Percolation of water from a pond may cause a landslide in a hilled rural area. Before measures to prevent such landslides can be planned, it is essential to identify reservoir-water influent from the pond. We describe an analytical method that uses ²²²Rn- and water-balance equations to quantify groundwater effluent and reservoir-water influent simultaneously, which is impossible using the conventional method that measures only surface-water inflow and outflow. We selected F pond, Nagano Prefecture, as the study site. We estimated the rate of ²²²Rn dispersion to the atmosphere by assuming a stagnant film between water and air with thickness inversely proportional to the rate of dispersion. By a laboratory experiment, we estimated the film to be about 830 ?m thick and found that the film thickness was not influenced by wind velocities less than 1.5 m s^-1. A preliminary investigation suggested that reservoir-water was mixed very well. The groundwater effluent and the reservoir-water influent during the investigation were calculated to be 0.67 × 10^-3 m³ s^-1 and 0.41 × 10^-3 m³ s^-1 respectively, by making ²²²Rn- and water-balance equations and solving them. This analytical method is expected to be useful for not only prevention of a landslide but also for effective use of water and prediction of water quality.