The hidden hydrology of Groenvlei, a lacustrine wetland near Sedgefield

Groenvlei_blog

Saturday 15  August: By Dr Roger Parsons

Dr Roger Parsons highlighted the need to embrace the scientific method when undertaking hydrological research and the need for all evidence to support the conceptual understanding of the functioning of the wetland system. The absence of surface run-off prevented rainfall – run-off models being used to quantify the hydrology of the 356 ha wetland located east of Sedgefield in the Garden Route. Rather, a simple water balance approach was used as the component parts (rainfall, evaporation and lake level) are measured daily. Also, the groundwater flow into and out of the wetland could be computed.

Hydrogeological evidence confirmed that Groenvlei is not endorheic in character; while there is no scientific evidence to support the contention that the wetland is fed by the underlying Table Mountain Group aquifer. It was found that direct rainfall and evapotranspiration are the key hydrological drivers of the system, while groundwater plays an important but secondary role. Groundwater accounts for 28% of the inflow into the system , while 17% of the losses from the wetland occur along the southern shores. The reed collar around the lake plays an important role in the hydrology of the system – with the reeds being dormant in the winter months.

Roger concluded by noting that Groenvlei is located in a similar hydrogeological setting as nine of the 21 Ramsar-designated wetlands in South Africa, and that a better understanding of Groenvlei can only contribute to understanding the hydrology of these wetland systems.