Using droughts indicators as triggers for water resources management in semiarid mountain regions

In the current context of global warming, droughts frequency and severity have increased in the Mediterranean Region. The past hydrological year, 2022-2023, was a clear example of water scarcity after some years with precipitation below the historical mean threshold. In mountain catchments, this reduction in precipitation has resulted in a significant decrease of the seasonal snow and a shift in the common snowfall patterns. The coastal-mountain catchments in the Sierra Nevada mountain range (southern Spain) exemplify this situation. 

The use of drought indices, which are defined using hydrometeorological information, has been the most used tool for the development of warning systems and the definition of adaptation strategies. Indexes like the Standardised Precipitation Index (SPI) or the Streamflow Drought Index (SSDI), have been widely used when characterising both meteorological and hydrological droughts. However, in high mountain areas, the role of snowfall should also be taken into account in this index definition. Snowfall patterns clearly modifies the precipitation-runoff response on a seasonal basis, changing the water balance at different time scales. Therefore, “snow drought” might result in scarcity conditions even though no warning stage has been reached regarding drought’s alerts yet, and it should also be taken into account in the defintion of these indexes. Furthermore, the intrinsic characteristics of the snow cover in these regions: seasonality, with snow generally present from mid-autumn to mid-spring; low thickness and high density; various accumulation-ablation cycles throughout the year; and, high losses due to evaposublimation, make the specific definition even more necessary.

This work aims to characterise snowfall droughts in semiarid mountains, understanding its connection to precipitation and hydrological droughts, assessing the viability of using drought indexes as tools for a better water-management decision-making. The Guadalfeo Catchment in the Sierra Nevada Mountain Range has been chosen as a representative coastal-mountain catchment of the Mediterranean basin to carry out this analysis.

Both SPI and a Standardised Snowfall Index (SSI, defined as SPI but using snowfall data) were calculated in the study area on different time scales for a reference period of 40 years (1960-2020), together with SSDI from the available streamflow time series. The joint analysis of SSI and SPI on each time scale has allowed us to classify the four potential situations in relation to the occurrence of hydrological drought in the study catchments. The results show the relevant seasonality of snowfall droughts in this area, and the importance of persistent precipitation drought as antecedent conditions for the impacts of low-snow years on the spring and summer streamflow. The validation performed points to an increase of the annual variability of the snowfall regime, very much related to a higher torrentiality of the precipitation regime on an annual basis than to changes in temperature.

Acknowledgement: This research was funded by the Spanish Ministry of Science and Innovation through the research project PID2021-12323SNB-I00, HYPOMED—“Incorporating hydrological uncertainty and risk analysis to the operation of hydropower facilities in Mediterranean mountain watersheds”.