Submitted to: Australian Journal of Water Resources
Publication Type: Peer reviewed journal
Publication Acceptance Date: 4/8/2006
Publication Date: 8/1/2006
Citation: Savige, C.L., French, A.N., Western, A.W., Walker, J.P., Abuzar, M., Hacker, J.M., Kalma, J.D. 2006. Remote sensing estimates of actual evapotranspiration in an irrigation district. Australian Journal of Soil Research. 10(2):207-212. Interpretive Summary: An evapotranspiration (ET) study was done over irrigated dairy and pasture lands in Nathalia, Victoria, Australia to measure the amount of water lost to the atmosphere from plants and the underlying soil. The experimental objective was to determine how accurately this water loss can be estimated using remotely sensed land surface temperatures and reflected light at 30-60 m resolutions. Previous research has shown that models can be accurate at coarser spatial scales, but few studies verify ET accuracies at resolutions most useful for managed agriculture because verification is experimentally difficult. This study accomplished model verification by employing an independent technique of measuring ET from low altitude aircraft transects. Using this technique and data sets from the ground and from the Landsat 7 satellite in January 2003, accuracies were good over well-irrigated pasture, but poor over bare soils. The results suggest that currently established remote sensing models can be relied upon for ET estimation over well-watered crops, but not over dry regions. These outcomes will be useful for farm and water district managers who seek improved tools for monitoring spatially distributed ET.
Technical Abstract: Accurate estimates of the spatial distribution of actual evapotranspiration (AET) are useful in hydrology, but can be difficult to obtain. Remote sensing provides a potential capability for routinely monitoring AET by combining remotely sensed surface temperature and vegetation cover observations with near surface meteorological data in a surface energy balance model. Results from two different energy balance models are compared to airborne and ground measurements of surface energy fluxes over an irrigation district in northern Victoria during January 2003. Ground data collected include eddy correlation measurements of latent and sensible heat fluxes and associated meteorological measurements. The airborne data include eddy correlation measurements of latent and sensible heat fluxes (35 m above ground level), surface temperature transects and normalized difference vegetation index (NDVI) imagery ( 1 m resolution. Surface temperature and NDVI maps were derived from Landsat ETM+ data and combined with the ground meteorological observations in both a one- and two-source energy balance model. The two models produced similar results over irrigated sites, but large discrepancies were present over sparsely vegetated and bare soil areas. Although both models overestimated the latent and sensible heat fluxes in comparison to the ground and airborne measured fluxes, it was found that the modeled and observed Bowen ratios compared well.