Jonas Götte

Floods following on streamflow droughts can have severe impacts. While they have been prominently featured by the media in recent years, we know little about their spatio-temporal variability. In this study, we analyze the occurrence and drivers of such drought-to-flood transitions by calculating transition lengths from droughts to floods for natural and regulated catchments across the Contiguous United States between 1970 and 2022. We find that drought-to-flood transitions strongly vary in their lengths and their spatial distribution. We identify snowmelt as the main driver of transitions in high-elevation catchments, while transitions in low-elevation catchments are more variable in their time of occurrence and drivers. Reservoir management reduces the number of short drought-to-flood transitions, particularly in catchments with a high amount of snow where snowmelt is crucial for filling reservoirs in early summer. These findings suggest that projected changes in the snowmelt season will lead to changes in transitions from streamflow droughts to floods and that reservoir management may be used to adapt to these changes.

Comprehensive large-sample hydrological datasets, particularly the CAMELS datasets (Catchment Attributes and MEteorology for Large-sample Studies), have advanced hydrological research and education in recent years. These datasets integrate extensive hydro-meteorological observations with landscape features, such as geology and land use, across numerous catchments within a national framework. They provide harmonised large-sample data for various purposes, such as assessing the impacts of climate change or testing hydrological models on a large number of catchments. Furthermore, these datasets are essential for the rapid progress of data-driven models in hydrology in recent years. Despite Germany's extensive hydro-meteorological measurement infrastructure, it has lacked a consistent, nationwide hydrological dataset, largely due to its decentralised management across different federal states. This fragmentation has hindered cross-state studies and made the preparation of hydrological data labour-intensive. The introduction of CAMELS-DE represents a step forward in bridging this gap. CAMELS-DE includes 1582 streamflow gauges with hydro-meteorological time series data covering up to 70 years (median length of 46 years and a minimum length of 10 years), from January 1951 to December 2020. It includes consistent catchment boundaries with areas ranging from 5 to 15 000 km² along with detailed catchment attributes covering soil, land cover, hydrogeologic properties, and data on human influences. Furthermore, it includes a regionally trained long short-term memory (LSTM) network and a locally trained HBV (Hydrologiska Byråns Vattenbalansavdelning) model that were used as quality control and that can be used to fill gaps in discharge data or act as baseline models for the development and testing of new hydrological models. Given the large number of catchments, including numerous relatively small ones (636 catchments < 100 km²), and the time series length of up to 70 years (166 catchments with 70 years of discharge data), CAMELS-DE is one of the most comprehensive national CAMELS datasets available and offers new opportunities for research, particularly in studying long-term trends and runoff formation in small catchments and in analysing catchments with strong human influences. This article describes CAMELS-DE version 1.0, which is available at https://doi.org/10.5281/zenodo.13837553 (Dolich et al., 2024).

Streamflow droughts are governed by different hydro-meteorological processes, whose relative importance may change over time, with potential impacts on drought severity. Here, we assess changes in the importance of different hydrological drought generation processes in the European Alps by applying a standardized drought type classification scheme to two time periods - one in the distant (1970-1993) and one in the recent past (1994-2017). Our findings show that changes in the relative importance of different drought generation processes are stronger in high-elevation catchments, where we detect clear changes in drought seasonality, than in low-elevation catchments. Furthermore, they suggest that changes in drought severity and generation processes are related because increasingly frequent snowmelt-deficit droughts in high-elevation catchments have larger deficits than droughts caused by decreasingly frequent cold temperatures. These changes might persist into the future, because of continuing decreases in snow cover and increases in evapotranspiration, with potential implications for water management.