ABSTRACT
Models are simplified representations of real-world processes, and model uncertainties can arise from oversimplification or from the failure to capture important characteristics of the process under investigation. The problem occurs when a model is used for a situation that lies outside the set of situations used in the process of model calibration and verification. Determining the objectives involves identifying the performance criteria of concern, deciding how these will be measured, and determining appropriate priorities and trade-offs between the criteria. The ways society manages risks appear to be dominated by considerations of perceived and subjective risks, while it is objective risks that kill people, damage the environment and create property loss. Probability is a concept widely accepted and practised in water resources systems management. Water resources systems usually include conveyance facilities such as canals, pipes and pumps, treatment facilities such as sedimentation tanks and filters, and storage facilities such as reservoirs and tanks.
