Efficacy of direct tidal methods for identifying hydraulic parameters of coastal aquifers

The analysis of tidal effects on groundwater systems plays an important role in coastal aquifer management. In this study, the efficacy of ‘tidal efficiency’ and ‘time lag’ models, which are also known as direct tidal methods, for determining hydraulic diffusivities has been assessed by using tide-aquifer interaction data from unconfined and confined coastal aquifers. The effect of spring and neap tidal data on aquifer parameter estimates was also analyzed for the unconfined aquifer. The hourly tide-aquifer interaction data for two unconfined sites of Konan groundwater basin, Japan and three confined sites of Dridrate groundwater basin, Morocco were used in this study. For all the five sites under study, the aquifer hydraulic diffusivities based on the ‘time lag’ model were found to be much larger (2 to 14 fold for the unconfined sites and 5 to 8 fold for the confined sites) than those based on the ‘tidal efficiency’ model. It is concluded that the ‘tidal efficiency’ model is superior to the ‘time lag’ model for both unconfined and confined aquifers, the numerically computed time lags are more reliable than the time lags obtained by graphical method, and that the spring and neap tidal data have a significant influence on the hydraulic diffusivity estimates.