The impact of declining rainfall and ocean forcing on morphology and dynamics of an island fresh groundwater lens, South-West Western Australia

• Winter rainfall decline is the primary cause of freshwater lens reduction • Seasonal sea-level fluctuations exacerbate the impact of recharge reduction. • Multiyear total dissolved solids data is required to understand lens response. Fresh groundwater lenses are an important natural source of potable water for communities on small oceanic islands but are highly vulnerable to climate variability and long-term trends such as prolonged decadal rainfall decline. This is particularly true of the islands along the coast of Southwest-Western Australia located in the Indian Ocean where substantial rainfall declines are the primary driver of a reduction in the volume of groundwater recharge. On these islands, the impact of these changes is further complicated by seawater mixing associated with sea level fluctuations operating on time scales ranging from hourly to seasonal, interannual, and decadal. The complex interaction between climatic and sea level variability highlights the need for well-constrained density-dependent groundwater models to understand changes to recharge on various timescales to manage groundwater resources. This study focused on Rottnest Island where groundwater age data was combined with water level and salinity measurements to develop a 3D density-dependent groundwater model. The steady state modelling of the fresh groundwater lens suggests a recharge rate of −41 % of the long-term historic annual rainfall, with the winter rainfall important for lens recharge, suppressing the upward movement of the saline transition zone groundwater associated with seasonal sea level fluctuations. A transient simulation reveals a substantial reduction of up to 50 % in the volume of potable groundwater (i.e. in the freshwater lens) in response to the prolonged rainfall decline that started in the late 1960s combined with groundwater abstraction. The sustained regional winter rainfall decline experienced in the Southwest Australia region accounts for most of this reduction when considering transient sea level boundary conditions. The modelling approach used in this study for Rottnest Island offers insights that can be applied to other oceanic islands experiencing changing climatic forcings, particularly in regions where sea level variability plays a significant role.