Repeated measurements of underground temperature were performed in the alluvial fan of the Nagara River to clarify the distribution of underground temperature, its seasonal change and its influence on the coefficient of performance (COP) of the geothermal heat pumps. The alluvial fan of the Nagara River is located in Gifu City, and 17 boreholes with 30 m or less length were selected for measurements. Underground temperatures were measured in an interval of 1 m along the boreholes once a month from May 2013 to May 2014. The characteristics of the temperature profile of the boreholes are different depending on the distribution of the groundwater flow layers. Type A of the temperature profile is characterized by the almost constant temperature along the borehole in each measurement and temperature differences among the seasons, indicating that a groundwater flow layer is distributed at least to the bottom of the borehole. Type B shows two different layers of temperature change, suggesting that two different groundwater flow layers are included in the boreholes.

Type C exhibits one layer of the temperature change in the middle part and the almost constant temperature along the borehole in all measurements in the deeper part, showing that one groundwater flow layer is included within the limited depth range. The seasonal temperature changes are assumed to be a sine curve with different parameters, i.e., an annual average temperature, an annual temperature fluctuation and a phase differences with the river temperature. In the southern side of the Nagara River, the increase of annual average temperature, the decrease of annual temperature fluctuation and the increase of phase differences are recognized toward the toe of the alluvial fan. These lines of evidence suggest that the masses of warm and cool water flow in the aquifer accompanied by heat transfer to the outside of the aquifer. These characteristics are not appeared clearly in the northern side of the river relative to the southern side. This implies that underground temperature distribution in the northern side is disturbed by the channel flows along the old river channels and the inflow of groundwater