Assessment of future water supply and demand in a river basin using water evaluation and planning (weap) system: Case study in South Africa

The Caledon catchment is one of 19 catchments in South Africa and one of three catchments in Lesotho. It covers part of South East of Free State in South Africa and North of Lesotho. Evaluation of the water resources of the catchment is important in order to plan for future water demand and make an informed decision. The objective of this study was to apply Water Evaluation and Planning Model (WEAP21) as a Decision Support System (DSS) tool for the allocation and planning of water resources in Caledon catchment.

The model was applied using three scenarios, with a current account (2014) and reference period (2015 – 2050), in order to predict possible impacts of the different scenarios on the water balance and its allocation among the regions. Scenario 1: increase in population growth rates; Scenario 2: irrigation activities in Lesotho; and Scenario 3: implementation of environmental flow requirement (EFR) on Caledon River at C6 site. The first two scenarios are consumptive scenarios whereas the third scenario is non-consumptive scenario. Scenario analysis answers “what if” questions for the future. In this context, Scenario 1 deals with the impact of increase in population growth on the water balance after 2020 by analyzing the unmet demands starting from the reference period. Scenario 2 deals with analyzing the unmet demands if the irrigation activities in Lesotho are considered after 2020. Scenario 3 considers the impacts of implementation of environmental flow requirement (EFR) at C6 site, which is situated downstream of all demand sites of the catchment, on upstream demands. Projected water demands and unmet demands were evaluated for four water use sectors, namely domestic, industry, irrigation and livestock. The catchment comprised of 46 demand sites which were categorised into four use sectors: 20 domestic demand sites, 11 irrigation sites, 10 livestock sites and 5 industrial sites in both rural and urban areas.

The model result showed that high population growth increases the water shortage to all water use sectors in the catchment. Under high population growth scenario, the unmet demand occurred from May to October whereas under reference, EFR and irrigation scenarios, the unmet demand occurred only from June to September. The annual unmet demand increased substantially after 2020 in high population growth scenario when the population growth rates were altered. The demand for irrigation sector is covered or no unmet demands were registered in all years. This is because of active irrigation activities that takes place from December to May when enough water is available with the rivers. The years 2025 and 2050 were chosen to evaluate the water balance situations in the middle and end of the referenced period under two water use scenarios (high population growth and irrigation activities). The result shows that the river flows meet the projected demand in 2025, however, most rivers including the main river (Caledon River) faces shortage in relation to the required demands.