River water temperature variability: classification, diurnal change, and precipitation impacts in Nagara River tributaries

Abstract

River water temperature patterns show diurnal and annual oscillatory behavior mainly due to the energy exchange between atmosphere and river water. In addition to this, rainfall runoff events can also contribute river temperature patterns. As a result of superposition of these factors, river water thermal regime shows complex patterns. In this study, we attempted to characterize the thermal regime of 16 tributaries of the Nagara River Basin by classifying them into atmospheric-dominated, groundwater-dominated, and intermediate. This classification was based on seasonal oscillatory behavior extracted from river and air temperature data during the period from 2017 to 2019, specifically using annual air–water temperature metrics, including the amplitude ratio and phase lag. Diurnal temperature variations standardized against air temperature, as well as seasonal responses to rainfall events, were then examined to validate and support the robustness of this classification, which revealed consistent thermal behavior across the identified classification types. A preliminary hydrogeographic interpretation indicates that land-use patterns and bedrock composition have a possible influence on the hydro-thermal regimes. The relative importance of groundwater inflow as a stable heat source and atmospheric heat exchange might be important in determining the thermal regime.