Did streamflow or suspended sediment concentration changes reduce sediment load in the middle reaches of the Yellow River?

Abstract

The continuous ecological restoration of the Loess Plateau, which aims to reduce the sediment entering into the Yellow River, is known throughout the world for two strategies: the integrated soil conservation project that began in the 1970s, and the “Grain for Green” project that began in the 1990s. However, the topic of whether the muddy water in the middle Yellow River run clearer remains debatable, and, in fact, response to the topic is reasonably well documented in regard to hydrological changes in the sediment source area. Six sub-catchments nested in the Beiluo River basin – one of the major sediment sources for the Yellow River – were selected, with data series ranging from 1957 to 2009. The Mann-Kendall and Pettitt tests were used for trend detection. A simple method was developed based on the distribution of suspended sediment concentration (SSC) versus water discharge. Using this method, we evaluated the quantities of sediment yield reduction attributed to streamflow and SSC changes due to the two strategies.The results showed that annual sediment yield in 5 out of 6 stations significantly decreased, with rates varying from −4 to −217t·km−2·yr−1. Significant decreases in daily and event streamflow and suspended sediment concentration were identified, especially at a high SSC (top 1–5%). During the integrated soil conservation period, the sediment yield was reduced mainly by decreases in high flow and high SSC conditions. In contrast, during the “Grain for Green” period, sediment yield was reduced due to decreases in streamflow and SSC at all magnitudes. It was concluded that rainfall-sediment load dynamics have changed in the context of ecological restoration. Changes in both streamflow and the SSC–water discharge relationship induced the sediment yield reduction over time; in other words, the streamflow in the middle reaches of Yellow River became clearer during periods of ecological restoration. Moreover, the increased annual sediment yield at the Zhangcunyi station exposed a risk of increased erosion in areas where forests had been well preserved.