Satellite-based estimates of evapotranspiration in major irrigated tree and vine crops grown in Australia

Satellite-based estimates of evapotranspiration (ET) in major irrigated tree and vine crops grown in SE Australia were made with the METRIC algorithm (Mapping ET with Internalised Calibration) applied to Landsat images acquired during peak irrigation periods (December – April) in years 2007 – 2010. Detailed contemporary land use maps were used to identify orchard and vine crops grown in major irrigation areas of South-East Australia. Field-scale estimates of ET were described in terms of the evapotranspiration ratio, ET r F = ET/ET r , where ET r was “tall”-crop reference ET. Image-specific values of NDVI extended over the approximate range, 0.15 ≤ NDVI ≤ 0.75, and ET r F showed strong consistent crop-specific positive relationships with NDVI within the dominant range, 0.1 ≤ ET r F ≤ 1.0. Maximal values of NDVI were truncated by comparison with reported values for irrigated field crops. An ET r F-NDVI reference triangle, derived here from published findings for irrigated field crops, was used as a reference framework for purposes of comparing ET r F-NDVI distributions of Australian tree/vine crops with field crops. NDVI limits of the triangle, 0.10 ≤ NDVI ≤ 0.85, corresponded with vegetation cover spanning the entire range, bare-ground – full-canopy. Bare-ground values of ET r F in the reference triangle ranged between 0 and the maximum, ET r F = 1, which extended over the full range of NDVI. Minimum values of ET r F, equated to “basal” rates of ET, increased linearly with NDVI. ET r F-NDVI distributions of Australian crops evidenced a mean 89% compliance with reference values, and a mean 91% exceedance of NDVI-dependent basal ET r F values.