Small Weirs Reshape the Hyporheic Zone: Cumulative Thermal Responses across Seasonal operation cycle

Small weirs are ubiquitous in agricultural watersheds, yet how their repeated seasonal operation reshapes groundwater–surface water interactions (GSI) remains poorly understood. This study explores the hydraulic, thermal, and geochemical responses to cyclic opening and closure of a small intake weir throughout the monitoring period along a 2.5-km reach of the Hyogyo Stream, South Korea. High-frequency monitoring of water level and electrical conductivity, together with multi-depth subsurface temperature observations, was conducted at three locations upstream of the weir within the study reach under both open and closed conditions. These continuous observations were complemented by four discrete δ¹⁸O–δD isotope surveys. Water level, electrical conductivity, isotope values, and subsurface temperature all captured individual weir-opening and -closure events. These event-scale responses included water-level rise, channel inundation, and transient flooding of streambanks and were accompanied by electrical-conductivity and isotope shifts indicative of enhanced surface water–subsurface water mixing. During the initial closure, diurnal thermal signals penetrated to depths of 3.5–4.0 m, indicating an expanded hyporheic exchange zone. Unlike other parameters, thermal signals at depth (3.5–4.0 m) progressively converged to a stable temperature with repeated weir opening and closure, implying contraction and reorganization of the active exchange zone despite comparable hydraulic forcing. These cumulative thermal responses may induce long-term restructuring of the hyporheic thermal regime. Our findings highlight the need to consider small weirs in integrated water management, particularly by emphasizing long-term thermal monitoring in agricultural watersheds.

Acknowledgement: This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2022R1A2C1006696). This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT)(No.RS-2022-NR070842). This work was supported by a grant from the National Institute of Environmental Research (NIER), funded by the Ministry of Environment (ME) of the Republic of Korea (NIER-2025-04-02-051).