Predicting wetland occurrence, main hydrogeomorphic type and vulnerability in the predominantly arid to semi-arid interior of the Western Cape, South Africa

As with drylands globally, there has been limited effort to map and characterize wetlands in the Western Cape interior of South Africa. Thus, the study assessed how probable wetland occurrence and type in the predominantly arid to semi-arid interior of the Western Cape relate to key biophysical drivers and physiographic zones, and, through modelling, to predict spatially their likelihood of occurrence. The study further aimed to qualitatively assess the vulnerabilities of the identified wetland types to human impacts. Field-verified test areas were selected to represent the aridity gradient, rainfall seasonality, hydrogeomorphic (HGM) types and physiographic zones encompassed in the study area. The Little Karoo and Great Karoo plains physiographic zones, which were predominantly arid, had: (1) a low (0.7%) proportional area of wetland; (2) an almost complete absence of seepage slope wetlands; and (3) much of the wetland associated with valley bottoms confined within a channel. The three mountainous physiographic zones, which were semi-arid to sub-humid, had: (1) a much higher (2.6 to 5.2%) proportional area of wetland; and (2) wetlands being predominantly hillslope seepages. A spatial probability surface of wetland occurrence was generated based on the statistical relationship of verified wetland presence and absence data points with a range of catchment-scale predictor variables, including topographic metrics and hydrological/climatic variables. This layer was combined with raster images of most likely HGM type within the landscape to provide a final product of wetland occurrence, attributed by HGM type. Vulnerabilities of the wetlands to human impacts were identified based on key attributes of the different wetland types, and recommendations were provided for assisting with future mapping and HGM type identification for the Western Cape interior and for predominantly arid to semi-arid areas generally.