Formulating an elasticity approach to quantify the effects of ecological restoration and climate variability on streamflow and sediment discharge changes in the Loess Plateau of China

Ecological restoration (ER) has strong consequences on hydrological responses. The China’s Loess Plateau (LP) contributed nearly 90% of sediment load in the Yellow River, which was once the world’s largest carrier of fluvial sediment. ER efforts including the soil and water conservation measures (SWCMs, especially terracing and construction of check dams) since 1950s and large-scale ecological restoration campaigns such as Grain-for-Green project (i.e., returning sloping cropland to afforestation and pasture reestablishment) in 1999, has resulted in extensive land use/cover change, leading to considerable decreases of streamflow (Q), suspended sediment yield (SSY) and sediment concentration (C) in the LP over the past 60 years. However, it remains challenging to quantify the impacts of ER and climate variability on declines of Q and especially SSY. In this study, we formulate the notion of elasticity of sediment discharge, by associating SSY change to climate variability and ER over the period 1950s to 2014. Our results strongly support the hypothesis that changes to both streamflow volumes and to the suspended sediment concentration versus water discharge (C-Q) relationships result in reduced SSY, so that streamflow is reduced but runs clearer. We find that two of the ER strategies resulted in weaker relative impacts of climate variability, largely by reducing streamflow (by 55% to 75%). Meanwhile, ER predominantly decreased SSY (by 63% to 81%). Regarding ER practices, (i) the predominant measure acting to reduce SSY changed, over time, from engineering to reforestation; (ii) check-dams preferentially act to regulate the C-Q relationships whereas reforestation preferentially acts to moderate streamflow. Overall, our results suggest that a combination of engineering and vegetation measures is critical to achieving high-efficiency ER. While change to the ER strategy increased the efficiency of streamflow for SSY control, the lost water discharge per unit SSY reduction increased from 5.2 to 6.4 m³·t^-1. Conflicting demands for water necessitate that further ER should target precision management by revegetation of targeted areas in the Loess Plateau.