Environmental Flow Assessment Based on Different Metrics of Hydrological Alteration

The concepts of hydrological alteration and the related natural flow paradigm conceive variable environmental flows that preserve as much as possible the natural variability of flows, with a particular focus on a suite of specific characteristics, the so-called indicators of hydrological alteration (IHA). In the paper we propose a simple simulation approach for a preliminary desk-top assessment environmental flows, whose principle is to maximize the possible utilization of water while complying with the alteration targets according to a global alteration metric. We investigate the use of three different alteration metrics, with the aim of measuring the sensitivity of environmental flow assessments respect to the index and the corresponding low and moderate alteration target thresholds. An application of the methodology to a case study area in Sicily, comprising several rivers sections, is carried out. Results show that a significant sensitivity of the optimal environmental flows to the alteration metric, both in the pattern and in the amount. While some metrics privilege environmental flow patterns that follow the natural variability of IHA parameters, other yield to optimal environmental flows that follow the long-term means of the IHA parameters. Results also show that in general the attainment of the low alteration target is quite demanding, since at least the 30 % of natural flows should be addressed to environmental flows, while for a moderate alteration hydrological status this percentage reduces to 15 %.