The Sentinel constellation of Earth Observation (EO) satellites provides unique and new possibilities to monitor European agricultural landscapes. The vast database of images acquired at a high temporal frequency of every 5- to 6-days can now enable identifying changes in land use management or intensity, specific cropping practices and land use dynamics, and harvests of various crop types multiple times in a single season. This offers offer unprecedented and unique opportunities for the monitoring of Ecological Focus Areas (EFAs).
Under the EFA rules, farmers with arable areas exceeding 15 ha must ensure that at least 5% of such areas are EFA, dedicated to ecologically beneficial elements. Many of the practices involved in the non-compliancy to EFA rules under the CAP, such as the non-maintenance of fallow lands, or cutting or disturbance of catch crops and nitrogen fixing crops, often manifest as very subtle changes in vegetation cover occurring over a short period of time in one season. Only frequent satellite-based monitoring provides sufficient data sources to monitor such subtleties efficiently and accurately. In support of these activities, the SEN4CAP has developed an Agricultural Practices Monitoring product, which compares satellite-based data time-series and rapid change detections to the farmer’s declarations. The methodology includes interpreting the EFA rules, extracting representative markers from the time series to validate or verify that the rule has been followed, and reaching a decision of the degree of compliancy of a parcel to the rules. Three specific practices were investigated within the Sen4CAP project - Land lying fallow, Catch crops and Nitrogen fixing crops.
The EFA practices include both direct greening by the maintenance of fallow lands and indirect greening in areas covered by catch crops (fast-growing crops grown between plantings of main crops) or nitrogen-fixing crops. The main challenge in developing a methodology in this domain is dealing with diverse EFA national implementations, which vary significantly amongst countries. The variations concern the definition of crops (crop groups) and activities (e.g. harvesting, mulching, sowing etc.) that are eligible for each practice, and the definition of the seasonal duration rules for the application of each practice. Hence, during the initial phase of the project, the Paying Agencies of all pilot countries were asked to provide the most detailed information about national rules related to the selected practices. For the years 2016 and 2017, a list of agricultural parcels with information on their associated practices declarations were also obtained. Analyses were carried out to identify what type of information related to particular practices could be derived from various satellite data indicators, including indices such as the normalized difference vegetation index (NDVI) from temporal profiles of optical data (Sentinel 2A), and backscatter and coherence from temporal profiles of Synthetic-aperture radar (Sentinel 1A/1B) imagery. The selected satellite data indicators were found to correctly inform about the following conditions – (1) the presence of low / high vegetation / bare soil; (2) the growth of vegetation; (3) the loss of vegetation (in the form of harvest, clearance); and (4) the changing or stable status of the parcel conditions within 6-day periods. Together, such markers, which characterise how the physical state of land is represented in satellite time-series, were able to aid informed decisions about the compliancy to various national regulations.