Extracting cropping index variations in northern China based on NDVI time-series

Multiple cropping system, characterized by cropping index, is of significant importance to Chinese food production and security. Owing to the changing nature conditions and human activity, cropping index could show remarkable inter-annual fluctuations, which in turn reflects arable land use intensity and indicates climate change impacts on agriculture system. NDVI (Normalized Difference Vegetation Index) time-series is an effective indicator of vegetation status at regional scale. We developed a new method for extracting cropping index from NDVI time-series, and employed it to extract cropping index and its inter-annual variations of northern China from 1982 to 2003. The results show: remotely sensed cropping index is high consistent with statistical data at province scale (R2=0.9213, P<0.001, slope=1.0775), demonstrating that this method can extract cropping index effectively and correctly. The average cropping index of northern China increased from 87.27 in 1982 to 115.98 in 2003, with an average change rate 1.3275 per year(R2=0.7955P<0.001). The areas displayed different changes of cropping index, with Huang-Huai-Hai drainage area experiencing a clear cropping index increase and other regions relatively less cropping index change.