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 on-line 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 focussed on 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.

Evapotranspiration (ET) is the process by which liquid water becomes water vapor and energetically this accounts for much of incoming solar radiation. If this ET did not occur temperatures would be higher, so understanding ET trends is crucial to predict future temperatures. Recent studies have reported prolonged declines in ET in recent decades, although these declines may relate to climate variability. Here, we used a well-validated diagnostic model to estimate daily ET during 1981–2012 and its three components: transpiration from vegetation (E_t), direct evaporation from the soil (E_s) and vaporization of intercepted rainfall from vegetation (E_i). During this period, ET over land has increased significantly (p < 0.01), caused by increases in E_t and E_i, which are partially counteracted by E_s decreasing. These contrasting trends are primarily driven by increases in vegetation leaf area index, dominated by greening. The overall increase in E_t over land is about twofold of the decrease in E_s. These opposing trends are not simulated by most Coupled Model Intercomparison Project phase 5 (CMIP5) models and highlight the importance of realistically representing vegetation changes in earth system models for predicting future changes in the energy and water cycle.

Extra water loss from earlier spring greening dries soils locally in summer, but can moisten soils in areas downwind.