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 interception process is responsible for the spatial and temporal redistribution of the precipitation that reaches the ground. The contact of the precipitation with the canopy influences on the water quality, increasing the concentration of various nutrients in the throughfall (Tf) and stemflow (Sf). The objective of this study was to assess the influence of the interception process on the precipitation quality in a catchment covered by Mixed Ombrophilous Forest. The precipitation (P) monitoring consisted of two rain gauges installed outside the basin. Six gauges were installed within the basin for Tf monitoring. The Sf monitoring was conducted in nine trees. Water sampled at all points was analyzed for color, conductivity, pH, turbidity, and total dissolved solids. The concentrations of Nitrate (NO3-), Chloride (Cl-), Phosphate (PO43-), Sulfate (SO42-), Acetate (CH3CO2-) and Calcium (Ca2+) ions were measured in five points, i.e., one precipitation, two throughfall and two stemflow. Measured precipitation, throughfall and stemflow during the period were 652.1 mm, 584.5 mm (89,6% P) and 2.6 mm (0,4% P), respectively. Total interception loss was 65 mm, corresponding to 10% of the total precipitation. The highest values of the physicochemical parameters were found in the Sf and the Tf. The pH was lower in the Sf, and it decreases with the diameter at breast height. There was no significant relationship between the physicochemical parameters and the canopy cover fraction. The analysis shows the significant difference in the water quality of the precipitation that reaches the ground after being intercepted.

Proper uncertainty estimation for data series with a high proportion of zero and near zero observations has been a challenge in hydrologic studies. This technical note proposes a modification to the Generalized Likelihood function that accounts for zero inflation of the error distribution (ZI‐GL). We compare the performance of the proposed ZI‐GL with the original Generalized Likelihood function using the entire data series (GL) and by simply suppressing zero observations (GLy>0). These approaches were applied to two interception modeling examples characterized by data series with a significant number of zeros. The ZI‐GL produced better uncertainty ranges than the GL as measured by the precision, reliability and volumetric bias metrics. The comparison between ZI‐GL and GLy>0 highlights the need for further improvement in the treatment of residuals from near zero simulations when a linear heteroscedastic error model is considered. Aside from the interception modeling examples illustrated herein, the proposed ZI‐GL may be useful for other hydrologic studies, such as for the modeling of the runoff generation in hillslopes and ephemeral catchments.

No abstract