Recent changes in runoff and sediment regimes and future projections in the Yellow River basin

The runoff and sediment regimes in the Yellow River Basin have been undergone significantly changes in recent years. This study analyzes the changes in runoff and sediment regimes during the period 1997—2006 using the combined approach of hydrological methods,soil and water conservation methods and mathematical modeling. The hydrological and sediment data measured at stations on the main stream and tributaries of the middle and upper reaches of Yellow River are used in the study. The mechanism of changes in runoff and sediment is analysed. Projections of future changes in runoff and sediment regimes are also provided. Results show that in comparison to the long-term average runoff from the source area of Yellow River,a reduction of 43. 90 billion m3 in the annual runoff has been found in recent years( 1997—2005). Among which,precipitation and other natural factors can account for 92. 26% of the reduction and the remaining is due to the effect of human activities. The observed value of Yellow River annual runoff during the same period has decreased 112. 1 billion m3compared to that before 1970. Human activities are responsible for 76. 50% of the decrease and the rest is due to the reduction of precipitation. At the same time,the observed annual sediment load has been reduced by 11. 80 million tons,49. 75% of which is due to the result of human activities such as the integrated control measures on soil and water loss and the remaining portion of the reduction can attribute to the reduction of precipitation. The impacts of human activities and precipitation reduction on the changes in runoff and sediment regimes vary significantly over space. For example,the effect of human activities on runoff reductions is much greater than that of precipitation reduction in the area along the middle reach of Yellow River. While for the reduction of sediment load,the human activities contribute nearly as much as precipitation does. Despite the fact that significant changes have been found in runoff and sediment regimes,the quantitative relationships generally remain unchanged between runoff and sediment load and between precipitation,floods and sediment load in most areas of Yellow River tributary basins. For example,the hyperconcentrated flow occurs during heavy storm events. There are few exceptions on the relationships between precipitation,floods and sediment load in individual tributary basins,which are often accompanied by a significant reduction in runoff and sediment yields. A slight downward trend is projected for both runoff amount and sediment load in the coming decades until 2050 with occasional occurrences of plentiful runoff and heavy sediment load.