Appropriate positioning of modelling as a decision support tool for surface water resources planning in South Africa

Abstract

The availability of adequate information is one of the basic requirements of sound water resources development. Simple water resource development options that required less detailed studies have already been developed, such that development proposals today require more detailed and comprehensive studies. Among other factors, these studies generate information on the hydrological risk of implementing water resources projects. The modelling tools used to generate water resources information are usually complicated by the many variables involved, which are inter-linked and usually unpredictable. The National Water Act (Republic of South Africa, 1998) emphasises the need for integrated water resources management, social equity, and ecological sustainability, which have added new dimensions to water resources planning. Water catchment simulation models that account for all the dimensions of water resources planning and bring more information than ever before to the decision-maker have become the preferred tools. Whilst earlier water resources planning tools are still in common use, this study found that these earlier tools lagged behind developments in important aspects such as national legislation, water stakeholders’ working environment, and rapid changes in computer software and hardware. The appropriateness of water resources modelling tools in South Africa was investigated in the light of a changing water environment as well as the need to address specific factors that are unique to South Africa. The water resources factors investigated included hydro-climatic, water institutional frameworks and stakeholder needs, available expertise and technological aspects of the available water management and planning tools. On the basis of the outcome of the investigation of South Africa’s unique water environment, recommendations and guidance were developed with the aim of developing a preferred local water resources modelling approach. This study investigated and recommended the use of water resources system models which are based on up to date modelling and Information Technology (IT) developments, such as HYDRO25, for multi-criteria planning of integrated water resources. In this study, the development of object oriented programming (OOP) models with visual interfaces that fit in the popular Windows operating environment was distinguished as a key aspect of water resources modelling. This modelling route was selected because it generates tools that are more user-friendly, have visual clues that relate closely with the physical system, including easy GIS integration, can handle the higher computer memory volume demands of longer time series data, and could handle a greater number of parameters as well as the increasingly more complex management scenarios. In the OOP approach, modelling tools are easily integrated with the input processing and output analysis objects that are developed separately before integration into the main model framework. All the separate software objects can easily be utilised in other models when the need arises. The HYDRO25 model uses modular objects and a visual-based programming language that easily accommodates integration with other software objects based on the component object model system. This has made further upgrading and redevelopment of the model easy to handle. In this study, the HYDRO25 model was developed and used in the Doring River catchment as a case study which was aimed at providing first-hand information about model development and application in South Africa. In the HYDRO25 model, computer code was used systematically to handle the catchment hydrology, geographical information, climatic factors, water use, catchment development proposals, the requirement of water legislation, and other factors to provide information that is useful for decision-making. In the Doring River case study, proposed irrigation developments in the Koue Bokkeveld and Aspoort area of the Western Cape were assessed using the HYDRO25 model to determine the most viable development options from a hydrological perspective. The study showed that the full irrigation potential of the catchment cannot be utilised with the available surface water resources in the catchment. The model simulation results showed that a maximum of 700 hectares can be irrigated in the Koue Bokkeveld area without creating additional water storage. Analysis of the Aspoort irrigation scheme showed that the irrigation area should be limited to 1000 hectares, with the proposed 178 million m3 Aspoort Dam being developed to support irrigation water demand and, to a small extent, to contribute to other water uses in the catchment, such as ecological flows and domestic uses.