Using nocturnal water level fluctuations for estimating seepage from stormwater detention systems

A simple method, modified from White's method, was developed and verified for estimating seepage from two stormwater detention areas (SDAs) for 2 years, using night‐time changes in surface water levels. The SDAs were located in warm sub‐tropical Florida where the assumption of negligible night‐time evaporation for White's method does not hold true. Daily seepage was estimated using the nocturnal water level fluctuations on no flow days during winter when evaporation losses were insignificant. Specific yield, rather than the composite specific yield, provided accurate seepage estimates. The average annual seepage from the two SDAs was 2.03 m/year. At almost 70% of surface flows, seepage is a significant contributor to regional surface and sub‐surface flows. Comparison of seepage estimates from the night‐time method (NM) and the water balance (WB) method showed that the NM‐based estimates were within the range of the estimates from the WB method. At SDA1, the differences between the NM and WB estimates were 1% and 11%, for the 2 years. The discrepancy between the two estimates became higher (27% and 23%) at SDA2. Larger differences at SDA2 were because of higher error in quantifying pumped inflows for the WB method. Successful performance of NM combined with its low resource (single well monitoring) requirements will help quantify seepage from detention areas and other similar features (e.g. ponds, constructed wetlands) in warmer climates. A scale‐up for the Caloosahatchee River basin showed that seepage from SDA's accounted for 15% of annual river flows indicating the importance of seepage in evaluating water and chemical balances. Copyright © 2015 John Wiley & Sons, Ltd.