Probabilistic definition of drought risk indicators for water supply systems in central Tuscany

Drought indices, particularly the meteorological ones, can describe the incoming and the persistency of droughts, especially in natural systems. However they have to be used cautiously when applied to water supply systems. They show little correlation with water shortage situations, since water storage play an important role in water resources management. For that reason a more complex system of indicators is required in order to identify situations when there is risk of water shortages. In water supply systems there is great variability on the natural water resources that flow in rivers, aquifers or into the reservoirs and also on the demands. These quantities can only be defined probabilistically. This great variability is faced defining some threshold values, expressed in probabilistic terms, which measure the hydrologic state of the system. They can identify specific actions in an operational context in different levels of severity, like the normal, pre-alert, alert and emergency scenarios. The threshold values are defined considering the probability to satisfy a given fraction of the demand in a certain time horizon, and are calibrated through discussion with water managers. A simplified model of every water resources system is built to evaluate the threshold values and the management rules. The probability distribution of demand shortages is estimated through Monte Carlo simulation, considering an ensemble of hydrological inflows for each storage level in the reservoirs. The threshold values are validated with a long term simulation that takes into account the characteristics of the evaluated system. The levels and volumes in the different reservoirs are simulated using 1970-2005 time series. 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. The methodology is applied to the urban area of Firenze in central Tuscany, in central Italy. The catchment of the investigated area has a surface of 1.230 km2. The considered demand centers are Firenze and Bagno a Ripoli that have, accordingly to the census ISTAT 2001, a total of 395.000 inhabitants.