Valuing hydrological alteration in multi-objective water resources management Simone Bizzi ⇑ , Francesca Pianosi, Rodolfo Soncini-Sessa Dipartimento di Elettronica e Informazione, Politecnico di Milano, Italy article info Article history: Received 11 July 2012 Received in revised form 14 September 2012 Accepted 17 September 2012 Available online 5 October 2012 This manuscript was handled by Geoff Syme, Editor-in-Chief, with the assistance of Paul Jeffrey, Associate Editor Keywords: Ecohydrology Hydrological alteration Reservoir operation Multi-objective optimization summary The management of water through the impoundment of rivers by dams and reservoirs is necessary to support key human activities such as hydropower production, agriculture and flood risk mitigation. Advances in multi-objective optimization techniques and ever growing computing power make it possi- ble to design reservoir operating policies that represent Pareto-optimal tradeoffs between multiple inter- ests. On the one hand, such optimization methods can enhance performances of commonly targeted objectives (such as hydropower production or water supply), on the other hand they risk strongly penalizing all the interests not directly (i.e. mathematically) included in the optimization algorithm. The alteration of the downstream hydrological regime is a well established cause of ecological degrada- tion and its evaluation and rehabilitation is commonly required by recent legislation (as the Water Framework Directive in Europe). However, it is rarely embedded in reservoir optimization routines and, even when explicitly considered, the criteria adopted for its evaluation are doubted and not com- monly trusted, undermining the possibility of real implementation of environmentally friendly policies. The main challenges in defining and assessing hydrological alterations are: how to define a reference state (referencing); how to define criteria upon which to build mathematical indicators of alteration (measuring); and finally how to aggregate the indicators in a single evaluation index (valuing) that can serve as objective function in the optimization problem. This paper aims to address these issues by: (i) discussing the benefits and constrains of different approaches to referencing, measuring and valuing hydrological alteration; (ii) testing two alternative indices of hydrological alteration, one based on the established framework of Indicators of Hydrological Alteration (Richter et al., 1996), and one satisfying the mathematical properties required by widely used optimization methods based on dynamic program- ming; (iii) demonstrating and discussing these indices by application River Ticino, in Italy; (iv) providing a framework to effectively include hydrological alteration within reservoir operation optimization. Ó 2012 Elsevier B.V. All rights reserved. 1. Introduction The regulation of rivers by dams and reservoirs is necessary to support key human activities including hydropower production, agricultural production, industrial and civil uses, and flood risk mitigation. However hydrological alteration induced by reservoirs and its effects on freshwater ecosystems are significant and well documented in the literature (McCartney, 2009). Various legisla- tion frameworks regarding catchment and water resources man- agement, e.g. the Water Framework Directive (WFD) in EU (EC, 2000), now require hydrological alteration appraisals, and in the future rehabilitation or compensation measures are likely to be re- quired. However, so far, the requirements imposed by legislators have been limited to the stipulation of a minimum environmental flow (MEF), often constant over the year. Although there is scien- tific evidence showing the inadequacy of MEFs in rehabilitating ecological processes (Jager and Rose, 2003), this type of legal requirement is well established because MEFs define a simple and measurable metric to evaluate environmental objectives and to compare them with other potentially conflicting ones. It is un- clear to what extent environmental objectives are really addressed using this approach (Loomis, 1998). The complexity of the freshwater ecosystems, where ecological and geomorphological processes interact at different spatial (unit, reach, catchment) and temporal (days, years, decades) scales, makes the evaluation of ecological impacts caused by a single pres- sure, such as hydrological alteration, a challenging task (Vaughan and Ormerod, 2010; Newson and Large, 2006). Several case study applications confirmed the existence of a causal relationship be- tween hydrological alteration and health of various biotic commu- nities (Monk et al., 2007, 2006; Shafroth et al., 2010). However these causal links are case-specific and their generalization first re- quires an advance in our understanding of freshwater ecosystems and is the focus of recent research interest (Palmer, 2009; Vaughan and Ormerod, 2010). A common approach to evaluating hydrological alteration is to define a reference period not affected by the pressure analyzed 0022-1694/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jhydrol.2012.09.033 ⇑ Corresponding author. Tel.: +39 02 2399 9616; fax: +39 02 2399 9611. E-mail address: bizzi@elet.polimi.it (S. Bizzi). Journal of Hydrology 472–473 (2012) 277–286 Contents lists available at SciVerse ScienceDirect Journal of Hydrology journal homepage: www.elsevier.com/locate/jhydrol