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Previous work demonstrated that whole-lagoon seepage rates could be determined by measuring the difference between the change in depth and cumulative evaporation over a 5-day period when waste inputs are withheld. However, faster techniques are needed to make the approach more cost effective and more logistically feasible at sites that can halt waste inputs for only 1 to 2 days. Research was conducted to develop a simplified overnight water balance test. Data were collected at several earthen-lined waste storages in Kansas. Evaporation was measured by eddy covariance and compared with that estimated by the bulk transfer equation; infrared measurements of waste surface temperature and weather data collected on the lagoon berm were used as inputs. Pressure probes and a float recorder were installed near the shoreline to measure depth changes. Data from berm-deployed weather stations were adequate for predicting evaporation with the bulk transfer equation, provided wind speed was downscaled by 27% to represent conditions near the waste surface. Depth sensors positioned on the upwind and downwind sides of the basin agreed when winds speeds were less than about 3 m s-1. Nighttime (2130 to 0630 h) evaporation ranged from 0.2 to 2.2 mm in May and June. Good agreement in the seepage estimates (e.g., 4.2 ±0.6 mm d-1) was found among tests conducted on consecutive nights at the same lagoon. Confidence in the seepage estimate was increased by repeating the overnight test for two consecutive or near-consecutive nights.