Investigation of High-Frequency Dynamics of Sea Surface Salinity in the Outer Shannon Estuary Using Numerical Model-Derived Data

Determining the most appropriate point for monitoring hourly sea surface salinity (SSS) in an outer estuary is a crucial requirement for operationalising a relatively proactive approach for monitoring and predicting upstream seawater intrusion (USI) successfully. However, our current knowledge of such high-frequency dynamics of SSS; and their spatial variability along the transect of the outer areas of European estuaries including Shannon Estuary, particularly when it comes to using relevant numerical model-derived (NM-D) data is still very limited. The study leveraged appropriate NM-D hourly SSS data to determine the daily, intraseasonal, and annual SSS variability at 4 different points in the outer part of Shannon Estuary; and to determine the most appropriate point for monitoring and predicting USI in the outer estuary. Descriptive statistics including measures of variability; and rigorous inferential statistics, pairwise Brown-Forsythe’s test were utilised for the study. Points A, B, C, and D show annual mean SSS (AMSSS) of 33.985, 33.881, 34.125, and 34.343; and annual mean CV (AMCV) of 0.086, 0.073, 0.094, and 0.106 % respectively. The lowest values exhibited by the point B in the results imply the highest level of annual freshwater availability (AFWA); and the most stable SSS on annual time scale. The Brown-Forsythe’s tests of the difference in the hourly SSS variability for points A vs B, A vs D, B vs C, B vs D, and C vs D show P-value of < 0.05, while A vs C shows P-value of > 0.05. This implies that the difference in the hourly SSS variability between the points of observation in each of the 5 pairs out of 6 is statistically significant. Instead of point A that is relatively close to the inner estuary, the results remarkably establish the point B as the most appropriate for monitoring and predicting USI in the outer estuary. The findings imply significant spatial and temporal dynamics, which underscore a complex hydrographic regime characterised by distinct geographic gradients and pronounced seasonal transitions in the outer parts of European estuaries.