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 highaltitude 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.

1 Impact categories and subtypes Table S1. All impact categories and subtypes defined by Stahl et al. (2016) and their assignment to soil-moisture drought (DSM) and hydrological drought (DH).

Impact category Impact subtype
Drought type ID Description DSM DH Agriculture and livestock farming 1.1 Reduced productivity of annual crop cultivation: crop losses, damage to crop quality or crop failure due to dieback, premature ripening, drought-induced pest infestations or diseases etc. X 1.2 Reduced productivity of permanent crop cultivation X 1.3 Agricultural yield losses ≥ 30% of normal production (EU compensation treshold) X 1.4 Reduced availability of irrigation water X 1.5 Reduced productivity of livestock farming (e.g. reduced yields or quality of milk, reduced stock weights) 1.6 Forced reduction of stock(early selling/slaughtering) 1.7 Regional shortage of feed/water for livestock X X 1.8 Other 1.9 Increased costs/economic losses Forestry 2.1 Reduced tree growth and vitality X 2.2 Decrease in annual non-timber products from forest trees (e.g. cork, pine nuts, mushrooms, berries, etc.) 2.3 Increased occurrence of water stress indicators and damage symptoms (e.g. premature ripening, seasoning checks, defoliation, worsened crown conditions etc.) X 2.4 Increase of pest/disease attacks on trees (please specify species in the description field!) 2.5 Increased dieback of trees X 2.6 Increased dieback of planted tree seedlings (in nurseries or afforested area) X 2.7 Damage to short rotation forestry plantations (energy forestry) X 2.8 Other 2.9 Increased costs/economic losses Freshwater aquaculture and fisheries 3.1 Reduced (freshwater) fishery production (please specify fish species in the description field) X 3.2 Reduced aquaculture production (please specify fish species in the description field) X 3.3 Other 3.4 Increased costs/economic losses Energy and industry 4.1 Reduced hydropower production 4.2 Impaired production/shut down of thermal/nuclear power plants (due to a lack of cooling water and/or environmental legislation for discharges into streams) 4.3 Restriction/disruption of industrial production process (due to a lack of process water and/or environmental legislation/restrictions for discharges into streams) 4.4 Other 4.5 Increased costs/economic losses Waterborne transportation 5.1 Impaired navigability of streams (reduction of load, increased need of interim storage of goods at ports) X 5.2 Stream closed for navigation X 5.3 Other 5.4 Increased costs/economic losses Tourism and recreation 6.1 Reduced number of short-stay-tourists 6.2 Reduced number of long-stay-tourists 6.3 Sport/recreation facilities affected by a lack of water X 6.4 Impaired use/navigability of surface waters for water sport activities (including bans) X 6.5 Other 6.6 Increased costs/economic losses Public water supply 7.1 Local water supply shortage / problems (drying up of springs/wells, reservoirs, streams) X 7.2 Regional/region-wide water supply shortage/problems (drying up of springs/wells, reservoirs, streams) X 7.3 Bans on domestic and public water use (e.g. car washing, watering the lawn/garden, irrigation of sport fields, filling of swimming pools) X 7.4 Limitations in water supply to households in rural areas (supply cuts, need to ensure water supply by emergency actions) X 7.5 Limitations in water supply to households in urban areas (supply cuts, need to ensure water supply by emergency actions) X 7.6 Other 7.7 Increased costs/economic losses Water quality 8.1 Increased temperature in surface waters (close to or exceeding critical values) X 8.2 (Temporary) water quality deterioration/problems of surface waters (natural & manmade); e.g. significant change of physio-chemical indicators, increased concentrations of pollutants, decreased oxygen saturation levels, eutrophication, algal bloom) X 8.3 (Temporary) impairment of ecological status of surface waters (according to EU Water Framework Directive) X 8.4 (Temporary) impairment of chemical status of surface waters (according to EU Water Framework Directive) X 8.5 Increased salinity of surface waters (saltwater intrusion and estuarine effects) X 8.6 Problems with groundwater quality X 8.7 Increased salinity of groundwater X 8.8 Problems with drinking water quality (e.g., increased treatment, violation of standards) X 8.9 Problems with bathing water quality X 8.10 Problems with irrigation water quality X 8.11 Problems with water quality for use in industrial production processes X 8.12 Other 8.13 Increased costs/economic losses Freshwater ecosystems: habitats, plants and wildlife 9.1 Increased mortality of aquatic species X 9.2 Increased species concentration near water 9.3 Migration and concentration (loss of wildlife in some areas and too many in others) 9.4 Increased populations of invasive (exotic) aquatic species 9.5 Observation of adverse impacts on populations of rare/endangered (protected) riparian species 9.6 Observation of adverse impacts on populations of rare/endangered (protected) species of wetlands 9.7 Loss of biodiversity (decrease in species diversity) 9.8 Danger for or actual violation of minimum flow or environmental flow requirements X 9.9 Drying up of shallow water areas, weed growth or algae bloom X 9.10 Drying up of perennial stream sections X 9.11 Drying up of lakes and reservoirs (which have a habitat function) X 9.12 (Mid-/Long-term) deterioration of wetlands X 9.13 Irreversible deterioration/loss of wetlands X 9.14 Other 9.15 Increased costs/economic losses

