The Detecting of Irrigated Croplands Changes in 1987–2015 in Zhangjiakou

The distribution of irrigated and rained croplands has a large impact on agricultural production and ecological systems. Zhangjiakou, which is the water conservation area of Jing-jin-ji agglomeration and the host city for the 2022 Winter Olympic Games, has large irrigated croplands and a fragile ecological environment. The methods of sample selection based on spectral analysis and partitioned extraction of irrigated croplands were proposed with medium-high-resolution images, then the spatiotemporal changes of irrigated and rainfed croplands were explored in Zhangjiakou. The growing season time series curves of normalized difference vegetation index (NDVI) and normalized difference water index (NDWI) based on Landsat images were employed to distinguish irrigated and rainfed croplands in 1987, 2000, and 2015. Through transfer matrix analysis of the three stages of classification results, it was found that the irrigated croplands of Bashang, a semi-arid region, increased sharply (66025 hectares, 7.55%), while the irrigated croplands of Baxia, a mountainous region with small basins and plains, decreased slightly (-8239 hectares, -0.60%). From the perspective of temporal changes, irrigated croplands grew rapidly in the early stage and slowly decline in the later stage, peaking at 377,117 hectares in 2000. According to the analysis, the decline of irrigated croplands was due to a project employed to quit irrigation and return to drought, and further analyses should be conducted in order to realize the rational utilization of water resources and guarantee ecological security. This paper is helpful in supporting extraction methods of irrigated croplands for semi-dry and arid areas, especially when historical samples are lacking. It is also helpful for policy adjustments of agricultural structure of Zhangjiakou and other similar areas.