Spatial Variability of Inter-row and Sub-canopy Measurements of Incident Solar Radiation within Vineyards

To effectively manage limited water resources and ensure the sustainability of the California wine industry, robust remote sensing-based approaches are needed for monitoring both evapotranspiration (ET) and vine water stress across the continuum from sub-field to regional scales. However, modeling ET accurately is contingent on the ability of the models to correctly describe radiation transfer through the vine canopy. The aim of this study is twofold: i. to evaluate the incident solar radiation measurements from different pyranometers: CMP6 and CMP11 (Kipp & Zonen, Delft, Netherlands), PSP (Eppley Laboratory, Newport, Rhode Island), and SP510 (Apogee, Logan, Utah); and, ii. to characterize the spatial variability in the incident solar radiation both below the vineyard canopy and in the inter-row space. Specifically, the data were collected as a part of the Grape Remote sensing Atmospheric Profile and Evapotranspiration eXperiment (GRAPEX), a multi-disciplinary field project ongoing in the Central Valley of California, during three intensive observation periods during the 2019 growing season. Additional data collected during 2021 were also used. Measurements were collected in two different vineyards with differing grape varieties, row orientations, and fractional canopy cover. The analysis suggests the measurement uncertainty for each of the pyranometers was between 5 and 15 W m-2. Compared to the other pyranometers, it was also found that the SP510 exhibited a bias that increased linearly with increasing incident solar radiation. After accounting for the differences in the response of the pyranometers, it was found that there was significant spatial and temporal variability in the below canopy measurements due to both changing solar angle throughout the day and intermittent shading caused by nonuniform canopy cover.