Initial Comparison 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 methods for monitoring both evapotranspiration (ET) and vine water stress across the continuum from sub-field to regional scales are needed. Remote sensing is the only viable means of addressing this need. However, modeling ET accurately requires the radiation transfer through the vine canopy to be modeled correctly. In turn, this requires accurate measurements of the components of the radiation budget. The aim of this study is to evaluate the incident solar radiation measurements both below the vineyard canopy and in the inter-row space using three different pyranometers: CMP11 (Kipp & Zonen, Delft, Netherlands), PSP (Eppley Laboratory, Newport, Rhode Island), and AP510 (Apogee, Logan, Utah). 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. Measurements were collected in two different vine varieties and row orientations, and under different fractional canopy cover conditions. In the inter-row, the measurements varied by 13.3% on average with PSP consistently providing the lowest measurement (422 W m-2, on average) and AP510 providing the highest measurement (487 W m-2, on average); the mean of the measurements from the CMP was 464.9 W m-2. Since only 15% of the incident solar radiation measured above the canopy is measured beneath the canopy, it is not unexpected that both the magnitude (72 W m-2, on average) and difference (6.3 W m-2, on average) of the measurements where much smaller. Nonetheless, the percent difference (10.0%) between the sub-canopy measurements from the three pyranometers is similar to that in the inter-row. This suggests that sensors perform similarly regardless of the environment employed. Additionally, since the magnitude of the differences in the measurements in the sub-canopy is similar to the measurement uncertainty for these sensors, any of them may be used to measure the incident solar radiation beneath the canopy.