Obtaining the Winkler Index for agricultural applications: a three-fold Assessment involving ground monitored data, MODIS-derived models and Copernicus-supplied data

This study focuses on the estimation of the Winkler Index by several sources in the Oltrepò Pavese (Northern Italy) region, identified as the study area of the NODES project. The Winkler Index, also known as Thermal Sum, is useful for assessing grape ripening: the index, based exclusively on temperature, is traditionally derived from in-situ air temperature measurements.
Within the NODES Project, rather than focusing on a few sites, which could be monitored locally, we are interested in the analysis of large-scale areas. For this reason, we took into consideration global Land Surface Temperature derived from satellite data.
Three data sources are focused, in this paper:
- Air temperature observations from the ARPA monitoring stations (ARPA is the Environmental Protection Agency of the Lombardy Region), which despite their dense temporal granularity have a low spatial resolution (about one station every 92 km² in the study area).
- Land Surface Temperature (LST) data from MODIS TERRA and AQUA satellite imagery, which provide a pixel-averaged Land Surface Temperature/Emissivity over 8 days with a spatial resolution of 1 km².
- Daily Copernicus air temperature data, which have a spatial resolution of 0.1° x 0.1° (approximately 11 km x 8 km).
Our main objective was to develop a robust methodology to estimate air temperature from MODIS Land Surface Temperature and then evaluate the applicability of this approach to calculate the Winkler Index, using ARPA temperature data as ground truth for calibration and validation.
MODIS satellite-derived LST data were processed to derive estimated air temperatures via regression-based calibration techniques: the calibrated models were validated using statistical metrics, including root mean square error (RMSE) and p-values, to verify the accuracy and reliability of the estimates.
Lastly, we used Copernicus air temperature data to directly compute the Winkler Index.
The Winkler Index was calculated for the study area over the years 2018-2022, capturing interannual variability and trends influenced by climate conditions.
The Winkler Indices derived from MODIS-calibrated air temperatures showed a strong overall agreement with those obtained from ARPA data, demonstrating the potential of this approach for areas without dense meteorological networks. On the other hand, the Winkler Indices calculated from Copernicus are not always in excellent agreement with the ones evaluated from monitoring stations, considered as true.
The results of this study highlight the feasibility of leveraging satellite-based datasets to complement traditional meteorological observations for agricultural and climate research. By combining MODIS and Copernicus data with in-situ measurements, the study provides a scalable and cost-effective framework to estimate air temperature and calculate the Winkler Index over large spatial extents.
This approach has significant implications beyond viticulture, enabling more precise assessments of regional suitability and supporting adaptive management strategies in the context of climate change