Identifying the water source of terrestrial vegetation by combining remote sensing and field based measurements - A case study from Barwon Downs

Full text: The risks posed to vegetation by groundwater extraction from the Barwon Downs borefield were investigated by a field and remote sensing investigation. The study aimed to determine if and to what extent terrestrial vegetation uses groundwater, and from which a likely risk to terrestrial vegetation can be established. The field study focused on thirteen sites within and outside borefield influence. During late summer 2015, a field-sampling program (soil, vegetation and groundwater) of water potential, stable isotope samples (δ18O and δ2H), soil and water chemistry was conducted to assess the likely water source for vegetation. NDVI analyses for four dates enabled spatial comparison of sites and provided an historical indication of change in vegetation activity. The findings from the field assessment indicate that at the time of sampling the vegetation is likely to be using soil water rather than groundwater. However, in areas of shallow watertable, opportunistic use of groundwater is likely. The study also found the stable isotopic signature was depleted and had little variation throughout the soil profile indicating rapid recharge that occurred mainly during high intensity rainfall events, providing sufficient soil water stores. The NDVI analysis suggests that there is some groundwater use by vegetation during prolonged periods of below average rainfall, where the groundwater is shallow. However, there is no evidence in the NDVI data that groundwater extraction has had a negative impact on vegetation activity/ condition. While groundwater pressures in the pumped aquifer have declined in aquifer outcrop areas, it is apparent that shallow groundwater has been buffered from the regional pressure decline and/or that vegetation has adapted to the decline in groundwater level. This study has shown how 'hard' field data, providing a snap shot of groundwater-vegetation interaction, can be supplemented by remote sensing to provide a fuller temporal picture of tree water use and condition. (author)