Developing a New Method to Identify Flowering Dynamics of Rapeseed Using Landsat 8 and Sentinel-1/2

Jichong Han,Zhao Zhang,Juan Cao

doi:10.3390/rs13010105

Abstract

Identifying the rapeseed (Brassica napus L.) flowering dates are important for planting area estimation, growth monitoring, and yield estimation. However, there is currently a lack of data on rapeseed flowering dates at the parcel scale. In this study, a new spectral index (Normalized Rapeseed Flowering Index, NRFI) is proposed to detect rapeseed flowering dates from time series data generated from Landsat 8 OLI and Sentinel-2 sensors. This study also analyzed the feasibility of using the backscattering coefficients (VV, VH, and VV/VH) of Sentinel-1 to detect the flowering dates of rapeseed at the parcel scale. Based on the spectral and polarization characteristics of 718 rapeseed parcels collected in 2018, we developed a method to automatically identify peak flowering dates by the local maximum of NRFI series and the local minimum of VH and VV, along with the maximum of VV/VH. The results show that most of the peak flowering dates derived from Sentinel-1 and Sentinel-2 can be confirmed by the in-situ phenological observations at the Deutscher Wetterdienst (DWD) stations in Germany. The NRFI outperforms the Normalized Difference Yellow Index (NDYI) in identifying the peak flowering dates from Landsat 8. The derived medians of peak flowering dates by NRFI, NDYI (Sentinel-2), and VH are similar, while a systematic delay is observed by NDYI (Landsat 8). The method with the spectrum and backscattering coefficients will be a potential tool to identify crop flowering dynamics and map crop planting area.

Highlights

Rapeseed (Brassica napus L.) is a staple oil crop used for cooking, high-quality animal feed, etc. [1]
Three different canopy morphologies are in the sequence indicated with the development of rapeseed, which is characterized by leaves, yellow flowers, and pods, respectively [35]
Corresponding to the true color images above, the time series of spectral reflectance is shown in Figure 4f,g, which represents the mean values of 718 rapeseed parcels in four bands including blue, green, red, and SWIR2

Summary

Introduction

Rapeseed (Brassica napus L.) is a staple oil crop used for cooking, high-quality animal feed, etc. [1]. Rapeseed (Brassica napus L.) is a staple oil crop used for cooking, high-quality animal feed, etc. The timing and intensity of flowering are closely related to the final yield of rapeseed [2,7,8,9]. The accurate flowering dates are key information to map rapeseed planting areas [15,16]. The peak flowering period is highly informative for monitoring growth status and predicting yield [2,17]. It is of vital importance to accurately identify the flowering dates of rapeseed. Given the high spatial heterogeneities of cultivars, farmers’ management practices, climate, and soil conditions, it is still challenging to detect the peak flowering dates of rapeseed timely and soundly [18]

Objectives

Methods

Results

Discussion

Conclusion