WATER RESOURCES MANAGEMENT IN SCARCITY CONDITIONS: FDC VARIABILITY ANALYSIS IMPLICATION ON ENVIRONMENTAL STREAMFLOW REQUIREMENTS

The management of water resources, more and more limited and poor in quality, represents a present key issue in hydrology. The development of a community is highly related to the management of available water resources and there is a need, for this reason, to rationalize them, to plan their conscious use and to preserve their quality and related wildlife. Environmental streamflow requirements represent then a particular aspect of water resources management. Environmental flow has been defined in many different ways but, on a general perspective, it can be defined as the river discharge that allows the sustainable maintenance of a river environment and, therefore, should allow protection of the natural river ecosystem. The scientific environmental flow estimating methodologies, perhaps different from country to country, are mainly based on analysis of historical flow data or on investigation of relationships between a particular aspect of the habitat and river discharge. It is possible to distinguish between theoretical and experimental methodologies. Examples of theoretical methods are the so known hydrological methods that estimate environmental flow as a function of catchment properties, such as drainage area, mean monthly discharge, mean annual discharge and so on. Among hydrological methods, statistical methods are based on the FDC flow duration curve estimation. Environmental flow is then defined as the discharge for a particular duration; generally large duration values are considered, corresponding to the lower tail of the FDC. FDCs have a broad use in hydrological and engineering application, which has been sometimes criticized because their representation, and therefore interpretation, tightly depends on the algorithm used to compute it. It is indeed possible to compute the FDC for the whole period of observation but it is also possible to derive a mean annual FDC. If n is the number of recorded year, it is possible to consider n different FDCs, each corresponding to a particular year. It is rather intuitive to imagine that the n FDCs would have different shapes between them, due to the well-known climate variability from year to year. This variability, in extreme climatic conditions makes questionable the definition of an average FDC. The empirical FDCs uncertainty analysis inevitably produces an uncertainty in the definition of the environmental streamflow requirements, which estimation should embed the considered elements. An application will be presented for a river basin located in a particular region of Southern Italy, experiencing a typical Mediterranean climate.