In ground source heat pump (GSHP) systems, large imbalances between cooling and heating loads cause a rise or decline in ground temperature because of thermal interference between multiple ground heat exchangers (GHEs). To evaluate annual changes in ground temperature, we applied a variable temperature penalty, which was simply obtained using measured data without computer simulation. First, we examined measured data for 3 years after completion of a hybrid GSHP system that had 70 borehole-type GHEs, combined with an air source heat pump unit. In the hybrid system, the GSHP showed high efficiency (coefficient of performance >5.0) throughout the year and had a variable contribution between years with regard to cooling/heating output and time of operation. The amount of heat rejected to the ground by cooling reached -4.8 times that of heat extracted from the ground by heating after 3 years of operation. This imbalance produced ground temperature increases of -3 degrees C in an internal borehole. The variable temperature penalty reproduced the measured temperature increase, suggesting that the index is appropriate for assessing long-term ground temperature changes in the operation phase. This simple index allows operational improvement onsite and will aid the sustainable operation of GSHP systems in cooling-dominant regions. (C) 2018 Elsevier Ltd. All rights reserved.