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Rapid warming has reduced seasonal snow cover and changed the hydrological regimes over many areas of the globe. Temperature increase cannot only stimulate snowmelt but also favors a shift of wintertime precipitation towards rainfall, especially in Western and Central Europe that has temperate and humid winters. We assume that the mid-season (January-February) instead of the end-season (March) snow is particularly sensitive to warming in the low mountain ranges of Europe, and increased rain-on-snow events are associated with the currently frequent winter floods. To validate this assumption, we simulated the snow water equivalent (SWE), rain-on-snow (ROS) events, evapotranspiration, runoff and percolation for the period 1961-2016 in Rhineland-Palatinate, southwestern Germany (< 820 m a.s.l.) using a distributed hydrological model (TRAIN). We analyzed the intra- and inter-annual variations of the simulated hydrological variables and the synchronous climate variables (air temperature and precipitation). Early spring (March) displayed the strongest warming and the January-February warming was less severe or insignificant. Precipitation amounts did not show significant trends. However, the snowpack in February experienced the most striking reductions, which is likely explained by both the highest susceptibility of snow to warming and the increased ROS occurrence in February since the early 1990s. An increment in runoff and percolation in the earlier winter is probably related to a combination of high temperatures and more rainfalls at that time. The expected changing climate towards warmer and wetter winters but drier springs will probably exacerbate the winter flooding and spring/summer drought risks in the future. [ABSTRACT FROM AUTHOR]

Droughts, such as the recent event of 2018 that hit many regions of Europe, have a large impact on environment and society. These droughts are often characterized and studied as slowly developing creeping phenomena. However, a root zone soil moisture shortage potentially affecting plant growth and agricultural production can develop rather quickly, e.g., under dry conditions that favor high evapotranspiration or in shallow root zones with a low available soil water content. In this study, we investigate which variables (climate and soil properties) exert a control on the likelihood and development time of simulated soil moisture shortage during drought across Baden-Württemberg (Southwestern Germany). First, the TRAIN model was used to simulate root zone soil moisture for a 1 km resolution grid over Baden-Württemberg. For major drought years (e.g., 2003 & 2018), soil moisture time series of each agricultural grid cell across the region were characterized according to 1) whether or not a soil moisture shortage was reached that year (here, defined as a level <30% of the available soil water content), and 2) the time it took to drop from field capacity to a soil moisture shortage. Finally, (logistic) regression analyses were applied to identify the dominant controls on the likelihood and development time of soil moisture shortage. Results show that the majority of the agricultural grid cells across the study region reach soil moisture shortage in major drought years such as 2018. The development time of soil moisture shortage varied substantially. The dominant control on the likelihood and development time of soil moisture shortage was found to be the available water content of the root zone, i.e., a higher likelihood and faster development for shallow root zones with a low available water content. The evapotranspiration- and precipitation- rate alone during the development of soil moisture shortage were only weakly related to the development time. However, these meteorological variables are of added explanatory value for the development time when considered in combination with the available water content of the soil. Overall, these results give insight in the variability and controls on the likelihood and development time of root zone soil moisture shortage during drought, which is important information for monitoring and early warning of the soil moisture drought hazard. [ABSTRACT FROM AUTHOR]

