Crop planting structure and water demand satisfaction degree in Manas River Basin from 2000 to 2020

Abstract: Efficient utilization of water and soil resources has been critical under the changing environmental conditions in the arid oasis. It is very necessary to understand the temporal and spatial evolution of crop planting structure and crop water demand satisfaction degree (i.e. the ratio of supplied water amount to water demand) for the efficient use of water and soil resources and sustainable development of agriculture in the arid Manas River Basin, including the largest typically agricultural oasis in Xinjiang of western China. In this study, a classification model was established for the surface features via the remote sensing inversion using the Google Earth Engine (GEE) cloud computing platform, together with the coupled Simple Non-Iterative Clustering super pixel image segmentation and Random Forest classifier. The classification model was verified using a regional survey at the fixing sampling positions with the GPS. Then, the model was applied to the crop planting structure in the Manas River Basin over the past 20 years from 2000 to 2020. The Penman-Monteith formula was used to calculate the effective precipitation in the irrigation schedule from the regional survey and the water demand of the main crop (drip-irrigated cotton under film mulch). A systematic investigation was made to explore the effect of the crop planting structure on the irrigation water supply and water demand satisfaction degree for the drip-irrigated cotton under film mulch. Results showed that the new remote sensing inversion model was reliable for the surface feature classification in the Manas River Basin, with an annual average overall accuracy of 90%. The planting crop of the basin was dominated by drip-irrigated and film-mulched cotton, accounting for more than 80% of the total planting area in all the last 21 years, and mainly distributed in the midstream and downstream regions, where the climate and hydrological conditions were more suitable for the cotton growth. Beneficial from the water-saving and salinization-alleviating characteristics of drip irrigation under the film mulch, a lot of mildly- and moderately- or even severely-salinized wastelands were continuously reclaimed into the cotton fields, resulting in an increased rate of about 101 km2 yr-1 in the area. There was an ever-increasing prominent contradiction between the expansion of cotton fields and the limited supply of irrigation water resources, particularly with a significant declining trend in the water demand satisfaction degree for cotton. The irrigation amount and water demand satisfaction degree in downstream were generally lower than those in the midstream, due to the low irrigation and drainage conditions, especially when the crop was in the peak water demand during the summer irrigation period. The average water demand satisfaction degree decreased even to less than 53% over the entire basin in the whole irrigation period by 2020. It is a high demand to optimize the crop planting structure in the arid Manas River Basin.