Explaining reported drought impacts in the European Alpine region with selected drought indices

Even across Europe’s generally water-rich Alpine region the number of reports on negative drought impacts increased recently. The Alpine Drought Impact report Inventory EDIIALPS archives information of more than 3,200 specifically reported impacts with a majority in the last decade underlining the need for region-specific drought monitoring and adaptation strategies. The relation between drought conditions and drought impact occurrence has not been analyzed systematically in this heterogeneous mountain terrain. This study aims to improve such systematic understanding through the analysis of selected drought characteristics and reported impacts. Therefore, we assigned EDIIALPS’ reported impacts as soil-moisture drought impacts (SMD) and hydrological drought impacts (HD) and explored statistically the relation of these two impact groups to the following drought indices: Soil Moisture Anomalies, Standardized Precipitation Index, Standardized Precipitation Evapotranspiration Index, Vegetation Condition Index and Vegetation Health Index. The density of the reported SMD impacts and HD impacts increased clearly, the stronger the index’ value indicates drought conditions - apart from the vegetation indices. However, the correlation tests between reported impacts and indices did not identify explicit linear relations. To capture non-linear effects and differences between reported SMD impacts and HD impacts we applied decision trees using recursive partitioning. This way, we identified the Standardized Precipitation and Evapotranspiration Index to be most important for reported HD impacts and the Soil Moisture Anomalies to be most important for reported SMD impacts. To predict impact occurrence we recommend to model and evaluate a combination of drought indices allowing non-linearities in order to improve drought impact monitoring and early warning.

Stephan, R., Dormann, C. F., and Stahl, K.: Explaining reported drought impacts in the European Alpine region with selected drought indices, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-240,, 2022.

An agricultural vulnerability assessment to droughts in the Alps: exploring indicators’ contributions at regional level

Recent drought events highlighted the vulnerability of the European Alps to unexpected conditions of reduced water availability. The drought conditions led to a wide range of impacts especially affecting agriculture. Impacts were not only triggered by the natural hazard itself but also by the level of regional exposure and vulnerability. Nevertheless, the characterization of the exposure and vulnerability in risk assessments still represents a challenging task due to the specific knowledge needed to depict regional conditions and its sparse quantitative high resolution data.

Our study aims to identify the main indicators affecting vulnerability and explore their contribution to the final drought risk in agriculture. We selected the Podravska region in Slovenia and the Thurgau canton in Switzerland. Both are case studies of the Alpine Drought Observatory Interreg project due to recent drought impacts in agriculture.

Overall, a total of 31 indicators describing vulnerability to agricultural drought impacts was identified by local experts with 12 common indicators for both study areas. The majority of the indicators was solely identified for either Thurgau or Podravska demonstrating each region's specific characteristics. The indicators covered a broad range of aspects, such as geographic conditions (e.g. elevation, south facing), hydrological aspects (e.g. distance to large water bodies), soil characteristics (e.g. water holding capacity), agricultural practices (e.g. intensive farming), agricultural infrastructure (e.g. irrigation infrastructure), farmers' education, and policies (e.g. compensations). For each indicator we collected quantitative spatial data, removing those for which no information was available. Moreover, we normalized the selected indicators and combined them into final regional maps following two weighting scenarios: the equal weighting scenario, with all indicators having the same weight and the expert weighting scenario, where weights were assigned by the involved experts. In the Thurgau case the experts assigned more weights to the indicators related to the soil characteristics (e.g. “water holding capacity” and “humus content”) while for the Podravska case indicators related to farms position and type (e.g. “accessibility to local food market” and “farm diversification”). Final vulnerability maps for the two weighting scenarios and case studies will provide insights into the main vulnerability hotspot to drought, highlighting the main contributing indicators as well as those indicators initially identified by the experts for which no regional data is available.

Overall, this study highlighted the need of integrating the widely used equal weighting scenarios with qualitative knowledge and narratives from key experts. This approach can improve the understanding of agricultural vulnerability assessments to drought events supporting the implementation of adaptation strategies and plans in the Alpine region.

Cocuccioni, S., Stephan, R., Terzi, S., Erfurt, M., Stahl, K., and Zebisch, M.: An agricultural vulnerability assessment to droughts in the Alps: exploring indicators’ contributions at regional level, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11185,, 2022.

