Blue water scarcity in the Black Sea catchment: Identifying key actors in the water-ecosystem-energy-food nexus M. Fasel a, *, C. Bréthaut a , E. Rouholahnejad b , M.A. Lacayo-Emery a , A. Lehmann a a University of Geneva, Institute for Environmental Sciences, Bd. Carl-Vogt 66, CH – 1211, Geneva, Switzerland b Department of Environmental Systems Science, ETH Zurich, Universitaetstrasse 16, CH – 8092, Zurich, Switzerland A R T I C L E I N F O Article history: Received 18 February 2016 Received in revised form 2 August 2016 Accepted 10 September 2016 Available online xxx Keywords: Water Scarcity Nexus Ecosystems Energy Food A B S T R A C T Large-scale water scarcity indicators have been widely used to map and inform decision makers and the public about the use of river flows, a vital and limited renewable resource. However, spatiotemporal interrelations among users and administrative entities are still lacking in most large-scale studies. Water scarcity and interrelations are at the core of the water-ecosystem-energy-food nexus. In this paper, we balance water availability in the Black Sea catchment with requirements and consumptive use of key water users, i.e., municipalities, power plants, manufacturing, irrigation and livestock breeding, accounting for evaporation from major reservoirs as well as environmental flow requirements. We use graph theory to highlight interrelations between users and countries along the hydrological network. The results show that water scarcity occurs mainly in the summer due to higher demand for irrigation and reservoir evaporation in conjunction with relatively lower water resources, and in the fall-winter period due to lower water resources and the relatively high demand for preserving ecosystems and from sectors other than irrigation. Cooling power plants and the demands of urban areas cause scarcity in many isolated locations in the winter and, to a far greater spatial extent, in the summer with the demands for irrigation. Interrelations in water scarcity-prone areas are mainly between relatively small, intra-national rivers, for which the underlying national and regional governments act as key players in mitigating water scarcity within the catchment. However, many interrelations exist for larger rivers, highlighting the need for international cooperation that could be achieved through a water-ecosystem-energy-food nexus. ã 2016 Elsevier Ltd. All rights reserved. 1. Introduction Water scarcity is a growing concern in many parts of the world as it leads to conflicts, overexploitation of aquifers, increased pollution, alteration of ecosystems, and various negative effects on human health, food and goods production, and economic well- being (Jury and Vaux, 2007; Kummu et al., 2010). Water scarcity is both a natural and anthropogenic phenome- non (Jaeger et al., 2013). It can be caused not only by unfavorable climate changes and the failure of a society to adapt but also by excessive and increasing water demand vital to sustaining population growth, industry and agriculture. Moreover, water infrastructure can change the temporal pattern of streamflow, and human activities that consume water through evaporation or incorporation into products eventually deplete a significant amount of resources from the hydrological system (Hoekstra et al., 2012). There has been a growing recognition that ecosystems are a central and legally established water user, and in 1982, the World Charter for Nature imposed further reductions to water use in nation states committed to their protection. Consequently, competition for available resources is likely to increase (FAO, 2012). This raises the need to decrease water demand, improve the efficiency of water consumption, and lessen the impacts of consumptive use and streamflow regulations to ensure a balance between socioeconomic development and environmental protec- tion (Hamdy et al., 2003). At the Bonn 2011 Nexus Conference, the concept of a water nexus, which allows for causal links between users and their common resources in water management plans (Hoff, 2011), was * Corresponding author. E-mail address: marc.fasel@unige.ch (M. Fasel). http://dx.doi.org/10.1016/j.envsci.2016.09.004 1462-9011/ã 2016 Elsevier Ltd. All rights reserved. Environmental Science & Policy 66 (2016) 140–150 Contents lists available at ScienceDirect Environmental Science & Policy journal homepage: www.elsevier.com/locate/env sci