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Droughts often have a severe impact on the environment, society, and the economy. The variables and scales that are relevant to understand the impact of drought motivated this study, which compared hazard and propagation characteristics, as well as impacts, of major droughts between 1990 and 2019 in southwestern Germany. We bring together high-resolution datasets of air temperature, precipitation, soil moisture simulations, and streamflow and groundwater level observations, as well as text-based information on drought impacts. Various drought characteristics were derived from the hydrometeorological and drought impact time series and compared across variables and spatial scales. Results revealed different drought types sharing similar hazard and impact characteristics. The most severe drought type identified is an intense multi-seasonal drought type peaking in summer, i.e., the events in 2003, 2015, and 2018. This drought type appeared in all domains of the hydrological cycle and coincided with high air temperatures, causing a high number of and variability in drought impacts. The regional average drought signals of this drought type exhibit typical drought propagation characteristics such as a time lag between meteorological and hydrological drought, whereas propagation characteristics of local drought signals are variable in space. This spatial variability in drought hazard increased when droughts propagated through the hydrological cycle, causing distinct differences among variables, as well as regional average and local drought information. Accordingly, single variable or regional average drought information is not sufficient to fully explain the variety of drought impacts that occurred, supporting the conclusion that in regions as diverse as the case study presented here, large-scale drought monitoring needs to be complemented by local drought information to assess the multifaceted impact of drought. [ABSTRACT FROM AUTHOR]

Droughts often have a severe impact on environment, society, and economy. Only a multifaceted assessment of such droughts and their impacts can provide insights in the variables and scales that are relevant for drought management. Motivated by this aim, we compared hazard and propagation characteristics as well as impacts of major droughts between 1990-2019 in Southwestern Germany. We bring together high-resolution datasets of air temperature, precipitation, soil moisture simulations, streamflow and groundwater level observations, as well as text-based information on drought impacts. Various drought characteristics were derived from the hydrometeorological and drought impact time series and compared across variables and spatial scales. Results revealed different drought types sharing similar hazard and impact characteristics. The most severe drought type identified is an intense multi-seasonal drought type peaking in summer, i.e. the events in 2003, 2015 and 2018. This drought type appeared in all domains of the hydrological cycle and coincided with high air temperatures, causing a high number and variability of drought impacts. The regional average drought signals of this drought type exhibit typical drought propagation characteristics such as a time lag between meteorological and hydrological drought, whereas propagation characteristics of local drought signals are variable in space. This spatial variability in drought hazard increased when droughts propagated through the hydrological cycle, causing distinct differences among variables, and regional average and local drought information. Accordingly, single variable or regional average drought information is considered to be not sufficient to fully explain the variety of drought impacts that occurred. In addition to large-scale drought monitoring, drought management needs to consider local drought information from different hydrometeorological variables and could be type based. [ABSTRACT FROM AUTHOR]

The drought of 2018 in central and northern Europe showed once more the large impact that this natural hazard can have on the environment and society. Such droughts are often seen as slowly developing phenomena. However, root zone soil moisture deficits can rapidly develop during periods lacking precipitation and meteorological conditions that favor high evapotranspiration rates. These periods of soil moisture stress can persist for as long as the meteorological drought conditions last, thereby negatively affecting vegetation and crop health. In this study, we aim to characterize past soil moisture stress events over the croplands of southwestern Germany and, furthermore, to relate the characteristics of these past events to different soil and climate properties. We first simulated daily soil moisture over the period 1989–2018 on a 1 km resolution grid, using the physically based hydrological model TRAIN. We then derived various soil moisture stress characteristics, including probability, development time, and persistence, from the simulated time series of all agricultural grid cells (n≈15000). Logistic regression and correlation were then applied to relate the derived characteristics to the plant-available storage capacity of the root zone and to the climatological setting. Finally, sensitivity analyses were carried out to investigate how results changed when using a different parameterization of the root zone, i.e., soil based or fixed, or when assessing soil moisture drought (anomaly) instead of stress. Results reveal that the majority of agricultural grid cells across the study region reached soil moisture stress during prominent drought years. The development time of these soil moisture stress events varied substantially, from as little as 10 d to over 4 months. The persistence of soil moisture stress varied as well and was especially high for the drought of 2018. A strong control on the probability and development time of soil moisture stress was found to be the storage capacity of the root zone, whereas the persistence was not strongly linearly related to any of the considered controls. On the other hand, the sensitivity analyses revealed the increased control of climate on soil moisture stress characteristics when using a fixed instead of a soil-based root zone storage. Thus, the strength of different controls depends on the assumptions made during modeling. Nonetheless, the storage capacity of the root zone, whether it is a characteristic of the soil or a difference between a shallow or deep rooting crop, remains an important control on soil moisture stress characteristics. This is different for SM drought characteristics, which have little or contrasting relation with the storage capacity of the root zone. Overall, the results give insight to the large spatial and temporal variability in soil moisture stress characteristics and suggest the importance of considering differences in root zone soil storage for agricultural drought assessments. [ABSTRACT FROM AUTHOR]

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]