Water temperature dynamics and heat transport in a typical Japanese river

摘要

The water temperature regime of the Oita River on Kyushu Island (33 degrees N latitude) in southern Japan is illustrated and analyzed, and related to the river's hydrology. The river is short (55 km) and steep (ave. slope = 0.088), and has a flashy flow regime. Precipitation is high (1653 mm year(-1)) and divided between a wet monsoon season, typhoons, and a dry season in winter. A base flow analysis showed that groundwater contributes about 50 % of the annual flow to the Oita River and controls the river's heat budget. At a measured mean annual temperature of similar to 16 degrees C, groundwater (base flow) input causes unexpected cool (<= 24 degrees C) river temperatures in summer and warm (>= 8 degrees C) river temperatures in winter. The Oita River is typical of many Japanese rivers. Its hydraulic residence time is very short (0.5-1.5 days) and limits the heat exchange with the atmosphere so that a significant deficit remains between observed river water temperatures and equilibrium temperatures in mid-summer and mid-winter. Correlations of recorded stream temperatures with atmospheric temperatures (air, dew point and equilibrium temperatures) were strong. Weekly air and stream temperature data (averaged from 10-min measurements) were fitted by linear regressions with R-2 = 0.98 and RMSE = 0.83 degrees C. Daily data were only slightly less well correlated with R-2 = 0.96 and RMSE = 1.17 degrees C. The relationships were highly linear. The slope of the stream temperature vs. air temperature correlation was in the range from 0.63 (weekly) to 0.58 (daily) indicating that stream temperatures are dominated by groundwater input. Air and stream temperatures have a strong correlation in both wet (monsoon) and dry seasons, and geothermal heat input has no basin-wide effect; the 50 % groundwater (base flow) input raises the annual mean and reduces seasonal temperature amplitude in the Oita River and provides a cool-water stream. Water temperatures were gathered from a long-term monitoring database and by recent (2012-2013) high-resolution instream monitoring. Key drivers of stream temperature were identified. Linear regressions were used to relate air temperature, dew point temperature, and equilibrium temperature with stream temperature. Groundwater was identified as a key input of water consistently around 16 degrees C, moderating temperatures in the summer and winter due to very short water residence times in the steep and short river. This paper contributes to the body of knowledge about global stream temperatures, and clarifies specific questions for Japanese rivers.