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

Observed phenotypic variation in the lateral root branching density (LRBD) in maize (Zea mays) is large (1-41 cm 21 major axis [i.e. brace, crown, seminal, and primary roots]), suggesting that LRBD has varying utility and tradeoffs in specific environments. Using the functional-structural plant model SimRoot, we simulated the three-dimensional development of maize root architectures with varying LRBD and quantified nitrate and phosphorus uptake, root competition, and whole-plant carbon balances in soils varying in the availability of these nutrients. Sparsely spaced (less than 7 branches cm 21 ), long laterals were optimal for nitrate acquisition, while densely spaced (more than 9 branches cm 21 ), short laterals were optimal for phosphorus acquisition. The nitrate results are mostly explained by the strong competition between lateral roots for nitrate, which causes increasing LRBD to decrease the uptake per unit root length, while the carbon budgets of the plant do not permit greater total root length (i.e. individual roots in the high-LRBD plants stay shorter). Competition and carbon limitations for growth play less of a role for phosphorus uptake, and consequently increasing LRBD results in greater root length and uptake. We conclude that the optimal LRBD depends on the relative availability of nitrate (a mobile soil resource) and phosphorus (an immobile soil resource) and is greater in environments with greater carbon fixation. The median LRBD reported in several field screens was 6 branches cm 21 , suggesting that most genotypes have an LRBD that balances the acquisition of both nutrients. LRBD merits additional investigation as a potential breeding target for greater nutrient acquisition.

Colloids have long been known to facilitate the transport of contaminants in soils, but few direct observations have been made of transport and retention in unsaturated porous media. Studies have typically been limited to evaluation of column breakthrough curves, resulting in differing and sometimes conflicting proposed retention mechanisms. We carried out pore scale visualization studies of colloid transport in unsaturated quartz sand to directly observe and characterize colloid retention phenomena. Synthetic hydrophilic (0.8, 2.6, and 4.8 microm carboxylated polystyrene latex) and relatively hydrophobic (5.2 microm polystyrene latex) colloidal microspheres were added to steady-state water flow (0.15 mm min(-1)) applied to an inclined infiltration chamber. Bright field microscopy was used to determine the positions and movement of water and colloids. Confocal laser scanning microscopy was used to determine water film geometry in an unsaturated horizontal chamber. We determined mechanisms of hydrophilic colloid retention at what is generally termed the air/water/solid (AWS) interface. Based on our observations, the AWS interface is here more accuratelytermed the air/water meniscus/solid (AWmS) interface, denoting the region where between-grain water meniscii diminish to thin water films on the grain surfaces. Colloids were retained at the AWmS interface where the film thickness approximately equaled colloid diameters. The greater retention for hydrophilic colloids at this interface (compared to elsewhere in the solid/water interface) can be explained by the additional surface tension capillary potentials exerted on colloids at the AWmS interface. While some 0.8-microm colloids were observed in thin water films, film straining played no significant role in the retention of larger colloids. Mechanisms for slightly hydrophobic colloids differed slightly. In addition to primary retention at the AWmS interface, hydrophobic colloids attached to others already present atthat interface resulting in apparent retention at the air/water (AW) interface. Attachment of hydrophobic colloids was also observed at water-solid interfaces, as hydrophobicity impelled the colloids to avoid water. Factors contributing to retention of slightly hydrophobic colloids were sand grain roughness and possibly a tendency for these colloids to flow near surfaces and interfaces, consonant with the enhanced retention of hydrophobic colloids (relative to hydrophilic colloids) observed in the literature.