Groundwater flow in Kumamoto Plain, Japan was studied using subsurface temperature as a tracer. The researchers found that surface warming led to an increase in subsurface temperature in the groundwater recharge area over the past 15 years, while the discharge area experienced a decrease due to reduced groundwater flow. These findings were supported by analytical solutions to the heat conduction–advection equation. Subsurface temperature changes can provide valuable insights into groundwater flow systems.