Ecohydraulically based environmental flow assessment in an alpine watershed. Water reosurces integrated sensitivity evaluation

The paper describes a methodology to evaluate, on a integrated ecohydraulic basis, the environmental flows of Alpine creeks. The study area is delimited by the alpine watershed of Stién creek (Piave basin) located on the southern side of Belluno Dolomites, within the National Park of the Eastern Alps (Northern Italy, province ofBelluno, Veneto region); the higher reaches of the basin pertains to the park area and are affected by hydraulic works for the diversion of water for hydroelectric purposes. Ecohydraulic evaluation of environmental flow was based on continuous monitoring of river discharge by a double level sensor connected to data-logger, coupled with macrobenthos analysis on sampling stations of the creek located up-stream and downstream in respect to the abstraction point. From the stream hydrogram, relative to a full hydrological year (from December 2000 to November 2001), by correlation with effective rainfall (rainfall minus actual evapotranspiration) either average annual or relative to the monitoring year, environmental flow (EF) was determined according a drought discharge methodology (Q7,10 – Q355). This type of EF derivation is well suited for mountain creeks for which data series on discharges are lacking. The macrobenthic community exhibited an evident loss of functionality as a consequence of the water abstraction; an index of ecological fluvial quality (REQ index) has been defined, integrating several existing indices related to biomonitoring of fluvial ecosystems. The opportunity to quantify in an objective manner the biological effects, and to link these with discharge measures at the sampling stations, allowed an integrated EF evaluation, taking into account physical and biological components. The water released by the plant was not sufficient for the needs of the community; the obtained integrated EF, being equal to 120 L/s, is a little lower than the Q7,10 hydraulically based EF (139 L/s). Such an integrated sensibility evaluation of a river ecosystem in an Alpine watershed cannot take into account the sensibility of major springs recharge areas, particularly if they contribute for the most part to the stream base-flow.