Rising global temperature has put increasing pressure on understanding the linkage between atmospheric warming and the occurrence of natural hazards. While the Paris Agreement has set the ambitious target to limiting global warming to 1.5 ∘ C compared to preindustrial levels, scientists are urged to explore scenarios for different warming thresholds and quantify ranges of socioeconomic impact. In this work, we present a framework to estimate the economic damage and population affected by river floods at global scale. It is based on a modeling cascade involving hydrological, hydraulic and socioeconomic impact simulations, and makes use of state-of-the-art global layers of hazard, exposure and vulnerability at 1-km grid resolution. An ensemble of seven high-resolution global climate projections based on Representative Concentration Pathways 8.5 is used to derive streamflow simulations in the present and in the future climate. Those were analyzed to assess the frequency and magnitude of river floods and their impacts under scenarios corresponding to 1.5 ∘ C, 2 ∘ C, and 4 ∘ C global warming. Results indicate a clear positive correlation between atmospheric warming and future flood risk at global scale. At 4 ∘ C global warming, countries representing more than 70% of the global population and global gross domestic product will face increases in flood risk in excess of 500%. Changes in flood risk are unevenly distributed, with the largest increases in Asia, U.S., and Europe. In contrast, changes are statistically not significant in most countries in Africa and Oceania for all considered warming levels.

Target 6.4 of the recently adopted Sustainable Development Goals (SDGs) deals with the reduction of water scarcity. To monitor progress towards this target, two indicators are used: Indicator 6.4.1 measuring water use efficiency and 6.4.2 measuring the level of water stress (WS). This paper aims to identify whether the currently proposed indicator 6.4.2 considers the different elements that need to be accounted for in a WS indicator. WS indicators compare water use with water availability. We identify seven essential elements: 1) both gross and net water abstraction (or withdrawal) provide important information to understand WS; 2) WS indicators need to incorporate environmental flow requirements (EFR); 3) temporal and 4) spatial disaggregation is required in a WS assessment; 5) both renewable surface water and groundwater resources, including their interaction, need to be accounted for as renewable water availability; 6) alternative available water resources need to be accounted for as well, like fossil groundwater and desalinated water; 7) WS indicators need to account for water storage in reservoirs, water recycling and managed aquifer recharge. Indicator 6.4.2 considers many of these elements, but there is need for improvement. It is recommended that WS is measured based on net abstraction as well, in addition to currently only measuring WS based on gross abstraction. It does incorporate EFR. Temporal and spatial disaggregation is indeed defined as a goal in more advanced monitoring levels, in which it is also called for a differentiation between surface and groundwater resources. However, regarding element 6 and 7 there are some shortcomings for which we provide recommendations. In addition, indicator 6.4.2 is only one indicator, which monitors blue WS, but does not give information on green or green-blue water scarcity or on water quality. Within the SDG indicator framework, some of these topics are covered with other indicators.

Abstract. Lakes and reservoirs are crucial elements of the hydrological and biochemical cycle and are a valuable resource for hydropower, domestic and industrial water use, and irrigation. Although their monitoring is crucial in times of increased pressure on water resources by both climate change and human interventions, publically available datasets of lake and reservoir levels and volumes are scarce. Within this study, a time series of variation in lake and reservoir volume between 1984 and 2015 were analysed for 137 lakes over all continents by combining the JRC Global Surface Water (GSW) dataset and the satellite altimetry database DAHITI. The GSW dataset is a highly accurate surface water dataset at 30 m resolution compromising the whole L1T Landsat 5, 7 and 8 archive, which allowed for detailed lake area calculations globally over a very long time period using Google Earth Engine. Therefore, the estimates in water volume fluctuations using the GSW dataset are expected to improve compared to current techniques as they are not constrained by complex and computationally intensive classification procedures. Lake areas and water levels were combined in a regression to derive the hypsometry relationship (dh ∕ dA) for all lakes. Nearly all lakes showed a linear regression, and 42 % of the lakes showed a strong linear relationship with a R2 > 0.8, an average R2 of 0.91 and a standard deviation of 0.05. For these lakes and for lakes with a nearly constant lake area (coefficient of variation < 0.008), volume variations were calculated. Lakes with a poor linear relationship were not considered. Reasons for low R2 values were found to be (1) a nearly constant lake area, (2) winter ice coverage and (3) a predominant lack of data within the GSW dataset for those lakes. Lake volume estimates were validated for 18 lakes in the US, Spain, Australia and Africa using in situ volume time series, and gave an excellent Pearson correlation coefficient of on average 0.97 with a standard deviation of 0.041, and a normalized RMSE of 7.42 %. These results show a high potential for measuring lake volume dynamics using a pre-classified GSW dataset, which easily allows the method to be scaled up to an extensive global volumetric dataset. This dataset will not only provide a historical lake and reservoir volume variation record, but will also help to improve our understanding of the behaviour of lakes and reservoirs and their representation in (large-scale) hydrological models.

