Study on The Extraction Method and Spatial-temporal Characteristics of Irrigated Land in Zhangjiakou City

In order to balance the economic and ecological interests, suitable farmland structures in different regions need be established which require understanding the current distribution pattern of irrigated and dry croplands, as well as the evolution rules and reasons of that. In this paper, irrigated croplands in 1985, 2000 and 2015 in Zhangjiakou city which is in the northwest from Beijing were extracted. The study area was divided into Bashang and Bxia aeras depending climate, terrain and agrotype. NDVIs and NDWIs from May to August reflecting vegetation growth and water indexes reflecting vegetation water content were adopted and decision tree classification method was employed. As a result, classification accuracies were high and meet the replying demand with 80.05% and 93.00% in Bashang and Baxia areas respectively. The classification results show that the area of irrigated lands was extended lightly, increasing about 12.73%, reached to 686127 km². Among them, there was 331438 km² converted from dryland with the proportions as 54.45%. By contrast, about 272419 km² irrigated croplands were transformed to drylands. But the plots areas of irrigated croplands were larger, showing a group development trend which is related to the large-scale development of the local vegetable industry in Bashang area. The total area of irrigated croplands was become bigger in intermontane plain around the rivers, while decreased in mountainous areas in Baxia area.