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This paper is the outcome of a community initiative to identify major unsolved scientific problems in hydrology motivated by a need for stronger harmonisation of research efforts. The procedure involved a public consultation through online media, followed by two workshops through which a large number of potential science questions were collated, prioritised, and synthesised. In spite of the diversity of the participants (230 scientists in total), the process revealed much about community priorities and the state of our science: a preference for continuity in research questions rather than radical departures or redirections from past and current work. Questions remain focused on the process-based understanding of hydrological variability and causality at all space and time scales. Increased attention to environmental change drives a new emphasis on understanding how change propagates across interfaces within the hydrological system and across disciplinary boundaries. In particular, the expansion of the human footprint raises a new set of questions related to human interactions with nature and water cycle feedbacks in the context of complex water management problems. We hope that this reflection and synthesis of the 23 unsolved problems in hydrology will help guide research efforts for some years to come. ARTICLE HISTORY

The Sustainable Development Goals (SDGs) of the United Nations Agenda 2030 represent an ambitious blueprint to reduce inequalities globally and achieve a sustainable future for all mankind. Meeting the SDGs for water requires an integrated approach to managing and allocating water resources, by involving all actors and stakeholders, and considering how water resources link different sectors of society. To date, water management practice is dominated by technocratic, scenario‐based approaches that may work well in the short term but can result in unintended consequences in the long term due to limited accounting of dynamic feedbacks between the natural, technical, and social dimensions of human‐water systems. The discipline of sociohydrology has an important role to play in informing policy by developing a generalizable understanding of phenomena that arise from interactions between water and human systems. To explain these phenomena, sociohydrology must address several scientific challenges to strengthen the field and broaden its scope. These include engagement with social scientists to accommodate social heterogeneity, power relations, trust, cultural beliefs, and cognitive biases, which strongly influence the way in which people alter, and adapt to, changing hydrological regimes. It also requires development of new methods to formulate and test alternative hypotheses for the explanation of emergent phenomena generated by feedbacks between water and society. Advancing sociohydrology in these ways therefore represents a major contribution toward meeting the targets set by the SDGs, the societal grand challenge of our time.

Abstract. Competition for water between humans and ecosystems is set to become a flash point in the coming decades in many parts of the world. An entirely new and comprehensive quantitative framework is needed to establish a holistic understanding of that competition, thereby enabling the development of effective mediation strategies. This paper presents a modeling study centered on the Murrumbidgee River basin (MRB). The MRB has witnessed a unique system dynamics over the last 100 years as a result of interactions between patterns of water management and climate driven hydrological variability. Data analysis has revealed a pendulum swing between agricultural development and restoration of environmental health and ecosystem services over different stages of basin-scale water resource development. A parsimonious, stylized, quasi-distributed coupled socio-hydrologic system model that simulates the twoway coupling between human and hydrological systems of the MRB is used to mimic and explain dominant features of the pendulum swing. The model consists of coupled nonlinear ordinary differential equations that describe the interaction between five state variables that govern the co-evolution: reservoir storage, irrigated area, human population, ecosystem health, and environmental awareness. The model simulations track the propagation of the external climatic and socio-economic drivers through this coupled, complex system to the emergence of the pendulum swing. The model results point to a competition between human "productive" and environmental "restorative" forces that underpin the pendulum swing. Both the forces are endogenous, i.e., generated by the system dynamics in response to external drivers and mediated by humans through technology change and environmental awareness, respectively. Sensitivity analysis carried out with the model further reveals that socio-hydrologic modeling can be used as a tool to explain or gain insight into observed co-evolutionary dynamics of diverse human-water coupled systems. This paper therefore contributes to the ultimate development of a generic modeling framework that can be applied to human-water coupled systems in different climatic and socio-economic settings.

Socio-hydrology was introduced 4 years ago into the scientific lexicon, and elicited several reactions about the meaning and originality of the concept. However, there has also been much activity triggered by the original paper, including further commentaries that clarified the definitions, and several papers that acted on the definitions, and through these activities further clarified and illustrated the meaning and usefulness of socio-hydrology for understanding coupled human-water systems and to assist with sustainable water management. This paper restates the case for socio-hydrology by articulating the need to consider the two-way feedbacks between human and water systems in order to explain puzzles, paradoxes, and unintended consequences that arise in the context of water management, and to suggest ways to avoid or overcome these challenges. The paper then presents a critical review of past research on socio-hydrology through the prism of historical, comparative, and process socio-hydrology, documenting both the progress made and the challenges faced. Much of the work done so far has involved studies of socio-hydrological systems in spatially isolated domains (e.g., river basins), and phenomena that involve emergent patterns in the time domain. The modeling studies so far have involved testing hypotheses about how these temporal patterns arise. An important feature that distinguishes socio-hydrology from other related fields is the importance of allowing human agency (e.g., socioeconomics, technology, norms, and values) to be endogenous to the systems. This paper articulates the need to extend sociohydrology to explore phenomena in space and in space-time, as the world becomes increasingly globalized and human-water systems become highly interconnected. The endogenization of human agency, in terms of values and norms, technology, economics, and trade must now be extended to space and to spacetime. This is a necessity, and a challenge, for water sustainability, but presents exciting opportunities for further research. © 2016 The Authors. WIREs Water published by Wiley Periodicals, Inc. How to cite this article:WIREs Water 2017, 4:e1193. doi: 10.1002/wat2.1193 INTRODUCTIONH umans have been exploiting the Earth's natural resources at an accelerating pace ever since intense economic activity was triggered by the industrial revolution. This exploitation of the Earth's natural resources facilitated, and was in turn facilitated by, technological innovations. These innovations included extraction of iron ore, the steam engine, and faster ships, railroads, and several other means This is an open access article under the terms of the Creative Commons Attribution-NonCommercial License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited and is not used for commercial purposes.subsequently that connected people worldwide through trade. Concerted ambitions of humans in different parts of the world with their unique niches, now connected through trade, fuelled the...