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Downscaling UAV land surface temperature using a coupled wavelet-machine learning-optimization algorithm and its impact on evapotranspiration

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Abstract

Monitoring evapotranspiration (ET) is possible through land surface temperature (LST) measured by satellites and unmanned aerial vehicles (UAV). The assumption that the higher resolution of LST may improve the performance of remote sensing ET models was verified in a recently published article showing that higher resolution LST led to increased performance of the Two-source Energy Balance Model (TSEB)—one of the well-known ET models. However, because of technology limitations, the spatial resolutions of satellite and UAV thermal imagery are coarser than those in optical and near-infrared (NIR) bands. Therefore, developing thermal sharpening techniques and assessing their impacts on ET models performance are imperative. Although previous studies have developed and evaluated downscaling LST methods for satellite imagery, implementation of those methods on UAV imagery is limited. In this study, a coupled wavelet, machine learning, and optimization algorithm was implemented for downscaling UAV thermal imagery from 60 cm to the resolution of UAV optical imagery (15 cm) because 60 cm pixel resolution still incorporate mixed temperatures from the soil, vine canopy, active cover crop and shaded regions. A 2D discrete wavelet transform (2D DWT) was employed for the decomposition of inputs to 60 cm and inverse transformation of low thermal resolution to higher resolution. Four machine-learning-based algorithms (Decision Tree Regression (DTR), Ensemble Decision Tree (DTER), Support Vector Machine (SVM), and Gaussian process regression (GPR)) along with four linear regression-based models (linear, interactions linear, robust linear and stepwise linear) are used as the potential fitting models, and a grid search algorithm is used for auto-tuning parameters of the machine learning algorithms. Additionally, a novel sampling technique was designed to provide more representative samples for training steps in the regressing models. Four sets of high-resolution images were provided by the Utah State University AggieAir sUAS Program as part of the ARS-USDA GRAPEX Project collected since 2014 over multiple vineyards located in California. After applying the proposed downscaling algorithm, a separation method was used for estimation of canopy and soil temperatures from the original and sharpened thermal imagery. Ultimately, the TSEB model was executed for these pairs of temperature components, and its performance compared to eddy covariance measurements. Results demonstrated that the proposed sampling algorithm can significantly accelerate the computation time for the UAV temperature sharpening efforts. Among all the fitting models, GPR, SVM and DTER were the most accurate in terms of R-square. GPR, SVM and DTER’s \(R^2\) were higher than 90% for all cases. The correlation between NDVI and radiometric temperature (Tr) was significantly improved when the downscaled Tr (DTr) was used in the NDVI–Tr domain for the separation procedure. Compared to additional IRT sensors temperatures, Tc and particularly Ts derived from the DTr were closer to the observed measurements. After feeding the TSEB model with DTr products, results demonstrated that estimations of soil heat flux (G) were significantly improved, while large LE differences were reduced (from 20.26 to 13.5 in terms of RRMSE).

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Acknowledgements

E.& J. Gallo Winery and Utah State Water Research Laboratory contributed towards the acquisition and processing of the ground truth and UAV imagery data collected during GRAPEX Intensive Observation Periods (IOPs). In addition, we would like to thank the staff of Viticulture, Chemistry and Enology Division of E. &J. Gallo Winery for the assistance in the collection and processing of field data during GRAPEX IOPs. Finally, this project would not have been possible without the cooperation of Ernie Dosio of Pacific Agri Lands Management, along with the Sierra Loma vineyard staff, for logistical support of GRAPEX field and research activities. The authors would like to thank Carri Richards for editing the manuscript. Finally, we would like to acknowledge the significant financial support for this research from the NASA Applied Sciences-Water Resources NNX17AF51G Project. USDA is an equal opportunity provider and employer.

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Correspondence to Mahyar Aboutalebi.

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Aboutalebi, M., Torres-Rua, A.F., McKee, M. et al. Downscaling UAV land surface temperature using a coupled wavelet-machine learning-optimization algorithm and its impact on evapotranspiration. Irrig Sci 40, 553–574 (2022). https://doi.org/10.1007/s00271-022-00801-2

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  • DOI: https://doi.org/10.1007/s00271-022-00801-2

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