Upstream-downstream asymmetries of drought impacts in major river basins of the European Alps

Despite their considerable water availability, the European Alps are increasingly affected by droughts. Especially in recent decades, drought impacts have illustrated the regions’ vulnerability, so improved knowledge on the spatial distribution of drought impacts from high elevation headwater regions down to plateau and foothill areas is of tremendous importance. The region has an exceptional data availability including archived drought impact information. It is therefore a good test bed for the often-assumed general hypothesis that drought impacts become more severe downstream. The aim of this study was to investigate whether upstream-downstream differences in the distribution of drought impacts exist in the four major river basins of the European Alps - Rhine, Rhone, Po and Danube. Two different classifications were developed to divide these basins in up- and downstream areas. We based the first classification on the distances to the main sink, and the second classification on human influence. The EDIIALPS database provided quantitative data to analyse the distribution patterns of reported drought impacts from 2000-2020. The results suggest a strong regional variability regarding the temporal and spatial distribution of drought impacts within the individual basins. But they support the general hypothesis: for both classifications the number of drought impacts per area is higher in downstream regions. For the classification based on distances differences are statistically significant for the Rhine and Danube basin. The study provides insight into the spatial distribution of drought impacts in the four major river basins of the European Alps and proves the existence of upstream-downstream asymmetries. The integration of drought indices indicating drought conditions might further explain these differences. Climate change and enhanced cascading effects likely increase these asymmetries and consequently future drought management strategies need to move from emergency actions to better preparedness.

Dahlmann, H., Stephan, R., and Stahl, K.: Upstream-downstream asymmetries of drought impacts in major river basins of the European Alps, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5841,, 2022.

Streamflow Reconstructions Using Tree-Ring Based Paleo Proxies for the Upper Adige River Basin (Italy)

The Adige River Basin (ARB) provides a vital water supply source for varying demands including agriculture (wine production), energy (hydropower) and municipal water supply. Given the importance of this river system, information about past (paleo) drought and pluvial (wet) periods would quantity risk to water managers and planners. Annual streamflow data were obtained for four gauges that were spatially located within the upper ARB. The Old World Drought Atlas (OWDA) provides an annual June–July–August (JJA) self-calibrating Palmer Drought Severity Index (scPDSI) derived from 106 tree-ring chronologies for 5414 grid points across Europe from 0 to 2012 AD. In lieu of tree-ring chronologies, the OWDA dataset was used as a proxy to reconstruct both individual gauge and ARB regional streamflow from 0 to 2012. Principal component analysis (PCA) was applied to the four ARB streamflow gauges to generate one representative vector of regional streamflow. This regional streamflow vector was highly correlated with the four individual gauges, as coefficient of determination (R2) values ranged from 85% to 96%. Prescreening methods included correlating annual streamflow and scPDSI cells (within a 450 km radius) in which significant (p ≤ 0.01 or 99% significance) scPDSI cells were identified. The significant scPDSI cells were then evaluated for temporal stability to ensure practical and reliable reconstructions. Statistically significant and temporally stable scPDSI cells were used as predictors (independent variables) to reconstruct streamflow (predictand or dependent variable) for both individual gauges and at the regional scale. This resulted in highly skillful reconstructions of upper ARB streamflow from 0 to 2012 AD. Multiple drought and pluvial periods were identified in the paleo record that exceed those observed in the recent, historic record. Moreover, this study concurred with streamflow reconstructions in nearby European watersheds.

Formetta G, Tootle G, Bertoldi G. Streamflow Reconstructions Using Tree-Ring Based Paleo Proxies for the Upper Adige River Basin (Italy). Hydrology. 2022; 9(1):8.

Evapotranspiration Changes over the European Alps: Consistency of Trends and Their Drivers between the MOD16 and SSEBop Algorithms

