Using stable isotopes to understand relative contributions of evaporation and evapotranspiration on groundwater recharge in the Verlorenvlei catchment, west coast, South Africa

Mafilika, Siyabulela Ben (2021-12)

Thesis (MSc)--Stellenbosch University, 2021.


ENGLISH ABSTRACT: In Southern Africa, especially in arid and semi-arid regions, there has been an increase in dependency on groundwater resources for both domestic and agricultural supplies. This has been caused by variations in regional precipitation patterns, climate change, increased abstraction rates, and increased population. These impacts have imposed significant pressure on available water resources and have also impacted recharge rates; therefore, it is very vital that proper management strategies are implemented to protect our groundwater resources. This has driven a variety of methods to be applied in hydrological investigations and assessments in support of sustainable development and management of groundwater resources. The stable isotope technique has been an area of significant scientific advancement, and has been applied successfully in semi-arid regions of South Africa, and where there is poor physical data coverage. This method is well-established, straightforward, and has been widely used in support of water resource management. The Verlorenvlei is a semi-arid region located in the west coast of South Africa, and has been faced with challenges of implementing water resource management due to its lack of information on temporal and spatial distribution of groundwater recharge across the catchment. This project aims to outline how the stable isotope technique can be used to provide an in-depth understanding of the hydrological system, how the processes of evaporation and evapotranspiration influence rainfall and groundwater recharge. To do this, groundwater samples, river water, springwater, and rainfall samples were collected across 10 locations (farms) within the catchment for a period of a year, and these samples were taken for stable isotope analysis. Based on the observed trends from the analysis of d-excess, δ¹⁸O, δ²H, and ¹⁷O-excess values, the rainfall across the catchment, rainfall was affected by the “seasonal effect” and “elevation differences”, resulting in spatial and temporal variation in δ¹⁸O and δ²H values. Furthermore, the rainfall was not heavily influenced by evaporation prior to its formation, but its source may have resulted from evapotranspiration of the surrounding natural vegetation, and this has been supported by high ¹⁷O-excess values. The groundwater isotopic composition had similar composition to that of the springwater and rainfall, suggesting that the rainfall could be their source. Groundwater has been recharged by local precipitation, with minimal evaporation prior to infiltration. There was no evidence of evapotranspiration influences on the isotopic composition of fraction of groundwater that has been recharged. Furthermore, this study has not been able to explain the evapotranspiration process the way it hoped it would, and this project has suggested that the method used should be used in conjunction with other methods to understand complex process like evapotranspiration.

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