Atmospheric moisture within a region is supplied by both local evaporation and advected from external sources. The contribution of local evaporation in a region to the precipitation in the same region is defined as "precipitation recycling." Precipitation recycling helps in defining the role of land-atmosphere interactions in regional climate. A dynamic precipitation recycling model, which includes the moisture storage term, has been applied to calculate summer variability of the precipitation recycling over Europe based on 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40) data. Time series for three subregions in Europe (central Europe, the Balkans, and Spain) are obtained to analyze the variability in recycling and to compare the potential in the subregions for interactions between land surface and atmospheric processes. In addition, the recycled precipitation and recycling ratios are linked to several components of the water vapor balance equation [precipitation, evaporation, precipitation minus evaporation (P Ϫ E ), and moisture transport]. It is found that precipitation recycling is large in dry summers for central Europe, while the opposite is true for the Balkans. Large precipitation recycling is determined in relation with weak moisture transport and high evaporation rates in central Europe. This occurs for dry summers. For the Balkans, precipitation recycling is large in wet summers when moisture transport is weak, and P Ϫ E and evaporation are large. Here, the recycling process intensifies the hydrological cycle due to a positive feedback via convective precipitation and therefore the amount of recycled precipitation is larger. For Spain, recycling is also larger when moisture transport is weak, but other correlations are not found. For regions such as central Europe in dry summers and the Balkans in wet summers, which are susceptible to land-atmosphere interactions, future climate and/or land use can have an impact on the regional climate conditions due to changes in evaporation.

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[1] Regional climate simulations over Europe were initialized with soil moisture derived from the Advanced Microwave Scanning Radiometer-Earth Observing System (AMSR-E) in order to assess the model accuracy in predicting soil moisture and other components of the hydrological cycle. The AMSR-E soil moisture initially showed systematic differences with model-predicted soil moisture. For proper initialization the AMSR-E product had to be rescaled and after that vertically profiled. To retrieve a root zone soil moisture profile, we tested the application of an exponential filter. The smoothing through the layers of the ERA-Interim soil moisture profile was applied to the rescaled AMSR-E surface soil moisture. The filter performed very well for that part of the data set where the top layer is positively correlated with the deeper layers. After the preparation of the soil moisture fields, several sensitivity simulations were performed. The model's soil moisture was replaced with the vertically profiled AMSR-E soil moisture at different initial times for a dry summer (2003) and a wet summer (2005). In general, the surplus of soil moisture in the AMSR-E data resulted in a better performance in predicting temperature when compared with observations. This finding was more pronounced in the dry summer of 2003 when the model results appeared very sensitive to land-atmosphere feedbacks. Our results suggest that in dry years, the use of appropriate observed soil moisture may help more to reduce modeled surface temperatures than inducing additional rainfall in the model. Using the AMSR-E product led to a decrease in areal extent sensitive to land-atmosphere interactions.Citation: Bisselink, B., E. van Meijgaard, A. J. Dolman, and R. A. M. de Jeu (2011), Initializing a regional climate model with satellite-derived soil moisture,

Abstract. Evaporation is a key parameter in the regional atmospheric water cycle. Precipitation recycling is defined as the contribution of local evaporation in a region to the precipitation in the same region. In this work, we apply a dynamic precipitation recycling mode, which includes the moisture storage term, to calculate the warm season variability of the precipitation recycling over central Europe at a daily scale for 2003 (dry) and 2006 (wet). For the central part of Europe advection is the most important contributor to precipitation. In dry spells in both years 2003 and 2006, when moisture of advective origin diminishes, the local evaporation becomes an important contributor to precipitation (negative feedback). In two periods (June 2003 and July 2006) where there is enough moisture storage in the soil to continue the evaporation the recycling is enhanced. In August 2003 the evaporation is affecting the recycling due the lack of water availability caused by the dryness of the preceding spring and summer season. According to a multi variance analyses the evaporation in 2003 is the most important factor to explain the variance in the recycling ratio. In 2006, the precipitable water and the moisture fluxes are more dominant and the evaporation becomes less important, except for the dry period in July. Not only evaporation is important for recycling, but also a mechanism to trigger precipitation. In case studies we follow the path of an air column of days with one of the highest recycling. At the 2 days with enough moisture availability (28 May 2003 and 5 July 2006) we see long path length due to weak winds. Following the paths, the air is transported over land for a very long distance before it precipitates and has a lot of time to traverse the region and capture moisture of evaporative origin. However, we hypothesize that the precipitation falling at those days originates (partly) from oceanic sources, but the triggering of precipitation may itself be a result of enhanced instability induced by soils, which still have enough moisture storage. In this way, the evaporation is an important driver in the recycling ratio variability. For the case study of 10 August 2003, the atmosphere is too dry to generate precipitation with exception of the mountainous regions due orographical lifting. However, the impact of land-use change in future climate will have the most impact on the evaporation in dry spells dominated by persistent blocking systems.

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