Orthotidal signal in the electrical conductivity of an inland river.

An orthotidal signal is a tidal component found in a streamwater parameter when there is no oceanic tidal input, i.e. when the streamwater monitoring point is located far inland and at high elevation. This study analyses various parameters of Cib River in Carpathian Mountains, Romania. This river receives water from a rich karst aquifer when crossing Cib Gorge. Streamwater level, temperature and electrical conductivity were measured in 270 days grouped in three time intervals of consecutive days. The measurements were done every 15 minutes in order to capture any significant periodic variation. The streamwater measurements were paired with air measurements and measurements done in a thermal spring. Solar semidiurnal oscillations were found in the streamwater electrical conductivity. In case study time series, selected based on their good signal to noise ratio, there are average semidiurnal oscillations of approximately 4 μS/cm, while the maximum amplitude rise up to 20 μS/cm. The semidiurnal peaks in water are generally in phase with the two atmospheric tide maxima, which are the cause of the studied phenomenon. The higher mineralisations of the thermal waters that rise from beneath the karst aquifer are the most probable cause of finding significant orthotides only in the electrical conductivity time series of the studied river.