Characteristics and variation law of wind-blown sand delivered to the Ningxia–Inner Mongolia reach of the Yellow River under a changing environment

Wind-blown sand is one of the key factors affecting the evolution of sediment transport, erosion, and deposition in rivers crossing desert areas. However, the differences and complex variations in the spatial and temporal distribution of the underlying surface conditions are seldom considered in research on the river inflow of wind-blown sand over a long time period. The Yellow River contains a large amount of sediment. The Ningxia–Inner Mongolia reach of the Yellow River was selected as the research area of the current study. The reach flows out of Heishanxia and then flows through the Tengger, Hedong, Ulan Buh, and Kubuqi Deserts. In the current study, the wind speed, vegetation coverage, and sand matter on the river basin's surface were analyzed from the perspectives of the river basin surface and riverbank line. The vegetation coverage of the river basin's surface was calculated using the normalized difference vegetation index. Based on the types of sand matter, vegetation coverage, and other underlying surface conditions, the loose particle sediment transport efficiency was determined, the Lettau and Lettau formula for the sediment transport rate was modified, a surface wind-erosion sand flux model was established, and the amount of wind-blown sand transported into the Ningxia–Inner Mongolia reach was calculated. The results show that, from 1981 to 2014, the annual average amount of wind-blown sand transported into the main stream and tributaries of the Ningxia–Inner Mongolia reach of the Yellow River were 7,310,000 and 13,190,000 t, respectively. The Shizuishan–Bayangole reach received 51% of the total wind-blown sand that transported into the main stream, while the tributaries in the Shidakongdui area were the most important source wind-blown sand, providing 74% of the total wind-blown sand inflow from the tributaries. In recent years, the amount of sand transported into the river of the mainstream and tributaries of the Ningxia–Inner Mongolia reach of the Yellow River has significantly decreased from 1981 to 2002, particularly in 1993–2002, which is mainly the result of the weakening wind speed, increasing vegetation coverage, and embankment construction. More specifically, environmental protection policies led by the government, such as “returning farmland to forest”, have played an important and positive role. Therefore, when regulating the water and sediment in the Ningxia–Inner Mongolia reach of the Yellow River, the issue of wind-blown sand deposition into the river should be fully considered in water and sediment regulation.