Applying satellite‐derived evapotranspiration rates to estimate the impact of vegetation on regional groundwater flux

Abstract

AbstractFreshwater resources are at a premium across the world, including many water‐limited areas across Australia. Ongoing water level decline in some groundwater systems suggests that the rate of loss may have reached unsustainable levels . However, identifying which factors are primarily responsible for the trend (lack of rainfall–recharge, presence of vegetation, and groundwater pumping) remains challenging using groundwater observations alone. We applied satellite‐derived estimates of evapotranspiration (ETa), which, when combined with local rainfall data and field‐based groundwater level observations, established a regional water balance spanning 1,500 km2 and seven groundwater lenses over a 10‐year period (2000 – 2010). Assuming that the extent of the freshwater isohaline represents the recharge area for a lens, the water balance suggests that the median annual groundwater recharge rate varied between 226 ± 92 mm year−1 (Mikkira) and −162 ± 194 mm year−1 (Uley East). Uley South is the most regionally significant lens in the system and recorded a median annual recharge rate of 91 ± 182 mm year−1. Overlaying vegetation highlighted the impact of woodland areas on groundwater recharge, where the trees were accessing groundwater to support ETa, provided the water table was <10 m. Areas of grassland demonstrated the highest median groundwater recharge rates of 151 mm year−1, followed by cropping (133 mm year−1) and pasture (95 mm year−1). Exploring the resilience of the groundwater system to variations in extraction (pumping) and woodland coverage suggests that resource managers must consider both systematic losses in order to maintain groundwater equilibrium.