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 objective of this study is to evaluate two satellite rainfall products Global Precipitation Measurement Integrated MultisatellitE Retrievals and Tropical Rainfall Measuring Mission 3B42V7 (GPM IMERG and TRMM 3B42V7) in southern Tibetan Plateau region, with special focus on the dependence of products' performance on topography and rainfall intensity. Over 500 in situ rain gauges constitute an unprecedentedly dense rain gauge network over this region and provide an exceptional resource for ground validation of satellite rainfall estimates. Our evaluation centers on the rainy season from May to October in 2014. Results indicate that (1) GPM product outperforms TRMM at all spatial scales and elevation ranges in detecting daily rainfall accumulation; (2) rainfall accumulation over the entire rainy season is negatively correlated with mean elevation for rain gauges and the two satellite rainfall products, while the performance of TRMM also significantly correlates with topographic variations; (3) in terms of the ability of rainfall detection, false alarming ratio of TRMM (21%) is larger than that of GPM (14%), while missing ratio of GPM (13%) is larger than that of TRMM (9%). GPM tends to underestimate the amount of light rain events of 0–1 mm/d, while the opposite (overestimation) is true for TRMM. GPM shows better detecting ability for light rainfall (0–5 mm/d) events but there is no detection skill for both GPM and TRMM at high‐elevation (>4500 m) regions. Our results not only highlight the superiority of GPM to TRMM in southern Tibetan Plateau region but also recommend that further improvement on the rainfall retrieval algorithm is needed by considering topographical influences for both GPM and TRMM rainfall products.

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.

The generalized complementary function to estimate actual evaporation, namely, the ratio of actual evaporation to Penman potential evaporation (E/EPen) as a function of the proportion of the radiation term in EPen (Erad/EPen), has been increasingly recognized. Existing analytical forms of this generalized function present deficiencies because they are limited by improper boundary conditions resulting from inadequate understanding of physical constraints. In this study, its zero‐ and first‐order boundary conditions were rigorously derived by adopting the physical constraints for E in Penman's combination theory, and a sigmoid feature of relationship between E/EPen and Erad/EPen was derived. Minimum and maximum limits of Erad/EPen were introduced based on the derived boundary conditions, and accordingly, a new sigmoid function was developed. By restricting it to be approximately equivalent to the linear advection‐aridity (Brutsaert & Stricker, 1979, https://doi.org/10.1029/WR015i002p00443) function under normal environments, the new sigmoid function satisfies the upper limits of Penman's open water evaporation and Priestley‐Taylor's minimal advection evaporation in parallel. The sigmoid feature and the new sigmoid function were validated by tower‐based data from FLUXNET. This work improves our understanding of the three‐stage pattern of the complementary behavior, and the new function demonstrates a favorable potential for use in evaporation estimation.

Abstract. This paper presents a historical socio-hydrological analysis of the Tarim River basin (TRB), Xinjiang Uyghur Autonomous Region, in Western China, from the time of the opening of the Silk Road to the present. The analysis is aimed at exploring the historical co-evolution of coupled human-water systems and at identifying common patterns or organizing principles underpinning socio-hydrological systems (SHS). As a self-organized entity, the evolution of the human-water system in the Tarim Basin reached stable states for long periods of time, but then was punctuated by sudden shifts due to internal or external disturbances. In this study, we discuss three stable periods (i.e., natural, human exploitation, and degradation and recovery) and the transitions in between during the past 2000 years. During the "natural" stage that existed pre-18th century, with small-scale human society and sound environment, evolution of the SHS was mainly driven by natural environmental changes such as river channel migration and climate change. During the human exploitation stage, especially in the 19th and 20th centuries, it experienced rapid population growth, massive land reclamation and fast socio-economic development, and humans became the principal players of system evolution. By the 1970s, the Tarim Basin had evolved into a new regime with a vulnerable eco-hydrological system seemingly populated beyond its carrying capacity, and a human society that began to suffer from serious water shortages, land salinization and desertification. With intensified deterioration of river health and increased recognition of unsustainability of traditional development patterns, human intervention and recovery measures have since been adopted. As a result, the basin has shown a reverse regime shift towards some healing of the environmental damage. Based on our analysis within TRB and a common theory of social development, four general types of SHSs are defined according to their characteristic spatio-temporal variations of historical co-evolution, including primitive agricultural, traditional agricultural, industrial agricultural, and urban SHSs. These co-evolutionary changes have been explained in the paper in terms of the Taiji-Tire model, a refinement of a special concept in Chinese philosophy, relating to the co-evolution of a system because of interactions among its components.