Estuarine Salinity Inversion Using Acoustic Doppler Velocimetry (ADV): Methodology, Sensitivity and Environmental Modulations

Retrieving the hydrophysical properties of water columns from acoustic backscatter signals is crucial for obtaining continuous and nonintrusive observations in estuarine environments. Utilizing the pulse coherent technology in the Acoustic Doppler Velocimeter (ADV), this study presents a methodology to estimate smoothed, low-frequency practical salinity in estuarine waters by integrating synchronized temperature and pressure datasets. Laboratory calibration experiments demonstrate that the Medwin formula achieves the highest inversion accuracy across a salinity range of 0 to 35 PSU, although data dispersion increases when salinity drops below 15 PSU. This methodology was further validated using field data, where acoustic intensity profiles were derived through multi-probe spatial averaging and quadratic polynomial fitting. Field application results show that a 300 min moving average filter extracts the lower frequency salt intrusion trend from turbulent noise, allowing the framework to track tidal-scale salinity variations with a low pass trend precision of ±3.07 PSU relative to reference instruments, whereas the raw unfiltered inversion exhibits a root-mean-square error (RMSE) of 5.68 PSU. The inversion performance is sensitive to ambient dynamics: the lowest error deviations occur within a moderate environmental window characterized by current velocities of 0.10–0.58 m/s and turbidities of 109.7–208.0 NTU. In contrast, the uncertainty increases during periods with higher velocities (up to 0.83 m/s) and severe turbidities (up to 278.1 NTU) or during slack water periods with current velocities below 0.10 m/s where the acoustic backscatter drops below 90 dB. These findings quantitatively define the environmental constraints for acoustic salinity estimations, providing a low-cost and non-intrusive methodological framework for recovering low-frequency salinity trends in dynamic estuaries.