Using remote sensing to determine spatio-temporal variations in winter wheat growing area in the North China Plain

Agricultural land uses affect land surface energy and water balance. The North China Plain (NCP) is one of the most important agricultural regions in China and is experiencing a severe water shortage due to decades of excessive extraction of groundwater for crop irrigation. The precise determination of the distribution of the land areas under different crops in NCP does not only provide the basic information needed for evaluating agricultural water consumption in space but also improve agricultural planning for sustainable water resources management. In this study, we established a method for agricultural land use classification using MODIS NDVI data time-series. Raw 16-day composite NDVI data were first processed using HANTS filtering and then sampled the time-series pattern of NDVI for different croplands based on ground truth data. The typical phenological characteristics of the main crops were determined based on field-monitored phenological data in agro-meteorological stations in the study area. Then multiple cropping index (MCI) was induced based on the NDVI time-series to distinguish land areas under wheat-corn double cropping system. Finally, the cultivated area under winter wheat in NCP for the period of 2000-2013 was extracted by using the CART algorithm. The result was validated with county agricultural statistics data, which showed a statistically significant correlation for the 13-year period with correlation coefficient of 0.94 at 95% confidence level. The probability of consistency less than 40% between the remote-sensing derived cultivated area and agricultural statistics data for the study period was less than 15% for the municipalities in the study area. Agricultural land use frequently changed (about yearly) due to crop rotation, price fluctuation, water limitation, etc. This made it difficult to evaluate agricultural land use change from only comparison of agricultural land use maps for any set of years. Thus in this study, the land use change for different crops was evaluated based on multi-year cropping probability comparison, which composed of actual counts of number of years of wheat cultivation in the study period, with high cultivation probability reflecting dominant crop distribution. Through comparison of the differences in wheat cultivated probability maps for the periods of 2000-2006 and 2007-2013, it was noted that wheat cultivated areas significantly increased in Henan and Shandong Provinces, but significantly decreased in the north part and Heilonggang region of Hebei Plain. These changes were mainly considered to be driven by groundwater conditions and national policies for increased grain production, leading to extensive land reclamation in Henan and Shandong.