Remote sensing applied to phenology monitoring in vineyards: testing through field observations

In this paper we study the relationship between vegetative phenology obtained from satellite-derived vegetation indices (VIs) and vegetative and floral phenology based on field observations. The work was conducted during 2023 and 2024 in vineyards belonging to the Designation of Origin Uclés, located in the west of Cuenca province (Castilla-La Mancha region, central Spain). The field work was carried out in seven plots that are frequently ploughed and lack cover crops and green covers. All plots grow under similar conditions and the maximum distance between plots is less than 2 kilometers. Phenological sampling was carried out weekly on 20 grapevines per plot, using the BBCH scale.

Different VIs (NDVI, EVI, SAVI and SAVI2) were calculated using Google Earth Engine (GEE) and Sentinel-2 data, but EVI was selected due to its greater amplitude in the index curves. The R package Phenofit was used to clean the data, curve fitting and extraction of phenology metrics. For curve fitting, the Elmore method was used, and for phenology metrics extraction, the Threshold, Inflection and Gu methods from the Phenofit package were applied. Although Inflection and Gu differ in their approach, they both divide the curve into four phenological metrics: greenup, when index starts to growth; maturity, when the index value remains stable; senescence, when it decreases; and dormancy, when it stops decreasing and remains at a low value. Threshold considers only greenup and dormancy.

The results show that greenup is associated with the inflorescence development. This phase starts in a similar day of the year (DOY) in all plots and in the two studied years. Maturity, marked by Inflection and Gu methods, occurs between flowering and fruit development stages, that is, between DOY 140–198. The senescence period is marked between fruit development and leaf discoloration (178–310 DOY), and despite its amplitude, 75% of the observations place senescence between the final stages of the fruit and leaf discoloration. Finally, dormancy occurs between leaf discoloration and the leaf fall which is correct but usually it is marked excessively late.

Phenological metrics derived from Vegetation Indices (VIs) such as greenup (potentially related to inflorescence development), senescence (potentially related to leaf discoloration), and dormancy (potentially related to leaf discoloration and fall) can be linked to the grapevine cycle on the BBCH scale. However, more studies are needed to accurately link field phenological observations with satellite-derived vegetation indices.

This work has been funded by the Junta de Comunidades de Castilla-La Mancha (JCCM) through the project SBPLY/ 21-180501-000172 and by the University of Castilla-La Mancha (UCLM) through the project 2022-GRIN-34507. EJJ thanks to the Investigo Program for a contract co-financed by the European Social Fund Plus.