Description: The state of Ceará is located in the semi-arid northeast of Brazil, where droughts and uncertain water supply threaten people living in one of the most densely populated dryland regions in the world. To store and supply water during dry periods, tens of thousands of (small) dams have been built over time. Except for 155 strategic reservoirs, there is hardly any systematic monitoring and management of water resources. In addition to comprehensive monitoring, a hydrological forecasting tool is also needed to better manage water resources in Ceará and reduce the impact of future droughts. We developed an innovative system for monitoring and forecasting hydrological dynamics in Ceará. The system is based on the integrated use of climate modeling, process-based hydrological modeling, remote sensing, and existing databases. Specifically, it consists of three complementary products: (1) Satellite-based monitoring of reservoir water storage: Water storage in reservoirs is monitored by evaluating weekly updating Sentinel-1 images together with satellite derived bathymetric information for 〉 40,000 reservoirs. (2) Modeling and forecasting of hydrological dynamics: The process-based hydrological model WASA-SED was adapted for the state area of Ceará. Information from satellite-based monitoring is dynamically assimilated in the simulations. Based on an ensemble of bias-corrected ECHAM4.6 climate simulations, seasonal hydrological forecasts with six months lead time are issued every month. (3) Web-based visualization of monitoring and forecast results: The results of satellite-based monitoring and dynamic hydrological forecasting are centrally managed in a database. Specific information is visualized online as maps and graphics for different user groups and decision makers.

Description: Flood risk assessments require different disciplines to understand and model the underlying components hazard, exposure, and vulnerability. Many methods and data sets have been refined considerably to cover more details of spatial, temporal, or process information. We compile case studies indicating that refined methods and data have a considerable effect on the overall assessment of flood risk. But are these improvements worth the effort? The adequate level of detail is typically unknown and prioritization of improvements in a specific component is hampered by the lack of an overarching view on flood risk. Consequently, creating the dilemma of potentially being too greedy or too wasteful with the resources available for a risk assessment. A “sweet spot” between those two would use methods and data sets that cover all relevant known processes without using resources inefficiently. We provide three key questions as a qualitative guidance toward this “sweet spot.” For quantitative decision support, more overarching case studies in various contexts are needed to reveal the sensitivity of the overall flood risk to individual components. This could also support the anticipation of unforeseen events like the flood event in Germany and Belgium in 2021 and increase the reliability of flood risk assessments.

Description: Glaciated alpine catchments feature high suspended sediment yields, with severe implications, e.g. for downstream water quality and flood hazard. However, the impact of past and future climate on these systems is not well understood, due to the limited availability of records and methods for future projections. We tested the feasibility of estimating sediment export at decadal scales using Quantile Regression Forest (QRF), a non-parametric data analysis method. For estimates of past sediment yields, we used short-term daily records (i.e. time series of a few years) of suspended sediment concentrations (SSC) and long records of the most important hydro-climatic drivers. The QRF models were trained independently for two nested and partially glaciated catchments, Vent and Vernagt, in the Upper Ötztal Alps in Tyrol, Austria. Themodels performed well in estimating daily sediment export, and outperformed traditional sediment rating curves. An analysis of the reconstructed time series indicated that there were significant positive trends in the annual suspended sediment yields at both gauges, with distinct step-like increases around 1981, which correspond to the onset of accelerated glacial melt and respective threshold behaviour. To estimate future sediment yields, we used hydroclimatic scenarios (2020-2100) as input. The method proved to be applicable, so the trends in sediment export of can be estimated for the next two or three decades. Modelling into the future is constricted, e.g. by projected glaciers disappearance around 2070 in some scenarios. Such major changes in the sedimentological functioning of the catchments that will likely not be represented adequately by our models.

Description: In recent years, urban and rural flash floods in Europe and abroad have gained considerable attention because of their sudden occurrence, severe material damages and even danger to life of inhabitants. This contribution addresses questions about possibly changing environmental conditions which might have altered the occurrence frequencies of such events and their consequences. We analyze the following major fields of environmental changes. Altered high intensity rain storm conditions, as a consequence of regional warming; Possibly altered runoff generation conditions in response to high intensity rainfall events; Possibly altered runoff concentration conditions in response to the usage and management of the landscape, such as agricultural, forest practices or rural roads; Effects of engineering measures in the catchment, such as retention basins, check dams, culverts, or river and geomorphological engineering measures. We take the flash-flood in Braunsbach, SW-Germany, as an example, where a particularly concise flash flood event occurred at the end of May 2016. This extreme cascading natural event led to immense damage in this particular village. The event is retrospectively analyzed with regard to meteorology, hydrology, geomorphology and damage to obtain a quantitative assessment of the processes and their development. The results show that it was a very rare rainfall event with extreme intensities, which in combination with catchment properties and altered environmental conditions led to extreme runoff, extreme debris flow and immense damages. Due to the complex and interacting processes, no single flood cause can be identified, since only the interplay of those led to such an event. We have shown that environmental changes are important, but—at least for this case study—even natural weather and hydrologic conditions would still have resulted in an extreme flash flood event.

