Optimal operation of multi-purpose reservoir systems during drought

The management for a multi-purpose water system comprising a set of reservoirs isgenerally a complex task, in particular during pervasive drought. Such management istypically further complicated by an increasing pressure on the system to provide forincreasing targets for water supply. Operation planning during (extreme) droughtconditions should form an integral part of the overall operation and must consider theimpacts on consumers, assess potential economic losses associated with an inability tomeet normal demands, and make trade-offs between competing users. A differentiationshould be made between a long term operation under average (normal) conditions andoperation during (extreme) drought conditions which should take into account thespecial conditions and information available at such time (real-time operation)Water rationing is a common technique for managing water supplies during (severe)drought, but forms a relatively little studied subject. The present study focuses on thecreation of a drought operation strategy for real-time management: a water rationingmechanism is derived for reducing demand in the early stage of impending drought toprevent drastic short-term damages associated with water shortage during the period ofdrought. The drought operation rule, which is to minimize shortage damages, is derivedconjunctively with the longer term operation rules that maximize the system benefits inthe longer term.The complexities of a multi-purpose, multiple reservoir system require mathematicalmodelling and a systematic (optimisation) procedure to determine the best operationamong a very large number of possibilities. In this study an optimisation model isdeveloped involving an implicit stochastic optimisation using 61 year of monthlyhistorical stream flows. The model is applied to the Citarum river basin, West Java,Indonesia. Currently most water is used for irrigation, a strong increase in urban waterdemand is foreseen; hydroelectric energy forms an important source of revenue. Due torapid development in the basin and neighbouring Jakarta , increasing water shortage isexpected in the near future; it is necessary to manage the supply capacity of this systemto meet demands in the best possible way; potential drought conditions, and the waythey can be handled play an important role in this .Application of the drought strategy to the case water system indicates a substantialincrease (5 %) in overall system benefits. This comes from a reduction of shortagedamage and the increase in firm water supply - and energy targets. A sensitivityanalysis indicates that application of the drought strategy becomes more effective (ormore needed) as the stress on water use in the system increases.The modelling approach uses a formulation of the model in an Excel spreadsheet, whichis then translated into an optimisation model using the extended Solver. The advantageof this modelling approach is that the model can be conveniently formulated inMicrosoft Excel providing a detailed representation of reservoir system using allavailable functions and is then automatically translated into a format suitable formathematical optimisation by the Solver