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

International audiencePopulations in Central Asia are heavily dependent on snow and glacier melt for their water supplies. Changes to the glaciers in the main mountain range in this region, the Tien Shan, have been reported over the past decade. However, reconstructions over longer, multi-decadal timescales and the mechanisms underlying these variations—both required for reliable future projections—are not well constrained. Here we use three ensembles of independent approaches based on satellite gravimetry, laser altimetry, and glaciological modelling to estimate the total glacier mass change in the Tien Shan. Results from the three approaches agree well, and allow us to reconstruct a consistent time series of annual mass changes for the past 50 years at the resolution of individual glaciers. We detect marked spatial and temporal variability in mass changes. We estimate the overall decrease in total glacier area and mass from 1961 to 2012 to be 18 ± 6% and 27 ± 15%, respectively. These values correspond to a total area loss of 2,960 ± 1,030 km2, and an average glacier mass-change rate of −5.4 ± 2.8 Gt yr−1. We suggest that the decline is driven primarily by summer melt and, possibly, linked to the combined effects of general climatic warming and circulation variability over the north Atlantic and north Pacific

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Observed streamflow of headwater catchments of the Tarim River (Central Asia) increased by about 30% over the period . This study aims at assessing to which extent these streamflow trends can be attributed to changes in air temperature or precipitation. The analysis includes a data-based approach using multiple linear regression and a simulation-based approach using a hydrological model. The hydrological model considers changes in both glacier area and surface elevation. It was calibrated using a multiobjective optimization algorithm with calibration criteria based on glacier mass balance and daily and interannual variations of discharge. The individual contributions to the overall streamflow trends from changes in glacier geometry, temperature, and precipitation were assessed using simulation experiments with a constant glacier geometry and with detrended temperature and precipitation time series. The results showed that the observed changes in streamflow were consistent with the changes in temperature and precipitation. In the Sari-Djaz catchment, increasing temperatures and related increase of glacier melt were identified as the dominant driver, while in the Kakshaal catchment, both increasing temperatures and increasing precipitation played a major role. Comparing the two approaches, an advantage of the simulation-based approach is the fact that it is based on process-based relationships implemented in the hydrological model instead of statistical links in the regression model. However, data-based approaches are less affected by model parameter and structural uncertainties and typically fast to apply. A complementary application of both approaches is recommended.

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Including satellite-derived snow cover data for hydrologic model calibration can be a good way to improve model internal consistency. This study applied a multiobjective genetic algorithm to characterize the trade-off curve between model performance in terms of discharge and snow cover area (SCA). Using a Monte Carlo-based approach, we further investigated the additional information content of an increasing number of SCA scenes used in the calibration period. The study was performed in six snowmeltdominated headwater catchments of the Karadarya Basin in Kyrgyzstan, Central Asia, using the hydrological model WASA and snow cover data from four melt seasons retrieved from AVHRR (Advanced Very High Resolution Radiometer). We generally found only small trade-offs between good simulations with respect to discharge and SCA, but good model performance with respect to discharge did not exclude low performance in terms of SCA. On average, the snow cover error in the validation period could be reduced by very few images in the calibration period. Increasing the number of images resulted in only small further improvements. However, using only a small number of images involves the risk that these particular images cause the selection of parameter sets which are not representative for the catchment. It is therefore advisable to use a larger number of images. In this study, it was necessary to include at least 10-16 images.

Abstract. Regional evaporation has increased in many parts of the world in the last decades, but the drivers of these increases are widely debated. Part of the difficulty lies in the scarcity of high-quality long-term data on evaporation. In this paper, we analyze changes in catchment evaporation estimated from the water balances of 156 catchments in Austria over the period 1977–2014 and attribute them to changes in atmospheric demand and available energy, vegetation, and precipitation as possible drivers. Trend analyses suggest that evaporation has significantly increased in 60 % of the catchments (p≤0.05) with an average increase of 29±14 mm yr−1 decade−1 (± standard deviation) or 4.9±2.3 % decade−1. Pan evaporation based on 24 stations has, on average, increased by 29±5 mm yr−1 decade−1 or 6.0±1.0 % decade−1. Reference evaporation over the 156 catchments estimated by the Penman–Monteith equation has increased by 18±5 mm yr−1 decade−1 or 2.8±0.7 % decade−1. Of these, 2.1 % are due to increased global radiation and 0.5 % due to increased air temperature according to the Penman–Monteith equation. A satellite-based vegetation index (NDVI) has increased by 0.02±0.01 decade−1 or 3.1±1.1 % decade−1. Estimates of reference evaporation accounting for changes in stomata resistance due to changes in the NDVI indicate that the increase in vegetation activity has led to a similar increase in reference evaporation as changes in the climate parameters. A regression between trends in evaporation and precipitation yields a sensitivity of a 0.22±0.05 mm yr−2 increase in evaporation to a 1 mm yr−2 increase in precipitation. A synthesis of the data analyses suggests that 43±15 % of the observed increase in catchment evaporation may be directly attributed to increased atmospheric demand and available energy, 34±14 % to increased vegetation activity, and 24±5 % to increases in precipitation.

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