Water supply systems failure and reservoir volumes interaction for the definition of drought risk

Identifying the onset, the extent and the magnitude of a drought is a delicate task. Drought by itself is not a disaster. Whether it becomes a disaster depends on its impact on people, economy, environment and specifically on the water supply systems. In water supply systems there is great variability on the natural water resources and also on the demands. This great variability is faced defining threshold values, expressed in probabilistic terms, that measure the hydrologic state of the system. The thresholds are defined considering the probability to satisfy a given fraction of demand in a certain time horizon. A simplified model of the water resources system is built to evaluate the interaction between the volumes stored in the reservoirs and the reliability to water supply systems in order to define the threshold values and the management rules. The simulations, based on a decision support system, are applied to the Firenze urban area in Tuscany (Italy). The critical situations are assessed month by month in order to evaluate optimal management rules during the year and avoid conditions of total water shortage.