Hydrologic connectivity and source heterogeneity control concentration–discharge relationships

Abstract

Changes in streamwater chemistry have frequently been used to understand the storage and release of water and solutes at the catchment scale. Streamwater chemistry typically varies in space and time, depending on sources, mobilization mechanisms, and pathways of water and solutes. However, less is known about the role of lateral hydrologic connectivity and how it may influence streamwater chemistry and solute export patterns under different wetness conditions. This study analyses long-term low-frequency data from four UK catchments using antecedent catchment wetness as proxy for lateral hydrologic connectivity. We demonstrate that solute mobilization mechanisms can vary depending on catchment wetness, as different catchment areas become hydrologically connected to or disconnected from streams. We show that flow and streamwater chemistry are mostly decoupled under dry conditions, leading to stronger impacts of the heterogeneity in solute sources on mobilization patterns during dry conditions compared to wet conditions. Our results demonstrate that the lateral and vertical distributions of solutes need to be integrated and considered together with the temporally variable hydrologic connectivity of these lateral areas to the stream when assessing streamwater chemistry. This combined analysis thus enables inferences regarding the lateral distribution of solutes throughout the catchment; it also indicates that a better understanding of the relationship between lateral hydrologic connectivity and the lateral and vertical distributions of solute concentrations can help to identify particularly vulnerable points in the catchment and their potential polluting effects on streams.