Seawater-freshwater interface dynamics driven by upstream freshwater discharge and tidal oscillations in highly stratified River Neretva estuary

Study region River Neretva estuary, Republic of Croatia Study focus The study is focused on understanding of the seawater-freshwater interface (SFI) dynamics driven simultaneously by upstream freshwater discharge (Q F ) and tidal oscillations (h SL ) along the dry hydrological period. Implementation of the novel, site specific, water column temperature based monitoring system, in combination with extensive hydrological monitoring, ensures comprehensive and relevant data sets along the 11-day period. Obtained time series are used to reveal frequency similarities and to establish functional dependencies between SFI depth (h SFI ) and its driving forces, Q F and h SL . New hydrological insights for the region Water column temperature profiling delineated upper freshwater layer from the bottom seawater layer, thus enabling the determination of the h SFI and correction of Q F in highly stratified conditions. Novel Q F offers a superior first order data based framework compared to present discharge assessment. Amplitude spectral density identified matching in frequency constituents, which led to functional interdependence of h SFI to Q F and h SL in trend and residual components. Linear regression model has been used to establish functional relationship between h SFI and its controlling mechanisms valid during dry season. SFI dynamics are simultaneously controlled by Q F and h SL , operating on different timescales. While Q F controls the overall h SFI trend feature, h SFI fluctuations at timescales smaller than daily have been identified to be in correspondence to diurnal and semidiurnal tidal components.