In the Alps, understanding how climate change is affecting evapotranspiration (ET) is relevant due to possible implications on water availability for large lowland areas of Europe. Here, changes in ET were studied based on 20 years of MODIS data. MOD16 and operational Simplified Surface Energy Balance (SSEBop) products were compared with eddy-covariance data and analyzed for trend detection. The two products showed a similar relationship with ground observations, with RMSE between 0.69 and 2 mm day−1, and a correlation coefficient between 0.6 and 0.83. A regression with the potential drivers of ET showed that, for climate variables, ground data were coherent with MOD16 at grassland sites, where r2 was 0.12 for potential ET, 0.17 for precipitation, and 0.57 for air temperature, whereas ground data agreed with SSEBop at forest sites, with an r2 of 0.46 for precipitation, no correlation with temperature, and negative correlation with potential ET. Interestingly, ground-based correlation corresponded to SSEBop for leaf area index (LAI), while it matched with MOD16 for land surface temperature (LST). Through the trend analysis, both MOD16 and SSEBop revealed positive trends in the south-west, and negative trends in the south and north-east. Moreover, in summer, positive trends prevailed at high elevations for grasslands and forests, while negative trends dominated at low elevations for croplands and grasslands. However, the Alpine area share with positive ET trends was 16.6% for MOD16 and 3.9% for SSEBop, while the share with negative trends was 1.2% for MOD16 and 15.3% for SSEBop. A regression between trends in ET and in climate variables, LST, and LAI indicated consistency, especially between ET, temperature, and LAI increase, but low correlation. Overall, the discrepancies in the trends, and the fact that none of the two products outperformed the other when compared to ground data, suggest that, in the Alps, SSEBop and MOD16 might not be accurate enough to be a robust basis to study ET changes.

Castelli M. Evapotranspiration Changes over the European Alps: Consistency of Trends and Their Drivers between the MOD16 and SSEBop Algorithms. Remote Sensing. 2021; 13(21):4316.

Agricultural Drought Detection with MODIS Based Vegetation Health Indices in Southeast Germany

Droughts during the growing season are projected to increase in frequency and severity in Central Europe in the future. Thus, area-wide monitoring of agricultural drought in this region is becoming more and more important. In this context, it is essential to know where and when vegetation growth is primarily water-limited and whether remote sensing-based drought indices can detect agricultural drought in these areas. To answer these questions, we conducted a correlation analysis between the Normalized Difference Vegetation Index (NDVI) and Land Surface Temperature (LST) within the growing season from 2001 to 2020 in Bavaria (Germany) and investigated the relationship with land cover and altitude. In the second step, we applied the drought indices Temperature Condition Index (TCI), Vegetation Condition Index (VCI), and Vegetation Health Index (VHI) to primarily water-limited areas and evaluated them with soil moisture and agricultural yield anomalies. We found that, especially in the summer months (July and August), on agricultural land and grassland and below 800 m, NDVI and LST are negatively correlated and thus, water is the primary limiting factor for vegetation growth here. Within these areas and periods, the TCI and VHI correlate strongly with soil moisture and agricultural yield anomalies, suggesting that both indices have the potential to detect agricultural drought in Bavaria.

Kloos S, Yuan Y, Castelli M, Menzel A. Agricultural Drought Detection with MODIS Based Vegetation Health Indices in Southeast Germany. Remote Sensing. 2021; 13(19):3907.

An inventory of Alpine drought impact reports to explore past droughts in a mountain region

Drought affects the European Alpine mountain region, despite a humid climate. Droughts' damaging character in the past and increasing probability in future projections call for an understanding of drought impacts in the mountain regions. The European Drought Impact report Inventory (EDII) collects text reports on negative drought impacts. This study presents a considerably updated EDII focusing on the Alpine region. This first version release of an Alpine Drought Impact report Inventory (EDIIALPS) classifies impact reports into categories covering various affected sectors and enables comparisons of the drought impact characteristics. We analysed the distribution of reported impacts on the spatial, temporal and seasonal scale and by drought type for soil moisture drought and hydrological drought. For the spatial analysis, we compared the impact data located in the Alpine region to the whole of Europe. Furthermore, we compared impact data between different climatic and altitudinal domains (the northern region vs. the southern region and the pre-Alpine region vs. the high-altitude region) and between the Alpine countries. Compared to the whole of Europe, in the Alpine region agriculture and livestock farming impacts are even more frequently reported, especially in the southern region. Public water supply is the second most relevant sector but overall less prominent compared to Europe, especially in spring when snowmelt mitigates water shortages. Impacts occur mostly in summer and early autumn, with a delay between those impacts initiated by soil moisture and those initiated by hydrological drought. The high-altitude region shows this delay the strongest. From 1975 to 2020, the number of archived reports increases, with substantially more impacts noted during the drought events of 1976, 2003, 2015 and 2018. Moreover, reported impacts diversify from agricultural dominance to multi-faceted impact types covering forestry, water quality, industry and so forth. Though EDIIALPS is biased by reporting behaviour, the region-specific results of negative drought impacts across the water-rich European mountain region demonstrate the need to move from emergency response to prevention and preparedness actions. These may be guided by EDIIALPS' insights to regional patterns, seasons and drought types.

