Long-term relationship between air and water temperatures in Lake Paldang, South Korea

A long-term investigation into the relationship between air and water temperatures was conducted in Lake Paldang, which is the largest water source in South Korea, by studying hysteresis. From 1973 to 2018, the annual mean air temperature increased by 0.05 degrees C/yr (seasonal Sen's slope). The results of a numerical model (R > 0.86) showed that the ratios of the air and water temperatures increased (0.71-0.77) in the rising limb and decreased (0.70-0.76) in the falling limb. However, the intercept values were 0.13-3.52 and 6.62-7.78 in the rising and falling limbs, respectively, and hence there was a 4-5 degrees C increase in temperature. In particular, in 2015, 2016, and 2018, the intercept values in the falling limb were >= 7, exhibiting hysteresis, whereby high water temperatures were slow to decline. Lake Paldang showed stronger water temperature hysteresis than its influent rivers and stream. The rising and falling limbs did not show a large difference in the extent of water temperature change (slope). However, the water temperature did not decrease rapidly, and the decrease continued for longer due to hysteresis, which is a type of inertia where the elevated temperature persists if the summer air temperature is significantly increased.