The hydrological characterisation and water budget of a South African rehabilitated headwater wetland system

Abstract

This paper presents a synopsis of the findings of a valley bottom wetland monitoring study in which dominant hydrological processes maintaining the system are quantitatively defined. The Craigieburn-Manalana is a wetland subjected to technical rehabilitation, at the headwaters of the Sand River in the lowveld savanna region of South Africa.
Findings include the identification of a rapid water delivery mechanism from the surrounding hillslopes to the wetland following a threshold-exceeding precipitation event, when hillslope-toe soil matric potential is close to 0, leading to a raising of the wetland water table by >0.7 m within 3 h. A summary of quantified fluxes and associated water budget of the wetland and its contributing catchment is developed. It is revealed that this wetland does not necessarily conform to the typical assumptions that wetlands augment low flows. Surface layer scintillometry shows actual wetland evapotranspiration to dominate the water budget during the dry season (2.3–3.5 mm/d) compared to its contributing catchment (0.9–2.2 mm/d), whilst stream discharge had ceased. Hydrograph separation, based on stable isotopes (18O), revealed that the wetland does not attenuate peak flows during the summer rains when the wetlands soil moisture deficit is close to 0, since more than 66% of stream discharge comprised event water. These results are discussed within the context of current hydrological understanding of southern African headwater wetlands, such as dambos.

Keywords: hillslope processes, hydro-geomorphology, water budget, dambos, rehabilitation, wetlands