Description: Hydrological and sediment transport regime are important in water resource management. Aim of this study is to identify the flow regime and the suspended sediment transport in a Mediterranean River Basin. Precipitation, temperature, soil, land use, discharges and suspended sediment concentration are used to quantify runoff and sediment yields at daily scales. WASA-SED (Water Availability in Semi-Arid environments – SEDiments) a spatially semi distributed model it is developed to simulate the flow and sediment transport in Seman Basin. Sediment deposits in Seman Basin contribute to a significant annual loss in the water storage capacity of the dams. Runoff and suspended sediments in Mediterranean hill slopes are closely related to rainfall intensities and land surface cover. This study gives a valuable approach in improving the prediction of flow and sediment transport in Mediterranean River Basin. Keywords:Flow, Sediment transport, WASA-SED, Mediterranean River Basin

Description: In July 2021, extreme weather conditions caused flooding in multiple tributaries of the Meuse and Rhine rivers. Particularly the Ahr Valley and the Erft Valley in Germany were greatly affected, where exceptional damage and severe human loss was registered. Since then, several studies have been conducted to understand the extremity of this event, the major driving forces, and the particular mechanisms of this flood. Furthermore, the possible impacts that climate change and human intervention may have contributed to the generation of such an event are discusses widely. In this study, we focus on the hydrological analysis of the July 2021 flood in the Ahr catchment, looking at the dominant hydrological processes that lead to rapid raising, extreme runoff rates and high water levels. For this purpose, we analyze the spatio-temporal distribution of the runoff coefficient using a semi-distributed hydrological catchment model, calibrated for this event based on the observed discharge measurements. The uncertainty in the rainfall input and the calibrated parameter set are as well incorporated in order to understand their influence on the spatio-temporal runoff yield patterns. Observed traces of erosion, surface water and flow paths are employed to shed light on the hydrological processes that took place. Furthermore, to understand how the dominant processes may have changed from previous events, the results of this analysis are compared to the 2016 flooding in Ahr Catchment. Lastly, the hydrologic response of this event is put on a broader perspective and compared with other extreme flood events in Europe.

Description: Extreme rainfall events in high mountain areas can trigger hydro-meteorological hazards like floods, debris-flows, and landslides, and affect water quality, flood hazards, and hydro-power production in downstream areas by mobilising and transporting substantial amounts of sediment. As such, there is a need to better understand rainstorm events in mountainous catchments, especially in light of the projected increases to their frequency and intensity with climate change. The quantification of rainfall extremes in high mountain areas is, however, challenging. Rain gauges are generally sparse and biased towards lower elevations, and are therefore often unable to adequately capture the spatial variability of heavy rainfall at higher elevations. Multi-source precipitation products that combine numerical weather prediction, radar and station data try to narrow this gap. Here, we analyse a multi-source gridded precipitation dataset, INCA, to detect and quantify heavy rainfall events in the High Ötztal Alps, Austria. Acknowledging that the dataset may be biased at higher elevations, we complement our analysis with rainfall data from a rather dense high-alpine network of independent weather stations maintained by various research institutions. We estimate event thresholds using a GEV distribution of catchment precipitation, which enables us to identify heavy rainfall event peaks. We constrain our analysis to the mainly snowfree period of April-October, as the catchment is hydrologically inactive outside of this period. Each rainstorm event is quantified in terms of intensity, duration, seasonality and spatiotemporal characteristics. Finally, we discuss observed hydro-morphological catchment responses to the detected occurrences of heavy rainfall.

Description: Hochwasser in Flussgebieten werden in lokale und plötzliche Sturzfluten in kleinen Gebieten und in Hochwasser an größeren Flüssen unterschieden. Für verschiedene Hochwasserindikatoren und Flusseinzugsgebiete ergeben sich erhebliche Unterschiede, wobei sowohl überwiegend aus Regen als auch überwiegend aus Schmelzwasser gespeiste Hochwasserereignisse betrachtet werden. Besondere Aufmerksamkeit finden Hochwasserereignisse in den großen Flussgebieten von Rhein, Donau, Elbe, Weser und Ems in Deutschland sowie die Entwicklung von Sturzfluten infolge von Extremniederschlägen kurzer Dauer, wobei die Beobachtungen und Trends zu Modellierungsergebnissen in Beziehung gesetzt werden. Auch die Notwendigkeit von Anpassungsmaßnahmen wird diskutiert.