Sediment yield of current underlying surface under simulated extreme rainstorm in middle reaches of Yellow River Basin

The flood control of the Yellow River Basin has been very difficult due to the heavy sedimentation of riverbed. So it is the very important to predict and evaluate the future situation of the sediment yield of the Yellow River Basin for deciding strategy of Yellow River governance and development, water resources allocation, plan and operation of the major water conservancy projects. Since 2000, the ecological environment of the Loess Plateau has been greatly improved and the sediment discharge of the Yellow River has decreased significantly. In this background, it is necessary to identify objectively the maximum possible sediment discharge of the Yellow River under the extreme rainstorm, so as to provide decision-making references for flood control and sedimentation reduction of the Yellow River. In this paper, the region between Hekouzhen and Tongguan in the middle reaches of the Yellow River basin was selected as the study area which contributes about 91% of the sediment discharge of the Yellow River Basin, and the underlying surface during 2010 to 2018 was selected as the current underlying surface because the corresponding environment situation of the Loess Plateau in the terms of sediment produce and transportation tend to be stable after experiencing rapid improvement of vegetation for years，and also because there were more rainstorms. Two extreme rainstorm scenarios were designed, one assumes that the rainstorms in 1933 have recurred, and other assumes that the maximum rainstorm in each tributary from 2010 to 2018 have occurred in the same year. Respectively using the method of summing up the annual measured sediment of the maximal rainstorm year of 2010-2018 in each tributary and using the hydrological method based on the current relationship between rainfall and sediment yield, the possible amount of sediment yield of the current underlying surface under extreme rainstorm in the study area was analyzed. In 1933, the Loess Plateau experienced the heaviest rainfall in the past 100 years. The magnitude of synthetic rainstorm was second only to the one in 1933. The difference of the two extreme rainstorm scenarios was the rainstorm area. The results revealed that if the maximum rainstorm of each tributary from 2010 to 2018 occurred in the same year under the current underlying surface condition, the sediment discharge poured into Yellow River in the study area would reach 620 million tons, and the corresponding regional sediment yield would be 990 million tons. If the rainfall scenario in 1933 reappeared, the annual sediment discharge would reach 940 million tons, based on the relationship between rainfall and sediment discharge under the current surface condition in study area, and the corresponding sediment yield would be 1.24 billion tons. The two extreme rainstorm scenarios used in this paper can only be regarded as the maximum rainfall in the past 100 years (not in hundreds of years), and also the calculation process took no account of the unfavorable conditions such as sediment releasing in case of check dams damage, or the situation which the extreme rainstorm occurs after successive years of drought. Otherwise, the sediment discharge would be even larger. Therefore, even if the forest and grass vegetation in the Loess Plateau has been greatly improved, and a large number of terraces and check dams have been built and put into operation, the Yellow River flood control and regulation of key reservoirs still needs to be prepared for large amount of sediment.