Including water quality in drought definition for a better assessment of water scarcity situation in Mediterranean highly human modified catchments.

Mediterranean river basins are characterized by high hydroclimatic variability and the recurrence of drought episodes that strongly condition water resources availability and management. In these systems, water scarcity situations are not only triggered by a reduction in water quantity but, may also be conditioned by a bad water quality that compromises multiple uses. This was the case of the water crisis experienced in the northern part of the Córdoba province during 2023, when, more than 80,000 people did not have running water at home for more than a year. The main reservoir supplying water to the population, Sierra Boyera Dam, was dried out and the safeguard water coming from La Colada reservoir did not fulfill the requirements for their consumption due to high rates of arsenic, TOC, and nitrogen. Therefore, early detection of these situations is particularly relevant in regulated basins, where drought develops progressively and with non-simultaneous responses across the system components due to storage effects and management practices.

This work uses this water crisis as case study to propose to include water quality in the definition of droughts. For that a Combined Water Scarcity Index (CWSI) that combines meteorological, agricultural and hydrological drought with a fourth component explicitly linked to water quality is proposed. Conventional drought indexes (i.e., Standardized Precipitation Index – SPI -, Standardize Soil Moisture Index – SSMI – and Standardized Streamflow Index – SSI) are used to account for the different types of droughts. In the case of water quality, the potential non-point pollution index modified monthly by precipitation patterns is adapted to be used as a fourth component of the combined drought index. The period 1960-2024 has been used for defining this new combined index. A comparative analysis has been carried out to assess the impact of including the potential affection of water quality in water scarcity definition and some specific cases know in the area were used as validation datasets for the methodology proposed.

The results show that the inclusion of water quality in drought assessment increases the number of water scarcity situations identified. Therefore, CWSI enables the early identification of alert states and tipping points relevant for decision-making to deal with water scarcity situations, highlighting its potential as a tool for foreseeing water scarcity episodes. In addition, our results showed that those areas with high water pollution potential risk, predominantly associated with zones of intensive agriculture, livestock farming, and urban development, had to face more frequent water scarcity situations and water shortages due to poor water quality in reservoirs.

Acknowledgements: This study has been funded by the call “Grants to develop innovative solutions to address drought, within the framework of the PLAnd Drought Andalusia. 2023 Call” through the project PLSQ-00172-F – “Service for the early detection of alert states in water management under scarcity conditions” (SEGA)