Hysteresis between groundwater and surface water levels indicates the states of hydrological turnover affecting solute transport and redox processes

Abstract. Small streams are highly sensitive to variations in discharge, a sensitivity predicted to increase in future climate scenarios, impacting ecological health of streams and water management practices. Prolonged low-flow conditions alter groundwater-surface water (GW-SW) exchange patterns, leading to extended losing phases and a reduced duration of gaining periods. This study examines the relationship between hydrological turnover (HT) and hysteresis patterns under various system states in a third-order tributary of the River Mosel in Trier, Germany, using high-resolution hydrological and chemical data collected over two years. Our results reveal distinct seasonal dynamics in GW-SW exchange. Counterclockwise hysteresis, prevalent during summer and drought conditions, was linked to the expansion of the hyporheic zone and bank storage, which reorganizes flow paths and influences redox dynamics. We established a strong correlation between HT and hysteresis characteristics, identifying the h-index as a valuable diagnostic tool for tracking seasonal changes in GW-SW connectivity, storage and hyporheic zone behavior based on hydraulic preconditions. As climate change intensifies drought conditions, the hyporheic zone will play a vital role in solute cycling and GW-SW connectivity. The h-index, combined with chemical and hydrological monitoring, provides a robust framework for understanding and predicting these dynamics in small stream ecosystems.