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. Observations indicate that the fortnightly fluctuations in the mean amplitude of water level increase in the upstream direction alongthe lower half of a tide-dominated estuary (the Guadiana Estuary), with negligibleriver discharge, but remain constant upstream. Analytical solutionsreproducing the semi-diurnal wave propagation shows that this patternresults from reflection effects at the estuary head. The phase differencebetween velocity and elevation increases from the mouth to the head (wherethe wave has a standing nature) as the timing of high and low water levelscome progressively closer to slack water. Thus, the tidal (flood–ebb)asymmetry in discharge is reduced in the upstream direction. It becomesnegligible along the upper estuary half as the mean sea level remainsconstant despite increased friction due to wave shoaling. Observations of aflat mean water level along a significant portion of an upper estuarysuggest a standing wave character and, thus, indicate significant reflection of the propagating semi-diurnal wave at the head. Details of the analytical model show that changes in the mean depth or length of semi-arid estuaries, in particular for macrotidal locations, affect the fortnightly tide amplitude and, thus, the upstream mass transport and inundation regime. This has significant potential impacts on the estuarine environment in terms of ecosystem management.

Dynamics of fortnightly water level variations along a tide-dominated estuary with negligible river discharge