Streamflow Generation Processes and Stream Intermittency in a Mediterranean Basin: Insights From End-Member Mixing Analysis

Abstract

Climate change, land use, and resource exploitation increasingly threaten river hydrology and water quality, leading to scarcity. Non-perennial or intermittent rivers are vulnerable systems characterised by highly variable flows. While recent research has advanced understanding of their natural variability, integrating the complex effects of multiple human impacts at the basin scale remains challenging. Aiming to improve this understanding and supporting effective management, this study uses end-member mixing analysis (EMMA) to identify streamflow sources in an intermittent Mediterranean river network, based on hydrochemical data from 23 sites over 14 monthly surveys and potentiometric data from six shallow wells. The analysis identifies three main contributors to streamflow: headwater runoff, groundwater baseflow from aquifers and hillslopes and treated wastewater. Their relative contributions varied spatially and seasonally. Upstream of urban areas, headwater runoff dominated (71%), while in the central catchment, groundwater contributed 15%–75%. Downstream of industrial areas, contributions were more balanced: 35% runoff, 27% groundwater and 38% wastewater. Alluvial groundwater was more important in small upstream subbasins in terms of maintaining the connectivity of the streamflow, whereas groundwater contribution from deeper aquifers dominated in lower reaches. EMMA results also highlighted discrepancies with point potentiometric data, emphasising the limitations of local measurements for understanding basin-scale hydrology. Under future climate scenarios, reduced headwater runoff and groundwater storage are expected to lower baseflow, increasing reliance on treated—but lower-quality—wastewater to sustain streamflow. This highlights the need for long-term monitoring and management, with a focus on protecting baseflow sources and improving wastewater quality to ensure water sustainability.