Abstract
Abstract. The management and allocation of water from multi-reservoir systems is complex and thus requires dynamic modelling systems to achieve optimality. A multi-reservoir system in the Southern Lowveld of Zimbabwe is used for irrigation of sugarcane estates that produce sugar for both local and export consumption. The system is burdened with water allocation problems, made worse by decommissioning of dams. Thus the aim of this research was to develop an operating policy model for the Lowveld multi-reservoir system.The Mann Kendall Trend and Wilcoxon Signed-Rank tests were used to assess the variability of historic monthly rainfall and dam inflows for the period 1899–2015. The WEAP model was set up to evaluate the water allocation system of the catchment and come-up with a reference scenario for the 2015/2016 hydrologic year. Stochastic Dynamic Programming approach was used for optimisation of the multi-reservoirs releases.Results showed no significant trend in the rainfall but a significantly decreasing trend in inflows (p < 0.05). The water allocation model (WEAP) showed significant deficits ( ∼ 40 %) in irrigation water allocation in the reference scenario. The optimal rule curves for all the twelve months for each reservoir were obtained and considered to be a proper guideline for solving multi- reservoir management problems within the catchment. The rule curves are effective tools in guiding decision makers in the release of water without emptying the reservoirs but at the same time satisfying the demands based on the inflow, initial storage and end of month storage.
Highlights
Effective water resources management requires optimal water resources allocation systems that maximizes set objectives such as crop yield (Reddy and Kumar, 2007); hydroelectric power generation (Mujumdar and Nirmala, 2007) and other uses such as flood control (Sharma et al, 2016)
Optimal water allocation is the consideration of all the water users and allocation of water equitably
The operation policies derived from the Stochastic Dynamic Programming (SDP) model for each month and for each reservoir are more stable and adaptable for release decisions due to fluctuations in the inflows into the reservoir
Summary
Effective water resources management requires optimal water resources allocation systems that maximizes set objectives such as crop yield (Reddy and Kumar, 2007); hydroelectric power generation (Mujumdar and Nirmala, 2007) and other uses such as flood control (Sharma et al, 2016). Man-made reservoirs are among the most efficient infrastructures to manage water supply as well as minimise the enormous impacts of droughts These infrastructures, coupled with the need for improved operational skills of watershed-scale reservoirs and water distribution networks necessitates the development and application of water allocation models (Namchaiswadwong, 2004; Sert et al, 2007; Juízo and Lidén, 2010). Simulation-based allocation models are used to meet this need and they simulate water allocation with pre-determined water use priorities and reservoir operation rules These models have poor flexibility if anthropogenic and hydro-climatic factors are taken into consideration as they operate in a “what-if-” scenario (Jha and Gupta, 2003). Considering these limitations, integrated optimal water resources allocation modelling should consider all the water users and equitably allocate water amongst users, whilst taking into consideration hydro-climatic variability.
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