Thermal regimes in a monsoon-influenced mountainous river basin: The Nagara River, Japan

Study region River water temperature ( T w ) variability was investigated in the Nagara River basin, central Japan, a monsoon-influenced mountainous watershed with a near-natural mainstem flow regime and intensive lowland paddy agriculture. Study focus From 2020–2021, T w was measured at 54 mainstem and tributary sites. These observations were integrated with daily air temperature ( T a ) and river discharge ( Q ) estimated using high-resolution climate data and a distributed hydrological model. We analysed air–water temperature coupling, tributary thermal effects, and discharge-dependent summer thermal responses. New Hydrological Insights for the Region T a – T w regression revealed increasing thermal coupling downstream along the mainstem, indicating reduced hydrological and groundwater buffering. Tributaries showed diverse thermal behaviours associated with groundwater contribution and land use. Groundwater-fed tributaries cooled and stabilised mainstem T w , whereas paddy irrigation return flows were associated with localised warming under summer low-flow conditions. Rainfall-driven high flows reduced T w and promoted spatial homogenisation across the river network, while summer low-flow and drought conditions amplified warming and increased spatial thermal heterogeneity, especially in downstream and agriculturally influenced reaches. These responses highlight the combined roles of climate, hydrology, groundwater, and agricultural water use in shaping river thermal regimes. The results support climate adaptation and river ecosystem management in monsoon-region basins by identifying contrasting thermal roles of groundwater-fed tributaries and agriculturally influenced lowland reaches.