Below canopy radiation divergence in a vineyard: implications on interrow surface energy balance

Vineyards' canopy architecture and row structure pose unique challenges in modeling the radiation partitioning and energy exchange between the vine canopy and the interrow area. The vines are often pruned and manipulated to be strongly clumped, while mechanical harvesting requires wide rows, often with vine height to vine spacing ratio >1. This paper estimates the intercepted radiation by the canopy, and the effect of this interception on the below canopy surface energy balance and evapotranspiration (ET). Measurements were conducted in an east-west oriented vineyard in CA during intensive observation periods as part of the grape remote sensing atmospheric profile and evapotrnspiration eXperiment (GRAPEX). Below canopy incoming shortwave radiation was measured at multiple positions across the interrow, and the surface energy balance/ET below the vine rows was measured for only one growing season (in 2015) using three micro-Bowen ratio (MBR) systems. These MBR systems were deployed across the interrow, in the north, center, and south of the interrow. A significant spatial and temporal variability in radiation was observed since the vines were not significantly pruned or manipulated and thus grew randomly into the interrow. However, when averaged across the interrow using the radiation sensor array, the values appeared to give reliable mean radiation extinction conditions that agreed with model estimates. The variation in the surface energy fluxes were dominated by the amount of transmitted radiation, while soil moisture was a second order effect. Daily estimates of ET from the three micro-Bowen ratio systems, weighted by their respective representative sampling area, yielded estimates similar to values computed by the correlation-based flux partitioning method, which utilizes high-frequency eddy covariance data measured above the canopy.