LONG-TERM TRENDS OF EQUILIBRIUM WATER TEMPERATURE AND THEIR FACTORS IN JAPAN

Abstract

 A long-term (63-year; 1961-2023) trend analysis of meteorological observation data in Japan revealed a statistically significant increase in equilibrium water temperature, which is an indicator of water temperature in rivers and lakes. This increase was found to be driven by two main factors: increases in solar radiation and air temperature. A seasonal analysis of the long-term trend of equilibrium water temperature and its contributing factors revealed that equilibrium water temperature exhibited an overall increase across all seasons. However, the magnitude of this increase varied considerably between seasons and regions. The contribution of increased solar radiation and increased air temperature to changes in equilibrium water temperature exhibited seasonal and regional variability.