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Abstract
Monitoring the phenological development of agricultural plants is of high importance for farmers to adapt their management strategies and estimate yields. The aim of this study is to analyze the sensitivity of remote sensing features to phenological development of winter wheat and winter barley and to test their transferability in two test sites in Northeast Germany and in two years. Local minima, local maxima and breakpoints of smoothed time series of synthetic aperture radar (SAR) data of the Sentinel-1 VH (vertical-horizontal) and VV (vertical-vertical) intensities and their ratio VH/VV; of the polarimetric features entropy, anisotropy and alpha derived from polarimetric decomposition; as well as of the vegetation index NDVI (Normalized Difference Vegetation Index) calculated using optical data of Sentinel-2 are compared with entry dates of phenological stages. The beginning of stem elongation produces a breakpoint in the time series of most parameters for wheat and barley. Furthermore, the beginning of heading could be detected by all parameters, whereas particularly a local minimum of VH and VV backscatter is observed less then 5 days before the entry date. The medium milk stage can not be detected reliably, whereas the hard dough stage of barley takes place approximately 6–8 days around a local maximum of VH backscatter in 2018. Harvest is detected for barley using the fourth breakpoint of most parameters. The study shows that backscatter and polarimetric parameters as well as the NDVI are sensitive to specific phenological developments. The transferability of the approach is demonstrated, whereas differences between test sites and years are mainly caused by meteorological differences.
Highlights
The adaption of agricultural production to climatic changes to ensure global food security as well as the simultaneous conservation of the environment in times of increasing land scarcity is one of the main tasks of agriculture today [1,2,3]
The aim of this study is to identify phenological entry dates of winter wheat and winter barley using smoothed time series of the Sentinel-1 radar backscatter parameters
This study confirms that time series features of different synthetic aperture radar (SAR) parameters as well as NDVI are sensitive to phenological changes and can detect phenological entry dates with different accuracies and transferabilites between years and test sites
Summary
The adaption of agricultural production to climatic changes to ensure global food security as well as the simultaneous conservation of the environment in times of increasing land scarcity is one of the main tasks of agriculture today [1,2,3]. In this context, the knowledge about prevailing phenological conditions of agricultural crops is of high importance for farmers. Brandenburg in a range of hills called “Fläming Heath” The climate in both test sites belongs to the transition zone between continental and maritime climate with mean annual temperatures of 8.8 ◦ C (DEMMIN) and 9.3 ◦ C (Blönsdorf). The two analyzed years 2017 and 2018 are meteorologically very different with remarkably lower precipitation sums and higher temperatures in 2018 (Figure 2)
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