Abstract:In order to explore variation characteristics of frequency coupling between streamflow and sediment in the period 1 of base(1960-1979),period 2 of comprehensive harness of soil and water conservation (1980-1999) and period 3 of the Grain for Green project (2000-2019) in the Loess Plateau, the study based on measured daily data of streamflow and sediment from 1960 to 2019 at Liujiahe hydrological station in the upper reaches of the Beiluo River Basin in the Loess hilly and gully region, and copula function were used for frequency statistics and simulation of streamflow and sediment.The results indicated that:(1) The Gumbel-Houggard Copula function could well simulate the joint probability distribution of streamflow and sediment. (2) Streamflow and sediment transport decreased significantly in each return period in 5 to 100 years. The asynchronous probability of both rich and poor enhanced at each design frequency from period 1 to period 3. (3) The impact of soil erosion control and ecological environment construction on the ecohydrological process of the Loess Plateau was mainly reflected in greatly reducing the probability of extreme runoff and sediment events in the return period of more than 20 years. Great improvement in the ecological environment of the Loess Plateau has already demonstrated the ecological functions on a regional scale in reducing streamflow and sediment and adjusting the probability of extreme runoff and sediment events.