Use of remote sensing to determine the impact of landuse on evapotranspiration and groundwater in Western Victoria, SE Australia

Full text: Shifts in agricultural landuse can disturb the hydrological balance and put stress on water resources which can then directly affect groundwater recharge. The aim of the current study is to investigate the response of evapotranspiration and groundwater availability to rainfed agricultural landuse in two adjacent catchments in western Victoria, SE Australia, one predominantly managed as a tree plantation (Eucalyptus globulus) and other as pasture for sheep grazing. The spatial variability of evapotranspiration within the catchments was estimated from Landsat and climate data by partitioning the surface energy balance components using SEBARA (Surface Energy Balance Algorithms for Rainfed Agriculture). The surface energy balance calculations show that the tree plantation, compared to the pasture, has a lower albedo, emissivity, and outgoing longwave radiation due to higher net radiation input, but the Eucalyptus trees show a lower surface temperature due to the cooling effect of the evapotranspirative water loss (average 6.6 ℃ less), and this results in a lower soil heat flux. The higher available net radiation and lower soil heat flux in the tree plantation translate into higher latent heat flux and evapotranspiration. As a result, the plantation has 15-20% higher evapotranspiration than the pasture. Comparison of the calculated evapotranspiration with estimates from a flux tower in the pasture catchment and sapflow measurements in the tree plantation shows that an estimation accuracy of more than 95% was achieved. The spatial variability in daily evapotranspiration across the plantation catchment is both positively and negatively correlated with groundwater depth and relief. In the higher elevation part of the catchment, where the groundwater is deeper, evapotranspiration is less; this is also true of the lower elevation area with shallow saline groundwater adversely affecting the tree health. In the pasture evapotranspiration is highly correlated with areas of shallow groundwater and low relief. Therefore, detailed mapping of evapotranspiration using remote sensing can provide useful information on recharge, through its relationship to groundwater usage by plants. (author)