Quantifying the contributions of climate variation, land use change, and engineering measures for dramatic reduction in streamflow and sediment in a typical loess watershed, China

The Loess Plateau (LP) in China is among the most seriously eroded areas on the earth. However, dramatic reduction of sediment load to rivers/tributaries on the LP has been observed, causing a record low level of sediment in the main channel of the Yellow River. Quantitative attribution of such a remarkable reduction in streamflow and sediment is important for designing sustainable watershed management strategies in the changing environment. This study used a typical loess watershed, the Zhou River, to quantitatively attribute the reduction of streamflow and sediment to climate variation, land use change characterized by revegetation, and landscape engineering measures (e.g., check-dams). In this study, we used the physically-based Soil and Water Assessment Tools (SWAT) and data covering 1971-2016 with the first 26 years (1971-1996) as the baseline period (1996 is the change-point year) and the remaining 20 years (1997-2016) as the changing period. The results suggested that streamflow and sediment load of the Zhou River Basin decreased by over 50% and 80%, respectively, since the change-point year 1996. The changing period (1997-2016) was further divided into two distinct periods in terms of the reduction rate of streamflow and sediment-a dramatic decrease from 1997 to 2006 (P-1) and a mild decrease from 2007 to 2016 (P-2). The attribution results showed that the landscape engineering measures were mainly responsible for both the streamflow and sediment load reduction for the last two decades, with an increasing contribution in streamflow reduction from 54.4% in P-1 to 94.7% in P-2 and no substantial change on sediment load reduction (61.6% in P-1 to 62.8% in P-2). Land use change was the second important contributor to sediment load reduction, and its contribution nearly doubled for the last two decades, from 23.0% in P-1 to 41.8% in P-2. However, the effects of climate variation on both streamflow and sediment load reduction varied greatly for the two decades. More interestingly, both land use change and landscape engineering measures, especially the latter, led to an increasing cost of water (i.e., reduced water availability) for sediment load control, alerting further expansion of these measures considering the conflict between water and sediment. Overall, this study has important implications for formulating the sustainable management strategies in this region.