Below canopy radiation divergence in a vineyard - implications on inter-row 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. Here we aimed to estimate 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 (IOPs) as part of the Grape Remote sensing Atmospheric Profile and Evapotranspiration eXperiment (GRAPEX). Below-canopy incoming shortwave radiation was measured at multiple positions across the interrow, and the surface energy balance/evapotranspiration (ET) below the vine rows was measured for one growing season using three micro-Bowen ratio (MBR) systems one in the center of the interrow and one underneath a vine row south and the other north of the MBR in the center of the interrow. A significant spatial and temporal variability in radiation was observed since the vines were not significantly pruned or manipulated so they were allowed to grow randomly into the interrow. However, when averaged over the sensor array, the values appeared to give reliable average 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 2nd order affect. 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.