Mapping daily evapotranspiration at field scale using the Harmonized Landsat and Sentinel-2 dataset, with sharpened VIIRS as a Sentinel-2 thermal proxy

Authors: XUE, JIE - US Department Of Agriculture (USDA); Anderson, Martha; Gao, Feng; HAIN, C. - National Aeronautics And Space Administration (NASA); Yang, Yun; Knipper, Kyle; Kustas, William - Bill; YANG, YANG - US Department Of Agriculture (USDA)

Submitted to: Remote Sensing
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/24/2021
Publication Date: 8/28/2021
Citation: Xue, J., Anderson, M.C., Gao, F.N., Hain, C., Yang, Y., Knipper, K.R., Kustas, W.P., Yang, Y. 2021. Mapping daily evapotranspiration at field scale using the Harmonized Landsat and Sentinel-2 dataset, with sharpened VIIRS as a Sentinel-2 thermal proxy. Remote Sensing. 13:3420. https://doi.org/10.3390/rs13173420.
DOI: https://doi.org/10.3390/rs13173420

Interpretive Summary: Satellite remote sensing methods have been developed to effectively map crop water use, or evapotranspiration (ET), down to sub-field spatial scales. The Landsat satellite is widely used for ET mapping, providing both the optical and thermal information needed as input to ET modeling systems at relatively high resolution (down to 30 m). However, Landsat overpasses are infrequent (8-day intervals for two satellites, combined), and clear-sky overpasses when the land can be viewed are even less frequent. This paper demonstrates a method for augmenting Landsat ET imaging frequency by combining optical data from the European Sentinel-2 satellites (with combined 5-day revisit) and sharpened thermal imagery from the U.S. Visible Infrared Imaging Radiometer Suite (VIIRS). Together, Landsat and Sentinel-2-VIIRS enable mapping of field-scale crop water use and stress every 2-3 days under clear skies. This capability will greatly improve our ability to provide actionable gridded water use information for irrigation scheduling and regional water management applications.

Technical Abstract: Accurate and frequent monitoring of evapotranspiration (ET) at sub-field scales can provide valuable information for agricultural water management, quantifying crop water use and stress toward increasing crop water use efficiency and production. Using land-surface temperature (LST) retrieved from Landsat thermal infrared (TIR) imagery, along with surface reflectance data, surface energy balance models can generate ET maps down to 30-m spatial resolution. However, the temporal sampling by such maps can be limited by the relatively infrequent revisit of Landsat data (8 days for combined Landsats 7 and 8), especially in cloudy areas experiencing rapid changes in moisture status. The Sentinel-2 (S2) satellites, as a good complement to the Landsat system, provide surface reflectance data at 10-20 m spatial resolution and 5-day revisit period but do not have a thermal sensor. On the other hand, the Visible Infrared Imaging Radiometer Suite (VIIRS) provides TIR data on a near-daily basis with 375-m resolution, which can be refined through thermal sharpening using S2 reflectances. This study assesses the value added by incorporating S2-sharpened VIIRS ET retrievals into the 30-m Landsat ET timeseries as a means of increasing temporal sampling, both fused to daily timesteps using a coarser resolution backbone of daily ET retrievals from MODIS. The value added by including VIIRS-S2 is assessed both retrospectively and operationally in comparison with flux tower observations collected from several U.S. agricultural sites covering a range of crop types. In particular, we evaluate the performance of VIIRS-S2 ET estimates as a function of VIIRS view angle and cloud masking approach. VIIRS-S2 ET retrievals (MAE of 0.49 mm d-1 against observations) generally show comparable accuracy to Landsat ET (0.45 mm d-1) on days of commensurate overpass, but with decreasing performance at large VIIRS view angles. Low-quality VIIRS-S2 ET retrievals linked to imperfect VIIRS/S2 cloud masking are also discussed, and caution is required when applying such data for generating ET timeseries. Fused daily ET time series benefited during the peak growing season from the improved multi-source temporal sampling afforded by VIIRS-S2. Results from this work demonstrate the improved capability of increased temporal frequency of ET in capturing rapid changes, and the incorporation of S2 and VIIRS data into the Landsat-based ET fusion system will facilitate the development of a prototype multi-source system for generating high spatiotemporal resolution ET products.