Using high-spatiotemporal thermal satellite ET retrievals to monitor vineyard water use and water stress across multiple California vineyards

Mapping the spatial variability of actual evapotranspiration (ET_a) across vineyards is necessary for optimizing irrigation scheduling and efficiency, subsequently leading to the conservation of water resources and maintaining sustainable wine grape production. To support efficient irrigation strategies, we investigate the utility of thermal infrared-based ET maps over a range of vineyards located throughout California and develop strategies for its integration into operational irrigation management frameworks. Daily and weekly total ET_a estimates at 30 m spatial resolution, coinciding with Variable Rate Drip Irrigation (VRDI) systems located in select vineyards, were derived from the combination of the Disaggregated Atmosphere Land Exchange Inverse (ALEXI/DisALEXI) surface energy balance model and the Spatial Temporal Adaptive Reflectance Fusion Model (STARFM). Model output is evaluated for years 2017 and 2018 over vineyard sites located in Sonoma, Sacramento and Madera counties in California that are being monitored as part of the Grape Remote sensing Atmospheric Profile and Evapotranspiration eXperiment (GRAPEX). Each vineyard is uniquely different, with variations in vine variety, soil physical properties, vine row orientation and climate. Additionally, we test the capacity of thermal-based ET_a to capture stress signals in near real-time by sub-dividing a select vineyard into 4 blocks with different irrigation management strategies and goals, inducing varying degrees of stress during the growing season. Results indicate sufficient model performance across all vineyards, demonstrating the portability of the thermal-based approach to categorically different regions. Results also show that the thermal-based approach could distinguish between stressed and non-stressed areas within a vineyard, while commonly applied approaches based on vegetation indices could not. Although the thermal-based ET_a approach shows promise in supporting efficient irrigation strategies by accurately quantifying time- and space varying water use and stress, latency in current satellite data availability and issues related to afternoon advection in more arid environments, impact applications in an operational setting.