Winter Rape Mapping On The Middle Reaches Of The Yangtze River Valley Based On Winter Rape Index

Rape,also known as rapeseed,is the third largest oil crop in the world following soybeans and palm and the fifth largest crop type in China.The production and planting area of winter rape accounts for more than 90%of total production of rape in China,and the Middle Reaches of the Yangtze River(MRYR)is one of the main production areas of rape in China.The continuous development of the market economy and urbanization in the region in recent years has been accompanied by a declining frequency of crops,a significant reduction in cropland and a dramatic change in the structure of crop cultivation.The efficient,accurate and timely acquisition of spatial distribution information of winter rape is of great importance for crop yield estimation,agricultural cultivation structure optimization and national food security.Based on the Google Earth Engine cloud computing platform and multi-source satellite remote sensing data,this study focuses on the differences in the time series characteristics of two key winter crops,winter rape and winter wheat,and proposes a new index,the winter rape index.And based on the winter rape index,a hierarchical classification method was designed to achieve remote sensing extraction of the spatial distribution of winter rape in the MRYR in 2020.Firstly,according to the land cover type of the study area,GlobeLand30 was introduced to obtain the distribution of arable land in the study area.Within the cropland,winter crop index was designed and combined with NDVI to obtain winter crop mask data in the study area based on the distribution of cropland.Secondly,the winter rape index was designed based on the differences in the phenological characteristics of winter rape and winter wheat,which are easily confused,to improve the separability of winter oilseed rape and winter wheat.Then,the feature images of winter rape index after cropland mask and winter crop mask were partitioned by dynamic threshold segmentation to obtain the spatial distribution of winter rape in the study area and analyze the distribution characteristics of winter rape.Finally,the experimental results are compared with the extraction results based on machine learning methods.The main conclusions of this study are as follows:(1)Combining the characteristic differences of two winter crops,winter rape and winter wheat,this paper proposes an automated extraction method of winter rape at a large scale based on winter rape index.Based on this method,the extraction of the spatial distribution of winter rape in the middle reaches of Yangtze River in 2020 was achieved.The extraction results achieved good accuracy both from qualitative(visual comparison analysis)and quantitative evaluation(confusion matrix analysis),which proved the feasibility of this paper’s method in conducting large regional winter rape mapping.(2)The winter crop index proposed in this paper has a better ability to distinguish winter rape from winter wheat compared with traditional vegetation indices such as NDVI and GCVI.Also,the winter crop index expands the time window for effective identification of winter rape remote sensing images acquisition and can better distinguish winter rape and winter wheat in a longer time window period,which can meet the needs of large-scale winter rape mapping.Compared with the machine learning algorithm,the method in this paper is more effective in distinguishing winter rape from winter wheat,and it can achieve fast extraction of winter rape in large areas without training samples.(3)The spatial distributions of winter rape on MRYR in the last three years were extracted using the method of this paper,and the spatial distribution of winter rape has obvious regional characteristics.The hot spots of winter rape cultivation are mainly located in Hubei and Hunan provinces,i.e.Dongting Lake Plain and Jianghan Plain areas(Jiangling County,Shayang County and Zhongxiang City,etc.),and there are also small distributions in the southeastern part of Hubei(Huangmei County and Herb Spring County),Anhui Province and along the rivers of Jiangxi Province.In addition,the total amount of winter oilseed rape in the Jianghan Plain area was significantly more than that in the Dongting Lake Plain area,and the distribution was more concentrated.