The hidden hydrology of Groenvlei, a lacustrine wetland on the southern Cape coast of South Africa

Abstract

Following Parsons' (2009) challenge of the hypothesis that Groenvlei is sustained by discharge from the underlying Table Mountain Group Aquifer, research was undertaken to better understand the hydrology of Groenvlei. Using a daily water balance model coupled to Darcian flow calculations it was possible to show that direct rainfall and evapotranspiration losses were the key hydrological drivers of the system, with groundwater playing an important but secondary role. It was found that Groenvlei is not endorheic and that S pan measurements are best upscaled to lake evaporation using coefficients offered by Midgley et al. (1994). The reed collar plays an important role in the functioning of the lake, but is dormant during winter. The outcome of the research demonstrated that sound conceptualisation using all available data, information and knowledge remains a critical aspect of any hydrological modelling. The improved understanding of the hydrology of Groenvlei is expected to facilitate a better understanding of the nine Ramsar wetlands located on similar, but more complicated, hydrogeological settings elsewhere on the South African coastline.

Using the water balance approach to understand pool dynamics along non-perennial rivers in the semi-arid areas of South Africa

“…Xiao et al, 2018). For instance, Parsons and Vermeulen (2017) found that ~16.9 % and 83.1 % of the water lost by the Groenvlei Lake were due to groundwater outflows and evaporation, respectively. In addition, the water balance method can be used to predict the responses of pools to changes in inputs or outflows.…”