Application of HEC-HMS Model for Evaluation of Rainwater Harvesting Potential in a Semi-arid City

With many countries facing water scarcity and the demand for water ever-increasing, more people are turning to rainwater harvesting (RWH) as a feasible way of supplementing supply. However, the success of RWH systems depends on several factors, including rainfall, catchment characteristics and socio-economic factors. Hence it is essential to carry out location-specific studies to evaluate the potential of RWH in a particular area. This study sought to assess the influence of seasonal rainfall and climate change, and socio-economic constraints on RWH in the City of Johannesburg. A GIS-based MCDA was run to estimate RWH suitable sites as influenced by various socio-economic factors. The RWH suitability map shows that at least 50% of the city is suitable for RWH. These, together with climate data was embedded into a rainfall-runoff model developed in HEC-HMS to simulate runoff volumes up to the year 2050. The Deficit and Constant loss method was employed to calculate infiltration losses. After calibration, the model results were within acceptable limits (R² = 0.68 and 0.79). The rainfall-runoff simulation results show that high runoff volumes are received during the wet season, which is more than double the non-potable water demand. These results indicate that, with ample storage, RWH systems are feasible despite the rainfall seasonality. Therefore, it is recommended to invest in diverse and multipurpose storage to improve RWH reliability.