A stochastic, daily time-step model for conjunctive water use at local authority level

South Africa has a broadly developed water resource infrastructure, based mainly on surface water, localised groundwater, and limited desalination. The Department of Water and Sanitation projects that increasing requirements will exceed the capacity of the country's bulk water supply systems by 2025. To mitigate water scarcity at a local authority level, more conjunctive water use solutions need to be investigated. A Microsoft Excel 2016 model, estimating supply capacity on a daily basis, was developed for local authorities to better manage their water supply systems. The model utilises synthetically generated stochastic streamflow sequences, based on historical data. Monthly streamflow is disaggregated into daily streamflow and a rainfall-runoff relationship is established to produce synthetic rainfall sequences. Groundwater is modelled using an approach similar to the Aquifer Firm Yield Model. Recharge percentages (rainfall data) are obtained from the Groundwater Resource Assessment Phase 2 project. The Cooper-Jacob model evaluates borehole drawdowns and their effect on surrounding boreholes. Desalination and water reclamation are modelled as one source. The model evaluates and compares the historical, as well as stochastic available yield of a conjunctive use water supply system, for different operational scenarios. The water supply system of Stellenbosch was used as a case study to illustrate the model's functionality.