This presentation describes the possible recharge process on North Stradbroke Island (NSI) and a possible explanation of why groundwater levels are receding. Groundwater levels on NSI have been monitored since the 1960s. Groundwater levels were at their highest in 1974 and for most bores, levels are now approaching their history low. There has been public concern over why groundwater levels and some perched lakes, are falling.
Recharge is derived mostly from infiltration of rainfall. In a few locations perched lakes and stream bed loss also contributes to recharge, and there is some localised anthropogenic recharge associated with sand mining. One of challenges in understanding recharge is the scant rainfall (spatial and temporal) data for the island. Long term rainfall data is restricted to Dunwich and Point Lookout. In recent years, daily rainfall is observed at a further six locations. While there is reasonable correlation at the annual scale, at the day scale it is poor.
An examination of bore hydrographs reveals the depth to regional water table varies for near ground level beneath the coast fringe, to over 100 metres below the highest dunes. Moreover there is an increasing delay in the water level response in the regional aquifer, with increasing depth. Delays of up to at least 130 days were identified, where the depth to the water table was nearly 100 m below ground level.
Daily estimates of recharge were derived from the SPLASH_Mult software which is a one dimensional model for simulating the temporal behaviour of soil moisture in the plant root zone and in the unsaturated zone below the root zone to the water table. Modelling has shown evapotranspiration from the root zone has a major influence on recharge, as well as antecedent condition and the magnitude of daily rainfall. About 80 percent of the island has an assigned root depth of 6m and 18 percent has a root depth of 1m
The average rainfall for Dunwich since 2011 is about 88% of the long term average rainfall of 1568mm. The model results how the corresponding average annual recharge for a root depth of six metres is 71% of the long term average annual recharge of 410mm. Modelling of recharge shows the low groundwater levels may be related to the below average recharge. A further consideration is the possible impact of fires and higher evapotranspiration due to regeneration.