To effectively manage scarce water resources, robust remote sensing-based approaches are needed to monitor both evapotranspiration (ET) and vine water stress. In turn, accuracy of the estimates of vine water status is contingent on the ability of the models to correctly describe the factors controlling ET. The aim of this study was to characterize the spatial variability in the incident solar radiation both below the vine canopy and in the inter-row space as a function of time of day using data collected in vineyards in California’s Central Valley during 2019 as a part of the Grape Remote sensing Atmospheric Profile and Evapotranspiration eXperiment (GRAPEX). Specifically, one-minute measurements of incident solar radiation were collected in two different vineyards with differing grape varieties, row orientations, and fractional canopy cover. 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 the changing solar angle throughout the day and intermittent shading caused by nonuniform canopy cover. During the day, under clear skies, the incident solar radiation varied by 560 Wm-2, on average, across the transect. Although the largest measurement range, on the order of 800 Wm-2 occurred during mid-day when the radiation region exhibited two distinct zones, one unshaded and one strongly shaded, the morning and afternoon periods demonstrated greater variability. The implications of this variability on the modeling of ET will also be discussed.
Prof. Joseph Alfieri
The Spatial Variability of the Incident Solar Radiation across a Vineyard Canopy
U.S. Department of Agriculture- Agricultural Research Service, USA