Application of the SWAT hydrological model for water balance assessment in an intensive irrigated agricultural catchment in the Western Cape, South Africa

Changing climate and expanding irrigated agriculture exert significant pressure on the already limited water resources in many regions of the world. This study is located in a South African strategic water source area called the Koue Bokkeveld (KBV). The area is characterised by a strongly seasonal flow regime, where winter flash flood flows dominate and long dry summer periods are experienced. Numerous small reservoirs have been constructed in the catchment to capture the extreme winter flows. The region is a biodiversity hotspot with a number of endemic species and forms part of the strategic water source area in South Africa (10% of South Africa’s land surface produces 80% of the country’s runoff).

During the long dry summer period, conflicts over water use and allocation among farmers in the study catchment emerge. When this happens, river flows are abstracted without considering ecological reserve (environmental flow) requirements.

This study quantifies the availability of water resources in the KBV catchment to contribute to creating an equitable and more sustainable water management plan that addresses water-related conflicts among farmers and ensures that ecological reserve requirements are met. SWAT+ has been set up for this study as it allows the representation of spatially distributed reservoirs and river diversion points within a catchment. This study also simulates future flow scenarios in the context of changing climate.

The outcomes of this project will be used to support decision-making regarding sustainable water management. This includes being used as input into an Agent-Based Model (ABM) that will be utilised to explore various management scenarios. Furthermore, the modelled current and future water availability information will assist catchment managers in making informed decisions about how water management could be adapted to changes in water availability.

The project is half way through, with SWAT setup complete. A meeting with the farmers in early 2022 will assist with further calibration of the model, in terms of reservoir and irrigation dynamics. The possibility of additional reservoirs will be explored to counter the run of river summer abstraction occurring that is currently negatively impacting the biodiversity.