Speaker
Description
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. Most agricultural practices 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. Dense time-series analysis of satellite imagery carry the advantage of being able to capture both highly dynamic and gradual or long-term change processes, compared to traditional multi-temporal image classifications alone.
The presented approach uses Sentinel time-series of pre-processed optical and SAR based biophysical indices (NDVI, SAR backscatter, SAR coherence) on agricultural parcel level. These time-series are interpreted, individually, to their expected trajectory in response to the agricultural practices to be monitored. Separated workflows for analysis of time-series of each biophysical indicator are constructed, so as to reflect different situations in availability of input data (e.g. optical data is likely to be relatively less dense time-series compared to SAR data). The time-series of all indices are tested to identify three transition periods in each agricultural parcel, including (i) onset of vegetation growth, (ii) a period where the maximum vegetation canopy is present and (iii) the onset of senescence or sudden occurrence of loss of vegetation. The last period is expected to be detected as a “break” in the time-series, whose occurrence is interpreted as either the presence (indicating undisturbed vegetation) or the absence (indicating harvest, clearance or disturbance) of vegetation at a given date. Such “breaks” could occur more than once a year, often indicating the repeated clearance or disturbances on the parcel in a single agricultural season. Using the trends in the biophysical indicators a set of conditions derived from the different biophysical indicators are established.
Various statistical analyses, regression analyses, trend analyses, and decision-tree thresholds for each conditions, and different logical combinations of the conditions, are tailored to best capture the agricultural practice (e.g. the harvest, mowing, presence/absence of catch crops) for the area of interest. In the case of ambiguous parcels with contrasting time-series, such individual conditions also allow for expert judgement to be called upon for specific observations of vegetation changes.
The above described method is based on the approach that has been developed by Gisat company within the DROMAS (Agricultural Crop Monitoring and Assessment driven by Satellites) project co-funded by ESA. The project aims at development of innovative tools for continuous monitoring of crop vegetation at parcel level. The first implementation has been done for grassland monitoring with the goal to detect grassland mowing & grazing events. The pilot testing and semi-operational automated production within the demonstration phase of the project have shown very promising performance providing high quality results comparing to other common classification or change detection approaches.
The further development and tailoring of this approach has been done internally and this method was selected for application within the Sen4CAP (Sentinels for Common Agriculture Policy) project funded by ESA. After the thorough benchmarking and prototyping activities done on selected sites in six European countries this method is being implemented as an operational tool for monitoring of agricultural practices. In particular the developed procedures aim at the identification of crop harvesting and compliancy assessment for farmer declarations of fallow lands or the growing of catch-crops and nitrogen fixing crops.
