Seepage from waste treatment lagoons has the potential to impact water quality and seepage from tailwater pits and stock-watering ponds affects water supply. Previous work by the author demonstrated that whole-pond seepage rates could be determined by measuring the difference between the changes in depth and cumulative evaporation over a 5- to 7-day interval. However, more rapid measurements techniques are needed to make the approach more feasible at sites that can only withhold waste inputs for 1 to 3 days. Also, faster and simpler procedures are needed to reduce costs and facilitate technology transfer to consulting engineers. Therefore, research was conducted to develop a simplified overnight water balance test for measuring lagoon seepage. The key to the technique was measuring net radiation and conductive heat flux at night using a custom built 25-cm diam. ring buoy deployed near the center of the basin. Depth changes were measured to within 0.1 mm using two pressure transducers. Results show that seepage can be measured to within 0.2 mm/day when using this method.