Assessing snowfall droughts in mediterranean mountain catchments

The Mediterranean Basin is a region naturally prone to drought due to its climatic variability. These variations are being exacerbated by the current climate change situation, with projections agree that the frequency and severity of these extreme events will increase. Paradigmatically, this region has based its socioeconomic development on activities directly related to water resources, such as agriculture and tourism. In addition, the Mediterranean basin is delimited by mountain ranges close to the sea that draw different-sized catchments where water resources availability is directly linked to snow presence. Therefore, snow dynamics need to be considered when analyzing droughts. Cold and warm winter droughts over these mountains have special characteristics since winter temperatures are around zero and conditions both snow accumulation and ablation directly.

However, the general drought indices, SPI (Standardized Precipitation Index) or SPEI (Standardized Precipitation-Evapotranspiration Index), which are the most widely used tools to characterize droughts, do not explicitly account for snow. New snow drought indices have been proposed, for instance, the Standardized Snow Water Equivalent Index (SSWEI). They are, on the one hand, based on snow variables typically derived from modelling and are subject to large uncertainties over Mediterranean mountain catchments; and, on the other hand, focus mainly on the ablation process. This work proposes to define a new drought index, introducing the concept of snowfall drought in Mediterranean mountain regions. Then, snowfall is the target variable used to define the drought index, the Standardized Snowfall Index (SSNI). The index definition is based on the methodology already proposed when defining other drought indices, evaluating, in this case, eight different candidate distributions, and standardizing their probability using a Normal distribution. The aggregation time selected for the snowfall series was 12 months. HydroGFD3 bias-adjusted reanalysis data (daily time-step and 25 km spatial resolution) for precipitation and temperature during the period 1980-2024 are used in the study. Snowfall is determined using a variable temperature thresholding over the whole Mediterranean Basin (2266 catchments). The SSNI was evaluated against the SPI index to assess the differences in detecting drought between the two indices.

The results show that the candidate distribution selected differed depending on the location of the catchment. That is, the Gamma distribution was the best at capturing the snowfall drought dynamics in high elevation catchments, Log-Logistic in the eastern Mediterranean catchments, and Weibull in the western ones. The number of drought periods also differed spatially, ranging from 0 to 22 episodes with a duration between 0 and 20 months, and a clear relationship between both: the longer the duration, the smaller its frequency. In addition, this new index helps better quantify the effect of a snow deficit in the meteorological drought definition - from all months identified with a drought, 38% of them would not have been classified as drought months, only accounting for precipitation and not for snowfall - and consequently to better understand the drought propagation cascade over the region.

Acknowledgments: This work is part of the project CNS2023-145125, funded by MCIN/AEI/10.13039/501100011033 and European Union “NextGenerationEU”/PRTR.