Abstract: In recent years, the runoff and sediment in wide-valleyed desert reach of Upper Yellow River has changed significantly, which would bring a great influence on practices of Yellow River harnessing such as river regulation and layout of large-scale hydraulic project. Based on data of runoff and sediment locating observation, the first-ever nationwide water resources survey and remote-sensing imagery interpretation, the features of runoff and sediment variations in the Upper Yellow River were analyzed by MWP, double mass-curve, hydrological method, soil conservation method and mathematical simulation. The contribution rate of driving factors for runoff and sediment variations in wide-valleyed desert reach from Xiaheyan to Toudaoguai during 2000-2012 was estimated. The results showed that: Compared with the standard period of less human activity before the 1970s, since 2000 the runoff and sediment discharge has been less than standard period, and sediment discharge per unit volume of runoff has decreased obviously due to the variation of water-sediment relationship. The reduction of runoff along the flow increased continuously while the reduction of sediment discharge along the flow showed little change in wide-valleyed desert reach, annual distribution of runoff and sediment were in contrast with that of standard period, and the incoming sediment coefficient decreased constantly. The main influence factors for runoff and sediment variations in wide-valleyed desert reach are the water and sediment diversion of irrigation district, soil and water conservation measures, sediment detention by reservoirs, incoming water and sediment from tributaries, reservoir regulation and channel scouring and deposition. The water demand of economic and social development contributed the most to reduction of runoff in the wide-valleyed desert reach, whose contribution rate was 81%, and the storage and discharge volume of reservoir was secondary, whose contribution rate was more than 15%. The sediment detention by reservoirs contributed the most to reduction of sediment discharge in Toudaoguai, whose contribution rate was 41%, the storage and discharge volume of reservoir was secondary, whose contribution rate was more than 13%, and the wind-blown sand into Yellow River contributed little, whose contribution rate was 6%, while the discharges of channel deposition and sediment diversion of irrigation district were less than those of the standard period, whose negative contribution rate were 41% and 8%, respectively. In recent 10 years, natural factors such as rainfall have contributed little to the runoff and sediment variations, while the factors of human activity such as reservoir operation, soil and water conservation and economic and social development have played a major role.

Key words: runoff and sediment, driving factors, wide-valleyed riverbed, Yellow River