Author(s): Morihiro Harada; Shigeya Nagayama; Ryoji Fujii; Masanao Sueyoshi

Linked Author(s): Morihiro Harada

Keywords: River water temperature thermal regime agricultural irrigation monsoon Asi

Abstract: This study explores the thermal regime characteristics of the Nagara River, a monsoon-influenced watershed in Japan with minimal flow regulation. By analyzing the relationships among air temperature (Ta), water temperature (Tw), and river discharge (Q), we identified key dynamics in river water temperature variations. The findings highlight that water temperature increases downstream along the mainstem, with a regression slope between Ta and Tw approaching 1.0. However, cooling effects were observed at confluences with relatively cold tributaries during summer, while some tributaries with extensive rice paddies increased mainstem temperatures. The study also reveals significant influences of watershed geology and land use. For instance, tributaries fed by limestone springs exhibited minimal Tw fluctuations, indicating a strong relationship with groundwater. During summer droughts, elevated river Tw approached Ta, attributed to water warmed in rice paddies discharged back into the river. These results underscore the diversity of thermal environments within a single river basin and the complex interplay of natural and anthropogenic factors shaping the thermal regime. This study contributes to a better understanding of river thermal dynamics, with implications for sustainable river management and ecosystem conservation in monsoon Asia.

DOI:

Year: 2025