ORIGINAL ARTICLE Multi-level interactions in a context of political decentralization and evolving water-policy goals: the case of Spain Lucia De Stefano 1,2 & Nuria Hernandez-Mora 3 Received: 19 July 2017 /Accepted: 7 March 2018 # Springer-Verlag GmbH Germany, part of Springer Nature 2018 Abstract Spain is a highly decentralized country where water governance is a multi-level institutional endeavor requiring effective intergovernmental coordination—in terms of objectives and actions. The paper revisits the evolution of vertical and horizontal intergovernmental interactions in Spain, with a special focus on four interregional river basins. We build on a historical analysis of the evolution of water governance institutions, a mapping of existing interactions over water, careful document analysis, and interviews with selected public officials that are at the interface between the political and the technical spheres. Intergovernmental interaction occurs through different mechanisms that are slowly evolving to adapt to new challenges posed by changing power dynamics and water policy goals. Since the start of political decentralization in 1978, key institutional reforms within and outside of the water sector have opened windows of opportunity for regions to seek new spheres of influence and power. Disputes over water allocation, environmental flows, inter-basin transfers, and even basin boundaries delineation emerge as an expression of a struggle over power distribution between the regions and the central government. The physical and institutional geography of water and diverging visions and priorities (over water and beyond) are among the factors that contribute to shape conflict and cooperation in intergovernmental relations over water. Keywords Subnational . Multilevel . Cooperation . Dispute . Spain . WFD Introduction Federal rivers are defined as Bmajor rivers within or shared by a federal political system^ (Garrick et al. 2013:12) and can be considered a special type of transboundary river, where the governments sharing waters are subnational levels with a strong level of autonomy instead of sovereign countries. While international and community-based levels of water governance have been extensively studied, literature about this meso-scale of water governance is still limited (Garrick and De Stefano 2016). Past evidence about relations over water between riparian countries in international rivers suggests that in general, posi- tive interactions outnumber negative ones (Wolf et al. 2003; De Stefano et al. 2010), whereas tensions in specific basins can reach high intensity and become a serious source of concern. Systematic studies focused on subnational intergovernmental conflicts over water are limited (for regional inventories, see, e.g., Bernauer et al. 2012; Eidem et al. 2012) but suggest that water can lead to heated intergovernmental disputes and political gridlock. Analyzing decentralized political systems, Moore (2017, p.15) observes that Bwhile scholars concerned with water resource management often direct their focus to the national level, […] subnational officials frequently determine the success or failure of national water resource policy reforms.^ This is especially true in federal political systems, where subnational governments usually have broad powers on key water-related policies. Intergovernmental interactions involve both vertical rela- tions between governments at the central and subnational Nuria Hernández-Mora is an Independent researcher, Madrid, Spain. Electronic supplementary material The online version of this article (https://doi.org/10.1007/s10113-018-1318-6) contains supplementary material, which is available to authorized users. * Lucia De Stefano luciads@geo.ucm.es Nuria Hernandez-Mora nuriahernandezmora@gmail.com 1 Facultad de Ciencias Geológicas, Universidad Complutense de Madrid, c/José Antonio Nováis, 12, 28040 Madrid, Spain 2 Water Observatory, Botín Foundation, Madrid, Spain 3 Fundación Nueva Cultura del Agua, Madrid, Spain Regional Environmental Change https://doi.org/10.1007/s10113-018-1318-6