The contribution of human activities to runoff and sediment changes in the Mang River basin of the Loess Plateau, China

Soil erosion changes after China's Grain-for-Green Program (GGP) have attracted widespread attention. Many studies have reported positive contributions of the GGP to sediment reduction; however, GGP effects on runoff remain under debate. The objective of this study was to identify runoff and sediment changes and estimate the contribution of human activities to soil erosion decreases in the Mang River basin (MRB) in the southeastern Loess Plateau. The temporal trends of the rainfall-runoff-sediment series between 1960 and 2017 and the contribution of human effects to runoff and sediment changes were determined using time-series analysis methods. The main transformations in land-use types were from farmland to forestland in the upstream MRB and farmland to built-up land in the downstream MRB. Rainfall remained in a relatively steady state, while runoff and sediment had descending trends, both of which began to mutate in 1990 and 1994, respectively. The average runoff (sediment load) decreased by 33 x 10(4) m(3) (5 x 10(4) t yr(-1)), of which the human effect contributed to 79% (93%) of the decreases. During the descending period, the human effects on runoff and sediment increased with time. Sediment experienced a much larger reduction effect than runoff because of increased forestland area, which can greatly reduce soil erosion, in the MRB upstream of our study site; however, runoff was increased in the MRB downstream of our study site because urbanization led to the expansion of impervious surfaces. Our results provide new lessons for assessing the reduction effects of the GGP on soil erosion in an urbanized basin.