Acceleration of rises in the level of rivers induced by rising air temperatures in a cold climate.

The effects of climate change on water discharge in rivers in cold climates were investigated. To quantify the impacts of air temperature rises on the promotion of snowmelt and associated acceleration of a rise in the level of a river, 10 rivers on Hokkaido, northwestern Japan were chosen. Available data of daily water discharge for more than half a century by the Ministry of Land, Infrastructure, Transport and Tourism (MLIT) were correlated with air temperatures observed at the 8 weather stations operated by the Japan Meteorological Agency (JMA). The analyses show that annual average air temperatures have risen at all 8 sites on Hokkaido for the period from 1954 to 2018. The trends for the 8 sites show a range from 0.119 °C/decade to 0.250 °C/decade with an average of 0.191 °C/decade. Annual snowfall has increased at Sutsu. The trend over the period from 1954 to 2018 was 45.2 cm/decade. Otherwise, annual snowfall has not changed significantly in the period (1954-2018). Accelerations of the rise in the level of the river induced by air temperature rises have been observed in the Teshio River, Ishikari River, Saru River, and Mu River. A delay in the rise in the level of the river caused by an increase in snowfall and the weak warming trend from January to April has been observed in the Shiribetsu River. Although air temperatures have risen year after year, a significant change in annual pattern of daily flow has not been observed in the Syokotsu River, Yubetsu River, Tokoro River, and Abashiri River located in eastern Hokkaido. This can be induced by the weak warming trends in April which may be due to the cold current of the sea of Okhotok.