Abstract. Hydro-meteorological risks due to natural hazards such as severe floods, storm surges, landslides and droughts are causing impacts on different sectors of society. Such risks are expected to become worse given projected changes in climate, degradation of ecosystems, population growth and urbanisation. In this respect, nature-based solutions (NBSs) have emerged as effective means to respond to such challenges. A NBS is a term used for innovative solutions that are based on natural processes and ecosystems to solve different types of societal and environmental challenges. The present paper provides a critical review of the literature concerning NBSs for hydro-meteorological risk reduction and identifies current knowledge gaps and future research prospects. There has been a considerable growth of scientific publications on this topic, with a more significant rise taking place from 2007 onwards. Hence, the review process presented in this paper starts by sourcing 1608 articles from Scopus and 1431 articles from the Web of Science. The full analysis was performed on 146 articles. The analysis confirmed that numerous advancements in the area of NBSs have been achieved to date. These solutions have already proven to be valuable in providing sustainable, cost-effective, multi-purpose and flexible means for hydro-meteorological risk reduction. However, there are still many areas where further research and demonstration are needed in order to promote their upscaling and replication and to make them become mainstream solutions.

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In many areas of the world, particularly in arid regions or in areas experiencing population growth, there is increased competition over scarce water resources. This is likely to increase in the future due to continued population growth, urban expansion and the challenge of the impact of climate change on water resource availability. In this context, groundwater is likely to play a pivotal role in facing water scarcity. When different users share a common-pool resource, basic rules are usually established to manage access to the resource and ensure balance between demand and supply. Water authorities worldwide are increasingly paying added attention towards regulating the use of groundwater because of its strategic value, e.g. in times of drought or as a natural reserve. In the case of groundwater, although regulatory measures exist, they are often difficult to enforce. This paper explores the situation with a discussion of two aspects: first (and in line with this special issue on water ethics), an examination of the fundamental individual values that underpin behavior in relation to water use, and second, an investigation of the typologies of unauthorized water use, its main potential impacts, potential root causes and reflections on imperfect institutions and social norms.

Based on a study conducted for the Environment Agency for England and Wales, we discuss the contribution that new alternative and complementary environmental policy instruments might make to the realization of the objectives of the EU's Water Framework Directive. Following a survey that identifi ed nearly 100 examples where alternative and complementary instruments are currently being applied in the UK, we categorize such instruments as information-based approaches, private and voluntary regulation or support and capacity building measures. Examples are given of each category of instrument before further fi ndings on the preconditions for the successful application of such measures are presented. These preconditions relate to levels of commitment from key groups, levels of stakeholder understanding, the role of the lead actor, the importance of timing, the need to deliver a clear message, the importance of enforcement, the role of the media and the importance of social capital in key networks. We conclude that alternative and complementary measures have signifi cant potential to contribute to the realization of the Water Framework Directive's objectives, but only where these preconditions are met.