Using ALEXI-DisALEXI for estimation of satellite-derived water use in a California almond orchard under spatially heterogeneous conditions

Authors: Knipper, Kyle; BAMBACH, NICOLAS - University Of California, Davis; Anderson, Martha; YANG, YUN - Mississippi State University; Kustas, William; McElrone, Andrew; NOCCO, MALLIKA - University Of California, Davis; TORRES-RUA, ALFONSO - Utah State University; Gao, Feng; HAIN, CHRISTOPHER - National Aeronautics And Space Administration (NASA); CASTRO, SEBASTIAN - University Of California, Davis; Crompton, Octavia; SAA, SEBASTIAN - Almond Board Of California

Submitted to: Acta Horticulture Proceedings
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/1/2024
Publication Date: 12/1/2024
Citation: Knipper, K.R., Bambach, N., Anderson, M.C., Yang, Y., Kustas, W.P., Mcelrone, A.J., Nocco, M., Torres-Rua, A., Gao, F.N., Hain, C., Castro, S., Crompton, O.V., Saa, S. 2024. Using ALEXI-DisALEXI for estimation of satellite-derived water use in a California almond orchard under spatially heterogeneous conditions . Acta Horticulture Proceedings. https://doi.org/10.17660/ActaHortic.2024.1409.20.
DOI: https://doi.org/10.17660/ActaHortic.2024.1409.20

Interpretive Summary: Accurate characterization of water use in the form of evapotranspiration (ET) is imperative in water-limited cropping systems such as California almond orchards. Satellite-based ET modeling techniques have proven reliable in determining field scale ET. However, drought conditions in the state of California are causing thousands of acres of land to be taken out of production and left fallow. In perennial cropping systems typical to California, this causes lush irrigated agriculture to be increasingly surrounded by dry, barren, and hot surfaces. Such a situation can cause horizontal advection of hot/dry air across the irrigated system, increasing evaporative demand. Moreover, increased fallowing of land causes an increase in thermal spatial heterogeneity of the surface as seen from satellite imagery. The combination of both is a cause for concern in satellitebased ET modeling. The current study was carried out to evaluate modifications to a satellite-based ET modeling framework to estimate ET over an almond orchard that neighbors a large patch of barren land. Modifications included providing more representative baseline values for the ET modeling framework that better described surface conditions at the field scale. Results suggest modifications provide a more representative baseline but do not improve field-scale estimates when applying the ET model.

Technical Abstract: A study was carried out to evaluate modifications to the ALEXI (Atmosphere-Land Exchange Inverse) and DisALEXI (associated DISaggregation technique) modeling framework to estimate water use or actual evapotranspiration (ETa) for a drip-irrigated almond orchard located in the Central Valley Region of California, USA (lat. 38°18’N; long. 121°55’S). Modifications included the creation of a “Synthetic” ALEXI ETa by edistributing coarse resolution (4 km) ALEXI ETa to higher spatial resolutions (2 km, 1 km, 0.5 km) using Leaf Area Index derived from Harmonized Landsat and Sentinel-2 datasets. This was done to provide more representative estimates of ETa for DisALEXI when applied over thermally heterogeneous landscapes. For the estimation of ETa using ALEXI/DisALEXI, X satellite images (Landsat 8 OLI/TIRS and Landsat 9 OLI-2/ TIR-2) acquired during clear sky days were used during the 2022 growing season. The performance of Synthetic ALEXI and subsequently DisALEXI was evaluated using measurements of ETa from an Eddy Covariance system (EC). Analysis indicated that Synthetic ALEXI provided more representative ETa estimates when applied over a region where a 4 km ALEXI pixel included mostly barren land and a small percentage of irrigated agriculture. However, more representative ETa did not provide substantial improvement to DisALEXI derived 30 m ETa estimates, suggesting Landsatscale input to DisALEXI remains the most important factor in the ALEXI/DisALEXI modeling scheme