins.
This process may be fairly common on ancient erosion surfaces that have been exposed to weathering processes for long periods of time. First described by Ellery for Shadow Vlei, in the Eastern Cape of South Africa, the resultant depression is surrounded by a ferricrete rim that is slightly elevated above the surrounding landscape. The same process can also result in the development of calcrete rims Alistoun, In contrast to redox depression wetlands, the soluble ions responsible for relief building in margin-aggradation depression wetlands are not derived in situ , but are supplied allochthonously by circulating waters rich in solutes.
The archetypal example of a margin-aggradation depression is described by McCarthy et al. Large, deep-rooted trees grow preferentially on the island margins, and are responsible for increasing groundwater salinity, leading to saturation of calcite and silica, by transpiration. Sub-surface saturation results in precipitation of calcite and amorphous silica, leading to vertical expansion on island margins.
The creation of topographic relief on the margin creates a central depression, characterised by sparse, salinity tolerant vegetation. On older islands, vegetation in the depression may be replaced with sodium carbonate deposits. In contrast to the long time frames required for the formation of subsidence and redox depressions, numerical modelling suggests that salinisation of the groundwater occurs over timespans between and years, but that aggradation of island margins takes place over much longer timespans.
The formation of margin-aggradation depression wetlands requires specific hydroclimatic conditions, such as low annual precipitation to allow local saturation of groundwater, as well as reliable surface water inflows.
Dissolution depression wetlands occur in areas underlain by water soluble and porous carbonate rocks and are synonymous with typical karst landscape features such as swallow holes, disappearing streams and solution sinkholes. Wolfe described two types of dissolution wetlands of karst landscapes, karst pans and compound sinks, which differ in terms of their degree of connection to the groundwater drainage system.
Karst pans are very shallow with no direct connection to groundwater. They are formed due to subsidence as the underlying calcareous rocks are dissolved. In some cases, they may be lined with clay, effectively sealing the base of the wetland to groundwater losses Greear, In contrast, compound sinks are deeper and linked to the underground drainage network.
As a result, sources and patterns of water inputs to karst depressions are highly varied. O'Driscoll and Parizek found that sinkhole wetlands in Pennsylvania, USA were fed primarily by direct precipitation as well as a perched groundwater table. As a result, dissolution depression wetlands may be seasonal e. For simplicity, we do not further subdivide dissolution depression wetlands. Dissolution features and the resultant ephemeral wetlands similar to those described above have been described in the Namib by Marker While the classification is based primarily on research from southern African wetlands, it is clear from the large and growing literature on wetlands in drylands across the globe see further examples from Africa, Australia and southwest USA in Lisenby et al.
The classification is physics-based and determined by processes of sediment deposition and erosion which apply globally, and it is therefore anticipated that the classification approach presented may be more widely applied and expanded than currently detailed. Wetland types that have not previously been defined with reference to a clear process-based mode of formation have been excluded e.
The advantage of this classification system is that all modes of dryland wetland formation identified in the literature are included.
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As research advances, additional wetland types may be added in accordance with the discriminatory framework developed. The implications of the genetic classification system for understanding spatial and temporal variability in form and process of wetlands in drylands are discussed in the sections that follow. Dryland wetlands are highly dynamic ecosystems, developing under extreme climatic conditions that are often characterised by alternating phases of floods and droughts.
This, in combination with their frequent occurrence on drainage lines, results in wetlands potentially shifting between alternate stable states e. While some wetlands may be quite resilient to environmental change Tooth, , geomorphic processes in others result in alternation from one state to another.
When classifying wetlands and developing catchment management plans, it is useful to be aware of the different forms that a wetland may take over time, and to consider the temporal scale at which such change takes place. Over short time scales seasons to years , if we consider a non-perennial river that flows seasonally or intermittently, during periods of no flow the river becomes akin to a discontinuous valley-bottom wetland with a series of disconnected pools and reed beds.
However, during flood events, the entire floodplain may become a macro channel for flow. There are thus two alternate transient states for this system depending on flow conditions. On a geomorphological time scale centuries to millennia , two prevailing geomorphic misconceptions Schumm, are frequently applied to dryland wetlands, potentially because they are often viewed exclusively as ecosystems rather than also as geomorphic systems.
For many ecologists, wetlands represent a stable habitat that supports obligate species that are adapted to that environment. The assumption is that the particular biophysical conditions that support those species have existed for a very long time.