The effects of the 2018 drought were significant in Southwestern Germany. The lack of precipitation caused the soils to dry out so that soil moisture often reached minimum levels and crop yield was well below average. However, the spottiness of the scarce precipitation led to strong regional differences. In the framework of a long-term monitoring program, we investigated soil moisture development at two agricultural sites in the province of Baden-Württemberg. The two field sites, which differ in terms of soil physical properties and cultivated crops, are located in the Upper Rhine Graben near Heidelberg ("Grenzhof") and in the Kraichgau near Pforzheim ("Katharinentaler Hof"). The Katharinentaler Hof site is on a loess plateau, where the soils are deep and show a cohesive structure, while the Grenzhof site is characterised by more sandy soils, which developed on quaternary gravel from the Rhine and its tributaries. Another interesting feature is the crop irrigation at the Grenzhof site during the drought. Our measurements were carried out with TDR and FDR probes at hourly intervals and in five different depths with triple replication. Furthermore, the observations were compared with soil moisture data from a forest monitoring site in Rhineland-Palatinate ("Merzalben"), approx. 60 km away from the Grenzhof site. Therefore, we investigated how drought affects two different kinds of ecosystems. In addition to the measurements, water balance and soil moisture were simulated with three different hydrological models: TRAIN was applied for the three sites, DAISY for the agricultural sites, and LWF-BROOK90 for the forest. The measurements at Grenzhof showed that the average soil moisture in the root zone during the main growing season in summer 2018 did not drop below the values of summer 2017. Here, irrigation prevented excessive soil water stress. In 2018, the mean soil moisture at Katharinentaler Hof only fell below the lowest values of summer 2017 at greater depths (90 cm). There, field capacity of the soil is higher than at Grenzhof and the few precipitation events during summer 2018 were sufficient to counteract excessive dehydration at all soil depths. The simulation of soil moisture with the TRAIN model showed good results for the Grenzhof site over the period 1st April to 27th September 2018. The natural development of the drought at Grenzhof without anthropogenic control was demonstrated by a scenario with the TRAIN model, in which the irrigation events were not taken into account. According to this, soil moisture would already have dropped to wilting point by the end of June 2018, which would have caused extreme drought stress for the plants. This clearly shows that agricultural irrigation can have a strong influence on soil moisture in soils with comparatively low field capacity during drought and thus should be considered in hydrological models. [ABSTRACT FROM AUTHOR]

The recharge of groundwater resources, which is an important source for drinking water supply in southwestern Germany, is affected by droughts. In order to manage groundwater resources sustainably, knowledge about groundwater recharge, especially under drought conditions, is indispensable. We investigate and compare groundwater recharge and groundwater levels during droughts in southwestern Germany with regard to their quantification, propagation and persistence. In particular, we investigate how groundwater recharge patterns change during droughts and how this affects groundwater levels. In addition, we investigate regional similarities of groundwater recharge patterns. Therefore, we compare measured groundwater levels with the model results of groundwater recharge of the spatial version of the hydrological model TRAIN. Groundwater droughts are detected with the help of a groundwater drought indicator and evaluated with regard to their spatial distribution. The incipient (modelled) groundwater recharge is displayed with heat map diagrams, which show the temporal anomalies of the incipient time. The dependency between groundwater recharge and groundwater drought is carried out by cross correlation analysis. We were able to show that the onset of the autumnal groundwater recharge in drought years is distinctly delayed. Another interesting finding is the spatial organization, where the model results show the strongest characteristic for the Upper Rhine Valley. According to our investigations, not only the time of onset of groundwater recharge in autumn, but also the extent of groundwater recharge in the following winter influences the propagation and persistence of a groundwater drought. Our findings will enable better decision-making processes for a sustainable water and aquifer management within the context of climate change. [ABSTRACT FROM AUTHOR]

