Drought in mountain regions

Abstract/Description

We assess the impact of two recent meteorological summer droughts ‒ 2018 and 2022 ‒ on the hydrology of Swiss catchments. Both summers featured precipitation deficits of ~30% and temperature anomalies of +1.5°C, relative to the 1994-2023 average. To analyse these events, we perform a detailed numerical simulation of Switzerland’s hydrological cycle from 2016 to 2023 using the mechanistic eco-hydrological model Tethys-Chloris (T&C). T&C calculates energy and mass fluxes using physics-based representations of hydrological, biospheric and cryospheric processes at 250 m spatial resolution and hourly time steps, providing spatially-distributed estimates of runoff, evapotranspiration, snowmelt, ice melt, vegetation productivity, and others. T&C is forced with observed meteorological hourly data, and uses up-to-date information on land cover, soil and glacier properties. Model results are consistent with observations of streamflow, snow variables, glacier mass balance, LAI, soil moisture, and fluxes of CO2, water and energy.

Although both summer droughts led to widespread runoff deficits, model results show that the relatively wet winter of 2018 mitigated summer drought impacts compared to 2022, which followed a snow-poor winter. Above-average snow accumulation in winter 2018 resulted in a +100 mm w.e. spring snowmelt anomaly across the Alpine region (relative to the 2016-2023 average), whereas spring 2022 had a −50 mm w.e. anomaly. The 2022 winter snow deficit triggered >50 mm w.e. of excess glacier meltwater in early summer, whereas only a weak positive anomaly was found in late summer 2018. Soil moisture deficits in 2018 were mostly limited to the Central Plateau, reaching values down to −35%, whereas in 2022 anomalies were more widespread ranging from −15% to −35%. Summer evapotranspiration remained near-average in 2018 but showed a +50 mm anomaly in 2022, especially above 1000 m a.s.l. We show that detailed representation of the hydrological cycle in T&C enables a comprehensive understanding of the widespread effects of a summer drought over Switzerland, including the complex and interconnected responses of rivers, vegetation and the cryosphere.

Abstract/Description

Another low snow year in the European Alps marks the rapid decline in Alpine snowpack and fuels the emerging topic of snow droughts. Such snow droughts can have serious hydrological consequences, as highlighted by the prolonged hydrological drought in northern Italy in 2022. Snow storage is expected to decrease and the number of snow droughts to increase in response to rising temperatures. However, it is not yet clear whether a decline in snow storage and the occurrence of low-snow years are solely caused by rising temperatures or whether changes in precipitation patterns also play an important role.
Here, we use gridded snow products for Switzerland and Austria to quantify (1) changes in catchment snow water equivalent (SWE), since 1961 and attribute these changes to SWE deficits from low to high elevation bands; (2) the occurrence, dynamics, and meteorologic drivers of low-snow years, i.e. years with annual maximum SWE below the 30th percentile; and (3) the persistence of low-snow conditions and the predictability of annual maximum SWE.
Our results show a median loss of total annual catchment SWE of approx. 20 % across 251 catchments in Switzerland and Austria over the period 1962-2023, with a marked regime shift at the end of the 1980s. All elevation ranges experience a loss in SWE, but most of the catchment SWE loss (approx. 60%) can be attributed to the loss in mid-elevation SWE (1200-2100m). Further, we see an increase in the fraction of area under low-snow conditions in all elevation bands. Thereby, low-snow years are connected to precipitation deficits, especially at higher elevations (>2100m). At lower and mid-elevations, warm temperature anomalies are additionally important to explain low-snow years. Further, low-snow years are characterized by a high persistence of low-snow conditions, starting three months (median) before the climatological period of maximum SWE. SWE deficits are to large parts accumulated during November-January and by January there already exists a 60% chance of predicting the SWE conditions at the end of winter . These results showcase the potential for a better seasonal snow prediction and will help to constrain future projections of SWE and associated hydrological impacts.

Abstract/Description

The Mediterranean basin is one of the areas most exposed to drought and paradigmaticly highly dependent on water resources in its socioeconomic development. Its climate is naturally characterized by a high variability that is being exacerbated by the current climate warming situation. In this sense, future projections agree that the frequency and severity of these extreme events will increase. In addition, the Mediterranean basin is delimitated by considerable mountain ranges close to the sea that draw different-sized catchments in which the presence of snow constitutes a significant proportion of their total water resources. Hence, it seems obvious that snow dynamics need to be considered when analyzing droughts. However, drought indexes, which are the most extended tool to characterize droughts, do not explicitly account for snow. This work aims to directly include snow dynamics in defining drought over these catchments. To achieve this, we introduce the concept of snow drought using snowfall as the target variable to propose a new standardized drought index, Standardized Snowfall Index (SSNI), in the whole region. The index definition is based on the methodology already proposed when defining other drought indexes, such as the Standardized Precipitation Index (SPI) or the Standardized Streamflow Index (SSI), evaluating different candidate distributions and aggregation times. HydroGFD3 bias-adjusted reanalysis data for precipitation and temperature for the last five decades are used in the study. The new index was evaluated in 18 pilot catchments along the region. The results show that the candidate distribution selected differed depending on the location and aggregation time. This new index helps better quantify the effect of a snow deficit in the meteorological drought definition and its implication throughout the drought propagation cascade over a region highly exposed to drought events.