The lunar semidiurnal influence is already known for tidal rivers. The moon also influences inland rivers at a monthly scale through precipitation. We show that, for some non-tidal rivers, with special geological conditions, the lunar semidiurnal tidal oscillation can be detected. The moon has semidiurnal tidal influence on groundwater, which will then export it to streamflow. Long time series with high frequency measurements were analysed by using standard wavelet analysis techniques. The lunar semidiurnal signal explains the daily double-peaked river level evolution of inland gauges. It is stronger where springs with high discharge occur, especially in the area of Edwards-Trinity and Great Artesian Basin aquifers and in areas with dolomite/limestone strata. The average maximum semidiurnal peaks range between 0.002 and 0.1 m. This secondary effect of the earth tides has important implications in predicting high resolution hydrographs, in the water cycle of wetlands and in water management.
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