There has been an accelerated and over-proportional rate of warming in the dry mid-latitudes and boreal forests of Southern Siberia. Associated changes affect permafrost distribution, the occurrence of wild fires and hydrological dynamics. Given that the Boreal ecotone occupies roughly 20 million km² of the global land surface, it can be assumed that these changes are of major interest with regard to the Earth's freshwater resources. However, since long-term hydro-meteorological time series are often not available, the impact of climate and environmental change on flow dynamics and water availability can only be roughly estimated. To gain insight into these challenging questions, we try to quantify the discharges and their temporal developments from two large-scale river basins. In a first step, we determine associated variabilities and trends of the major influencing factors, such as air temperature, precipitation and evapotranspiration. The selected catchments represent the variety of the climatic, landscape and permafrost patterns of that region: (1) Selenga River is the largest sub-basin of Lake Baikal, with a catchment size of about 440,000 km². The basin covers the transition belt between the dry mid-latitudes and the Boreal Zone and is mostly underlain by discontinuous permafrost. (2) Aldan River is a tributary of the Lena River. It drains an area of approx. 750,000 km², which is completely attributable to the Boreal ecotone and the continuous permafrost zone. Our analysis is based on long-term historical (1946-2015) hydro-meteorological time series, large-scale gridded data sets, as well as modeling simulations. The data undergoes extensive testing to detect anomalies, trends and multiple dependencies. The strong increase in air temperature during last 60 years may result in permafrost degradation and a decrease in seasonal frost occurrence, which is evident from the decreasing intra-annual variability of discharge in both catchments. However, the changes in river discharge show different directions: while the mean discharge of Selenga has decreased significantly, the mean discharge of Aldan shows a significant increase over the long-term period (based on the Mann Kendall test at a 95% confidence level). There also exist substantial seasonal shifts in the flow dynamics over both areas. These drastic and highly significant changes can be partly (but not fully) explained by precipitation and air temperature variabilities and trends. It is therefore likely that changes in landscape patterns, such as permafrost degradation and large-scale forest fires, are of great significance for the hydrological changes. Moreover, increasing water withdrawals due to a growing population, accelerated urbanization and a highly dynamic mining sector in the Selenga basin are factors that contribute to decreasing streamflow trends. To further clarify these relationships, our investigations will be extended to neighboring catchments, and our simulation studies will include an ensemble of hydrological models. [ABSTRACT FROM AUTHOR]

The drought of 2018 caused a wide range of impacts on environment and economy. This contribution explores how different drought impacts were linked to the occurrence of extreme values of hydrometeorological indices. Focusing on Germany, this study is based on drought impact data collected from text reports that are archived in the European Drought Impact report Inventory (EDII) and on standardized meteorological drought indices, modelled water balance and soil moisture, and observations of streamflow and groundwater. Our hypotheses included that impacts in particular sectors depended on the season (e.g. agricultural impacts), the combination of drought with heatwaves (e.g. impacts on river ecology), or other particular drought characteristics such as the prolonged duration of low flows (e.g. impacts on riverborne transportation). We therefore expected a particular temporal sequence of impact occurrences in relation to the drought evolution. Based on the currently available data there is little support of a temporal sequencing of impacts, but there are indications for drought characteristic specific impact profiles. Compared to recent previous drought events, impacts in different sectors appear to have co-occurred to a higher degree in 2018. However, results may still change as reporting and data collection are ongoing. Effects on groundwater levels and forests, for example, may lag behind by up to a year and many reports on impact-related statistics are only published with delay. Nevertheless, the combined data analysis of impacts and indices suggests that there may be potential for impact-specific drought monitoring of the multi-sectorial impacts that contributed to the event's high visibility. [ABSTRACT FROM AUTHOR]

Knowledge of the snow cover dynamics in mountains of the semi-arid regions on Earth is essential for an improved understanding of the hydrological regime and for human water supply in a water scarce environment. In the semi-arid Sugnugur headwater catchment (in the Khentii Mountains, northern Mongolia), a spatial analysis of seasonal snow cover duration (SCD) was achieved on a 30 m grid by integrating the spatial resolution of Landsat-7, Landsat-8 and Sentinel-2A images with the daily temporal resolution of MODIS snow products (2000-2017). Validation was achieved using in situ measurements from winter field campaigns and spatially distributed surface temperature recordings. We found a mean increase of SCD with altitude at approximately +6 days/100m. However, no altitude-dependent changes in snow depth could be observed during the field campaigns. The southern exposed valley slopes are either snow free or covered by intermittent snow throughout the winter months due to high sublimation rates and prevailing wind. The estimated mean SCD ranges from 124 in the lower parts of the catchment to 226 days on the mountain peaks, with a mean underestimation of 12-13 days. Snow onset and melt dates exhibited large inter-annual variability, but no significant trend in the seasonal SCD was evident. Our method supports the better quantification of snow melt rates and thus water availability in this semi-arid environment. It can be applied to high-resolution snow mapping in similar mountainous regions. [ABSTRACT FROM AUTHOR]