Stephan, R., Erfurt, M., Terzi, S., Žun, M., Kristan, B., Haslinger, K., and Stahl, K.: An inventory of Alpine drought impact reports to explore past droughts in a mountain region, Nat. Hazards Earth Syst. Sci., 21, 2485–2501,, 2021.

Alpine drought impact chains for sector-based climate-risk assessments

Droughts are slow and silent natural hazards that can lead to long-lasting environmental, societal and economic impacts. Mountain regions are also experiencing drought conditions with climate change affecting their environments more rapidly than other places and reducing water availability well beyond their geographical locations. These conditions call for better understanding of drought events in mountains with innovative methodologies able to capture their complex interplays.

Within this context, the Alpine Drought Observatory (ADO) Interreg Project aims to further improve the understanding of drought conditions in the Alpine Space, with activities spanning from the characterization of drought types’ components in five heterogeneous case studies in Austria, France, Italy, Slovenia and Switzerland. For each case study, different sectors exposed to drought, ranging from hydropower, agriculture to tourism are considered. Moreover, specific socio-economic characteristics were collected for each sector in order to better understand the main drivers leading to drought impacts.

Starting from the risk concept in the IPCC AR5, the Impact Chains (IC) methodology has been applied to characterize the hazard, exposure and vulnerability components in the ADO case studies. IC allowed to pinpoint the main factors affecting drought risk and the relevant socio-economic sectors integrating a mixed-method approach. Quantitative data collection on the hazard and exposure components were coupled with local experts’ knowledge on the main vulnerability factors (e.g., through a questionnaire). Although validation represents a critical part of drought modelling, IC analysis and results were therefor compared with information from the European Drought Impact Inventory (EDII) on local drought impacts collected from scientific publications, unions press releases and newspaper articles over a long time period.

While drought risk assessment through IC can improve the understanding of the main drought events and their underlying factors, they also provide insights to improve planning and management of future drought events in the Alpine Space.


Terzi, S., Erfurt, M., Stephan, R., Stahl, K., and Zebisch, M.: Alpine drought impact chains for sector-based climate-risk assessments, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-205,, 2020.

Assessing the options for the operational mapping of the soil moisture content in the European Alps

Soil Moisture (SM) is one of the key observable variables of the hydrological cycle and therefore of high importance for many disciplines, from meteorology to agriculture. This contribution presents a comparison of different products for the mapping of SM. The aim was to identify the best available solution for the operational monitoring of SM as a drought indicator for the entire area of the European Alps, to be applied in the context of the Interreg Alpine Space project, the Alpine Drought Observatory.

The following datasets were considered: Soil Water Index (SWI) of the Copernicus Global Land Service [1]; ERA5 [2]; ERA5-Land [3]; UERRA MESCAN-SURFEX land-surface component [4]. All four datasets offer a different set of advantages and disadvantages related to their spatial resolution, update frequency and latency. As a reference, modelled SM time-series for 307 catchments in Switzerland were used [5]. Switzerland is well suited as a test case for the Alps, due to its different landscapes, from lowlands to high mountain.

The intercomparison was based on a correlation analysis of daily absolute SM values and the daily anomalies. Furthermore, the probability to detect certain events, such as persistent dry conditions, was evaluated for each of the SM datasets. First results showed that the temporal dynamics (both in terms of absolute values as well as anomalies) of the re-analysis datasets show a high correlation to the reference. A clear gradient, from the lowlands in the north to the high mountains in the south, with decreasing correlation is evident. The SWI data showed weak correlations to the temporal dynamics of the reference in general. Especially, during spring and the first part of the summer SM is significantly underestimated. This might be related to the influence of snowmelt, which is not taken into account in the two-layer water balance model used to model SM for deeper soil layers. Low coverage in the high mountain areas hampered a thorough comparison with the reference in these areas.

The results presented here are the foundation for selecting a suitable source for the operational mapping of SM for the Alpine Drought Observatory. The next steps will be to test the potential of MESCAN-SURFEX and ERA5-Land for the downscaling of ERA5 to take advantage of the low latency of ERA5 and the improved spatial detail of the other two datasets. 

