Assessing stream water scarcity and groundwater roles under global change in a Mediterranean watershed: the Onyar River basin (NE Catalonia, Spain)

Mediterranean basins face significant water scarcity which requires examining long-term data to evaluate their trends in water availability and quality and assess management options. In this presentation, we explore the historical streamflow changes, the influencing climatic —streamflow, precipitation, temperature, and evapotranspiration (PET and AET)— and land-use factors, and the evolution of surface water quality in the Onyar River (Inner Catalan basins, NE Spain; 295 km²) during the last decades (1960-2020).

Results highlight a consistent decline in streamflow, most pronounced over the last two decades, accompanied by an increase in PET, and a probable decrease in groundwater recharge. These changes co-occurred with higher concentrations of river water ammonium and nitrate. We attribute these patterns to changes in land use such as afforestation and intensive fertilization, as well as increased groundwater withdrawal, particularly during irrigation seasons. Additional factors include growing urban water demand and the discharges of treated wastewater back into the river system. Evaluation of the relationship between groundwater and surface water using end-member mixing analysis of hydrochemical data points out an interesting scale-dependence behaviour: groundwater baseflow from alluvial formations was relevant in the smallest subbasins, whereas regional groundwater flow involving deeper aquifers could significantly contribute to stream discharge in the lowest zones of the basin. Since water balance alteration in the future climate scenarios will reduce the contribution of the headwater flow as well as groundwater storage and baseflow generation, reclaimed wastewater shows up as a relevant source to maintain stream runoff, yet its quality is low and might not be properly diluted by rainfall originated runoff.

These observations provide a comprehensive overview of the declining water quantity and quality in the Onyar River network, attributing these trends to an interplay of climatic and anthropogenic factors. They urge for integrated water resources management strategies to mitigate the implications of these environmental changes, such as protecting baseflow generating areas as well as controlling reclaimed wastewater quality.

Funding: G. Córdoba-Ariza acknowledges funding from Secretariat of Universities and Research from Generalitat de Catalunya and European Social Fund for her FI fellowship (2022 FI_B1 00105).