How advection affects the surface energy balance and its closure at an irrigated alfalfa field

Authors: WANG, T - University Of California; Alfieri, Joseph; MALLICK, K - Luxembourg Institute Of Science & Technology; ORTIZ, A - Autonomous University Of Barcelona; Anderson, Martha; FISHER, J - California Institute Of Technology; GIROTTO, M - University Of California; SZUTU, D - University Of California Berkeley; VERFAILLIE, J - University Of California Berkeley; BALDOCCHI, D - University Of California Berkeley

Submitted to: Agricultural and Forest Meteorology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/12/2024
Publication Date: 8/18/2024
Citation: Wang, T., Alfieri, J.G., Mallick, K., Ortiz, A., Anderson, M.C., Fisher, J., Girotto, M., Szutu, D., Verfaillie, J., Baldocchi, D. 2024. How advection affects the surface energy balance and its closure at an irrigated alfalfa field. Agricultural and Forest Meteorology. 357. https://doi.org/10.1016/j.agrformet.2024.110196.
DOI: https://doi.org/10.1016/j.agrformet.2024.110196

Interpretive Summary: Water is a scarce resource in California and other semi-arid and arid environments that needs to be managed effectively to ensure the longterm sustainability of agriculture. In turn, effective water management requires accurate measurement and modeling techniques to monitor water loss from croplands and other ecosystems. Focusing on alfalfa, this study demonstrated the effects of advections, i.e., the horizontal transport of warm dry air, on the quality of field measurements of evaporative water loss and provided improved methods for accounting for the measurement error introduced by advection. This work yielded more accurate datasets that can be used to develop more accurate remote sensing tools for monitoring water loss and guiding irrigation decisions. It also provided insights that could facilitate the integration of advection directly into the models.

Technical Abstract: Orbiting around the non-closure problem in eddy covariance, a new generation of high-resolution thermal imagery has revealed that advection may be more common than previously expected. To investigate this, we conducted an extensive study over an irrigated alfalfa field that experienced both heat and moisture advection. Over the course of five analysis periods (37 days), tower arrays and profile measurements were deployed to measure the horizontal advection and vertical heat flux divergence. Measured latent heat flux ('E) at the anchor tower showed an enhancement due to both local and non-local processes. Locally, as a result of the upwind 'E, advection humidified the atmosphere and increased stomatal opening, enhancing the downwind 'E. With lowered atmospheric demand, 'E was suppressed. Our results here suggest that stomatal opening played a dominant role in the enhancement, but not by itself. From spectral analysis, large contributions in the low frequency revealed the effect of large eddies. In combination with thermal remote sensing observations from ECOSTRESS and Landsat, we found that these large eddies were generated over the upwind surface, and they are independent of the local boundary layer conditions. Consequently, spatiotemporal heterogeneity induced large eddies, which further enhanced 'E through non-local transport of heat and moisture. Lastly, by conditionally including the advective fluxes, the energy balance closure improved from 89% to 97% (r2 = 0.97, p<0.001) over the five analysis periods. Results from this improved energy balance closure offer additional validation dataset for remote sensing evapotranspiration (ET) models other than forcing closure with Bowen-ratio. Furthermore, our findings provide insights for algorithms, which can improve remote sensing ET products that treat pixels as isolated columns rather than also considering the lateral effects of heat and moisture.