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

Rp Parsons,Pd Vermeulen

doi:10.4314/wsa.v43i1.07

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.

Highlights

Parsons (2009) disagreed with Roets (2008) and Roets et al (2008a, 2008b) by asking whether Groenvlei was really fed by groundwater discharged from the Table Mountain Group (TMG) Aquifer
Analysis of groundwater level data showed that Groenvlei is not endorheic in character, with groundwater discharging into the lake along its northern shore and water discharging into the subsurface along the southern shore (Fig. 2). This interpretation was supported by salinity and isotope data (Parsons, 2014) and highlights the importance of considering an integrated hydrological cycle, as has been embedded in the National Water Act (Act No 36 of 1998)
It points to the hidden hydrology of systems often not being appreciated

Summary

Introduction

Parsons (2009) disagreed with Roets (2008) and Roets et al (2008a, 2008b) by asking whether Groenvlei was really fed by groundwater discharged from the Table Mountain Group (TMG) Aquifer. He found no credible scientific evidence to support Roets’ hypothesis. Groenvlei is located directly east of the village of Sedgefield on the southern Cape coast of South Africa (Fig. 1) It is one of a series of five brackish coastal lakes in the area, but is the only one disconnected from the sea. Rain falls throughout the year, with the area experiencing a mean annual rainfall of 653 mm

Methods

Results

Discussion

Conclusion