Assessing the suitability of using Sentinel-1A SAR multi-temporal imagery to detect fallow periods between rice crops

Sammanfattning: Fallow, understood as a period where no crop is cultivated during a growing season, is an important crop management practice. Fallow plays an important role in pest and disease management, and monitoring the presence and duration of fallow can have implications for understanding the extent and effectiveness of this agricultural practice. Therefore, fallow period mapping using remotely sensed data is a widely researched topic. However, its identification using remotely sensed data is often problematic due to the fallow backscatter values being similar to other land cover classes. SAR data tends to be applied to crop identification, paying little to no attention to fallow. This thesis investigated fallow backscatter characteristics and detection methods that could be used for identification of fallow periods between rice crops. Timeseries of backscatter from Sentinel-1A data were plotted for 83 fields in two areas in the Philippines. Rice crop and fallow temporal characteristics were derived from field data and correlated with the plotted backscatter. The start and end of fallow showed significant differences that could be used for detecting fallow periods. However, the detection gave acceptable results only using VH polarisation and the VV/VH ratio, while VV polarisation resulted in the lowest accuracy. A backward fallow detection method was also employed, where the end of the rice season indicated the fallow start, and the start of the rice season marked the fallow end. This method proved more successful in determining fallow duration than using the fallow itself. Fallow duration was also categorised according to its length into short and long fallow. In short fallow periods, backscatter was mainly decreasing during the whole fallow period, while in long fallow periods, some variation was observed approximately halfway through the fallow duration, which were attributed to short-term drought tolerant crops. Fallow backscatter values for irrigated and rainfed fields were also compared. Ecosystem type was found to have a significant effect on fallow backscatter only in one of the two study areas, indicating that water availability may affect fallow duration. The study found that SAR backscatter can be used to map fallow by detecting rice seasons, although the detected fallow duration was slightly overestimated. The study lacked information about the conditions of the fields during the fallow periods; therefore, future studies could benefit from more information about weed prevalence, planting of non-rice crops, water or rainfall deficit, and flooding/tillage activities during or after fallow.