Two parameter analytical framework for surface layer salinity assessment in highly stratified microtidal estuary

Seawater intrusion threatens coastal water resources and agricultural sustainability, particularly in microtidal highly stratified estuaries where analytical prediction tools remain limited. This study presents a novel analytical framework, based on in situ observations, for assessing the salinity distribution in the surface layer of highly stratified salt-wedge estuaries. Building upon Xu et al.’s analytical model for well-mixed estuaries, we introduce four key modifications enabling application to highly stratified systems: (1) isolation of the surface layer through percentile-based definition with min-max normalization; (2) reformulation using freshwater velocity at the estuary mouth instead of the discharge; (3) direct derivation of tidal excursion from water level observations addressing upstream tidal amplification; and (4) coherence-based selection of boundary conditions capturing characteristic time lags between seawater level (SWL), freshwater velocity, and salinity response along the estuarine surface layer. Main contribution of the work is found in estuarine salinity indicator (kforc10) derived from observed SWL at the estuary mouth. When combined with the salinity profile at the estuary mouth, (kforc10) leads to two-parametric framework for surface water column layer salinity assessment. Calibration is performed by using nine field campaigns in the Neretva River estuary (Croatia) during 2021–2022 period with R² = 0.83, RMSE = 0.40, MAE = 0.32. Validation is performed on 2023 campaigns with R² = 0.89–0.94, RMSE = 0.002–0.005, MAE = 0.001–0.005, demonstrating applicative potential across varying conditions characterizing highly stratified to salt wedge conditions. This computationally efficient framework can provide coastal management with practical tool for operational decision-making and assessment of surface-layer salinity response in vulnerable agricultural regions.