Hydrological regime alteration and ecological flow evaluation in mountain rivers

In the Alpine region, the hydropower production has caused alterations in the hydrological regime of most rivers, due to the massive withdrawal of water and to the construction of dams and reservoirs, which modify the natural flow duration curve of the river. These alterations may result in a decrease in hydro-morphological and ecological quality of the river and limitations are considered, usually expressed as an ecological flow that has to be guaranteed. Many types of approaches are suggested in literature to define this flow, taking into account hydrological, physical, chemical and biological factors. Methods based only on hydraulic/hydrological parameters are often preferred, due to simplicity and robustness. More, with methods based only on hydrological parameters is simpler to take into account changes in river regime due to anthropic activities. However, some ecological issues are also important for river quality, such as the lentic-lotic characteristics of flow, but they are difficult to be expressed in crisp numerical values. In the last years more complex methods have been proposed as an alternative to hydrological models. One of them is the method CARAVAGGIO (Core Assessment of River hAbitat VAlue and hydro-morpholoGIcal cOndition). Although this model is able to take into account a great number of different aspects, its application is not always possible due to the difficulties in entering the river in some sites. In this paper a procedure able to merge the ease of use of hydrological methods and the multi-factor approach of CARAVAGGIO is proposed. Ecological flow is estimated from basic hydrological information, using a simple set of coefficients to take into account other quality issues, particularly the lentic-lotic features of flow regime. Application to a case study in the Province of Verbano-Cusio-Ossola, in the north-western part of Piedmont Region in Italy, is presented.