Effects of episodic rainfall on a subterranean estuary

Numerical simulations were conducted to examine the effect of episodic rainfall on nearshore groundwater dynamics in a tidally influenced unconfined coastal aquifer, with a focus on both long- (yearly) and short-term (daily) behaviour of submarine groundwater discharge (SGD) and seawater intrusion (SWI). The results showed non-linear interactions among the processes driven by rainfall, tides and density gradients. Rainfall-induced infiltration increased the yearly averaged fresh groundwater discharge to the ocean but reduced the extents of the saltwater wedge and upper saline plume as well as the total rate of seawater circulation through both zones. Overall, the net effect of the interactions led to an increase of the SGD. The nearshore groundwater responded to individual rainfall events in a delayed and cumulative fashion, as evident in the variations of daily averaged SGD and salt stored in the saltwater wedge (quantifying the extent of SWI). A generalized linear model (GLM) along with a Gamma distribution function was developed to describe the delayed and prolonged effect of rainfall events on short-term groundwater behaviour. This model, validated with results of daily averaged SGD and SWI from the simulations of groundwater and solute transport using independent rainfall datasets, performed well in predicting the behaviour of the nearshore groundwater system under the combined influence of episodic rainfall, tides and density gradients. The findings and developed GLM form a basis for evaluating and predicting SGD, SWI and associated mass fluxes from unconfined coastal aquifers under natural conditions, including episodic rainfall.