Greifeneder, F., Haslinger, K., Seyerl, G., Notarnicola, C., Zappa, M., and Zebisch, M.: Assessing the options for the operational mapping of the soil moisture content in the European Alps, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7445,, 2021.

Delayed soil-moisture and hydrological drought impacts in mountain regions

Droughts are among the most damaging natural hazards leading to a wide range of impacts also in the water-rich mountain regions. In recent years, drought conditions and impacts have been reported frequently in the European Alpine region, possibly because climate change affects mountain environments more rapidly than others. However, in mountain regions the propagation of drought signals through the hydrological cycle and into different environmental and societal impact occurrence are poorly understood, especially regarding the impacts’ seasonal patterns and delayed effects. This study aims to improve the understanding of the droughts’ characteristics and their impacts from the high elevation headwater regions down to plateau and foothill areas. Specific climate conditions in high elevation regions determine an alpine environment, economy and society that differs from the pre-Alpine regions. Subsequently, impacts are expected to vary as well and indices for drought monitoring may have to be selected specifically for such a region. The European Drought Impact Report Inventory (EDII) compiles text-reports on negative drought impacts across Europe and classifies them into 15 categories with 108 subtypes. An updated version focusing on the ‘Alpine Space’ released as EDIIALPS V1.0 contains more than 3,200 reports about drought impacts. The most relevant categories are Agriculture and livestock farming and Public water supply. This data allowed an analysis of the seasonal patterns of drought impacts in several categories for four sub-regions in the Alpine Space: pre-Alpine vs. high-elevated region, Northern vs. Southern region. Assigning the impacts subtypes to drought type, soil-moisture drought (SMD impacts) and hydrological drought (HD impacts) allowed the derivation of smoothed seasonal “impact regimes”. The peak of HD impacts occurred later in the year than the SMD impact peak, most clearly in the high-elevation region, with the latest increase of HD impacts in May and strongest decrease between November-December. This pattern is less clear for the Southern region. SMD indices and HD indices that may be used for monitoring and early warning need to be targeted to and tested for capturing these delays.


Stephan, R., Erfurt, M., and Stahl, K.: Delayed soil-moisture and hydrological drought impacts in mountain regions, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-355,, 2021.

A Remote-sensing Perspective on Evapotranspiration Trends in the Alps

This study analyzes 20 years of changes in evapotranspiration (ET) over the entire Alpine region by exploiting MODIS timeseries, with the aim to detect significant trends and examine their main drivers. The Alps are considered the “water towers of Europe” because of the ecosystem services that they provide, sustaining socio-economic security in large lowland areas. Worryingly, Alpine climate is changing fast, with temperature increasing more than twice the global average during the 20th century. This can affect the hydrological cycle, not only by causing the decrease of snow and glacier cover, but also by altering ET. In fact, ET is a key component of the water budget, as it feeds back to the atmosphere 60-80% of precipitation. Recently it has been shown that both actual ET and the evaporative demand of the atmosphere are changing in several regions, including some Alpine catchments. However, decadal studies at Alpine scale are still missing, and the drivers of these changes are not fully understood. Given the impacts that changes in ET can produce on the hydrological cycle and on the human activities that depend on water availability, it is important to quantify the entity of these changes and estimate their spatial distribution. Remote sensing offers the possibility to monitor ET on wide areas. This work exploits the 2000-2020 timeseries of the MOD16A2/A3 operative product at 500 m x 500 m spatial resolution, which is first validated at Alpine eddy-covariance stations, and then statistically analyzed for trend detection. The trend variability with altitude and land-cover is investigated. Furthermore, the influence of atmospheric demand and vegetation activity, for which Leaf Area Index (LAI) is considered here as a proxy, is studied. First results show a significant positive yearly ET trend in 32% of the Alpine vegetated area, up to 6.4 mm year-1 at altitudes between 1000 and 1500 m. Monthly ET is increasing up to 1.3 mm month-1 year-1 in summer months, especially for grasslands and shrublands. Trends in yearly atmospheric demand explain up to 80% of ET increase, but they are occurring only in limited areas (6% of the area with positive ET trend). Conversely, LAI trends are less influential, but they are concurrent with ET trends all over the Alpine region. Next step will be the analysis of the influence of water supply on ET trends.


Castelli M.: A Remote-sensing Perspective on Evapotranspiration Trends in the Alps, AGU fall meeting 2020, online, 1-17 Dec