Title: Comparison of two temperature differencing methods to estimate daily evapotranspiration over a Mediterranean vineyard watershed from ASTER data Authors
|Galleguilos, Mauricio -|
|Jacob, Frederic -|
|Prevot, Laurent -|
|Lagacherie, Philippe -|
Submitted to: Remote Sensing of Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: January 23, 2011
Publication Date: N/A
Interpretive Summary: Measuring and managing daily water use by vineyards is important for ensuring high quality and high value grape harvests. However, patterns of soil types and topography are highly variable, making water management difficult. Satellite-based remote sensing of land surface temperatures may ease this problem by providing comprehensive views of entire farms throughout any growing season. These temperatures are closely related evapotranspiration (ET) and therefore to crop water use. Because accurate ET estimates are often difficult to acquire from satellites, ways are needed to minimize factors that degrade temperature observations. Using ASTER satellite data collected over the Languedoc-Rousillion region in southern France, two temperature differencing techniques were evaluated for ET estimation accuracy. One is a physically-based method known as a Water Deficit Index (WDI), the other is a more empirical method called the Simplified Surface Energy Balance Index (S-SEBI). Both techniques estimated daily vineyard ET to better than 1 mm d-1 , although the S-SEBI approach appears to outperform WDI when considering crop row orientation. Results from this research will be helpful to scientists and engineers in the creation of decision support tools for agricultural water management.
Technical Abstract: Daily evapo-transpiration (ET) was mapped at the regional extent over a Mediterranean vineyard watershed, by using ASTER imagery along with two temperature differencing methods: the Simplified Surface Energy Balance Index (S-SEBI) and the Water Deficit Index (WDI). Validation of remotely sensed estimates was conducted during almost two growth cycles (August 2007 – October 2008) over seven sites that differed in soil properties, water status and canopy. In order to alleviate the experimental efforts, ground truthing relied in situ estimates from the HYDRUS-1D model that simulates water transfers within the vadose zone after calibration against measured soil moisture profiles. The quality of the HYDRUS-1D simulations was beforehand controlled against direct measurements from the Eddy Covariance devices, where it was shown these simulations could be used for ground truthing. Despite the use of simple differencing methods over a complex row-structured landscape, the obtained accuracies (0.8 mm d-1 and 1.1 mm d-1) were similar to those reported in the literature for simpler canopies, and fulfilled requirements for further applications in agronomy and hydrology. WDI performed worse than S-SEBI, in spite of more determinism with the derivation of evaporative extremes used for temperature differencing, which raised the question of compromising between process description and information availability. Analyzing validation results suggested that amongst the possible factors that could affect model performances (spatial variability, soil type and color, row orientation), the first-order influence was row orientation, a property that can be characterized from very high spatial resolution remote sensing data. Finally, intercomparing S-SEBI and WDI at the watershed extent showed estimates from both models agreed within 1 mm d-1. Then, time averaged maps suggested the existence of spatial patterns at the watershed extent, which were ascribed to landscape conditions in relation with pedology and watertable level.