Wide-area estimates of evapotranspiration by red gum (Eucalyptus camaldulensis) and associated vegetation in the Murray-Darling River Basin, Australia

Background/Question/Methods:  Riparian red gum forests (Eucalyptus camaldulensis plus associated grasses, reeds and sedges) in the Murray-Darling River Basin in Australia are important components of the natural ecosystem, but are also consumers of groundwater; hence there is a need to quantify their water requirements. We used sap flux sensors to measure transpiration (E_G) of red gum trees and an algorithm based on the MODIS Enhanced Vegetation Index (EVI) from the Terra satellite to estimate total ET for the years 2000-2013 before and after a 10-year drought for a large area of red gum forest fed by the Murrumbidgee River, a tributary of the Murray River. For nine field sites where sap flux was measured, red gum accounted for 69% of ground cover, and 72% of total ET as estimated by MODIS. 

Results/Conclusions: Total ET had a steeper response to temperature and net radiation than red gum E_G, possibly reflecting differences between grasses and trees.  Red gum forest covered 38,134 ha of riparian area in the Yanga National Forest near the town of Balranald, New South Wales.  Mean ET estimated by MODIS imagery and ET_o was 795 mm yr^-1 and precipitation was 351 mm yr^-1.  The difference, projected over the area of red gum forest, was 169 mcm yr^-1, presumably derived from groundwater.  About 17% of river inflows were consumed by red gums and associated vegetation. This study supports ground studies suggesting that red gum trees are resilient to drought conditions and that floods at five year intervals are sufficient to maintain healthy trees.