Potential climate change impacts on groundwater resources of south-western Australia

Abstract

Summary About three – quarters of all water used in the south-western Australia is from groundwater. A decline in rainfall since about 1975 and increased abstraction has resulted in some groundwater levels declining and groundwater dependent ecosystems decreasing in health and extent. Levels are rising under some areas used for dryland (rainfed) agriculture because crops and pastures are shallow rooted. Almost all global climate models (GCMs) project a drier and hotter climate for the region by 2030. In this project, five climate scenarios were applied to groundwater models to estimate groundwater levels in the region in 2030. The climate scenarios were (i) a continuation of the historical climate of 1975–2007; (ii) a continuation of the more recent climate of 1997–2007 until 2030; and (iii–v) three climate scenarios derived by applying the GCM projected climate under three global warming scenarios of 0.7, 1.0 and 1.3 °C by 2030. A sixth scenario considered increasing abstraction levels to maximum allowed levels under a median future climate (1.0 °C warming). Groundwater levels were found to be much less affected than surface water resources by a future drier climate as well as for a continuation of the climate experienced since 1975. For a fixed rainfall, recharge was highest where soils were sandy, there was little or no perennial vegetation and the watertable was neither very shallow nor very deep. A feature of the project area is that about half has a watertable within 10 m of the soil surface, and about a quarter within 3 m. Levels were not as affected by a decline in rainfall when reduced groundwater drainage and evapotranspiration losses offset the reduced rainfall amounts. However once a threshold groundwater level is exceeded, the rainfall fails to refill the available seasonal storage and groundwater levels decline. Projected watertables declined in all areas under a drier climate where perennial vegetation was present and able to intercept recharge or use groundwater directly. In areas under dryland agriculture, projected groundwater levels continue to rise even under a drier future climate. The climate change effects on confined groundwater systems are expected to be modest. This is due to the longer times required for the changed recharge and water level conditions in the overlying aquifers to propagate to the confined aquifers. All water balance components are projected to be impacted by climate change to a greater or lesser extent. This has consequences for the amount of extractable water from both the unconfined and confined aquifers, changes the risk of sea-water intrusion, and has implications for the groundwater dependent ecosystems.