Status of EDII from September 2019
This part presents the applied analysis described in the manuscript with the EDII before our update (i.e. status of the EDII from September 2019) to enable insights of the effect of our impact report collection focused on the Alpine Space and therefore presenting EDIIALPS. We raised the number of reported drought impacts located in the Alpine Space from 1412 to 3243. Further EDIIEU, was updated with more than 3,300 reported impacts with more than half of them by our focus on the Alpine Space (compare Fig. S1b with 2b). For the EDIIALPS we could add missing drought impact reports especially located in Austria, Slovenia, Italy and France (compare Fig. S1a with 2a, and Fig. S2b with Fig. 3b) with most reports from newspaper articles, web pages, governmental reports, scientific articles and the investigated specialty databases Unwetterchronik, DMCSEE, Drought.ch, and Propluvia). Our update altered the impact categories fractions especially in the French part (compare Fig. S1a with 2a). Reports located there were mainly on Agriculture and livestock farming, as well as on Public water supply, as the French platform "Propluvia" informed on water restrictions in drought periods most often affecting the agriculture and water supply.
Further, we added previously missed impact reports for several years, especially for the periods in-between the selected drought events and after 2010 to better cover the more recent years (compare Fig. S2b with Fig. 3b).
Regarding the selected drought events 1976, 2003, 2015, and 2018, the majority of our added reports related to 2018 and to the subtype ´Reduced availability of irrigation water' (1.4) from the category Agriculture and livestock farming. The better impact data completeness enabled the applied seasonal and spatial analysis across the study region (compare Fig. S3 with Fig.  4). Due to fewer reports before our update, the existence of one report in another subtype or category could affect the whole season. For example, before our update EDII archived one report about 'air quality pollution effects/problems' (13.1) and two reports affecting 'Changes in species biology/ecology' (10.2) in winter in the Southern region. As only six reported impacts had been archived in total for the winter season, these impacts had been represented with 50 % (see Fig. S3f). During the update, we classified almost 300 reported impacts in the Southern region occurring in winter mainly from the categories Agriculture and livestock farming, followed by Public water supply and Tourism. Subsequently, the fraction of the categories Terrestrial ecosystems and Air quality decreased substantially. Subsequently, the added impact reports strengthened the categorical representation of drought impacts especially in subregions and seasons. In addition, the updated impact collection supported the analysis on the drought type impact regimes (compare Fig. S4 with Fig. 5). Especially, the added impacts located in the Highaltitude and Southern region increased the number of DSM and DH impacts with smaller effects on the relative fraction of the different impact categories and subtypes, but with a substantial effect on the monthly development of DSM impacts and DH impacts (compare Fig. S4d,f with Fig. 5d,f). Due to our update, the maximum of the DSM impact regime shifted from autumn to summer in both domains, the DH impact regimes started already in January and both regimes showed a clearer pattern by the loess curves with smaller standard errors (compare Fig. S4d,f with Fig. 5d,f).
To conclude, the update during this study was substantial as it supported the analyses of the EDIIALPS and the comparison with the EDIIEU in all parts. Therefore, we aim to add further drought impact reports fulfilling all requirements in order to strengthen the value of the inventory further and especially at the regional and local scale to reduce potential gaps of missed impact information.   Monthly percentages relate to the sum of all impacts per month and category with 100 % corresponding to the month with most impacts. Total counts of each season are given on top of the bars, the fraction in brackets relates to the amount of impacts assigned to the season. Subtypes with a fraction ≥ 10 % per season are labeled. Colours see Fig. S1. Figure S4. Reported impacts archived in the EDII before our update (i.e. status of the EDII from September 2019). Impact subtypes assigned to DSM (yellow) or DH types (blue). DSM and DH impacts aggregated per month (line diagram) and drought type (bar plot) for the (a)EDIIALPS, (b) EDIIEU, (c) pre-Alpine region, (d) high-altitude region, (e) Northern region, (f) Southern region. Seasonal regimes for DSM (yellow lines) and DH impacts (blue lines) are loess curves with standard errors (dotted line with coloured shape). Monthly percentages (solid lines) relate to the sum of all impacts per month and subtype with 100 % corresponding to the month with most impacts. Total counts of DSM and DH impacts are given on top of the bars, the fraction in brackets relates to the amount of impacts assigned to the season. Subtypes with a fraction ≥ 5 % are labeled. Colours see Fig. S1.