Abstract
Abstract. Suspended sediment transport in rivers is controlled by terrain, climate, and human activities. These variables affect hillslope and riverbank erosion at the source, transport velocities and sedimentation opportunities in the river channel, and trapping in reservoirs. The relative importance of those factors varies by context, but the specific attribution to sediment transfer is important for policymaking, and has wide implications on watershed management. In our research, we analyzed data from the Kejie watershed in the upper Salween River (Yunnan Province, China), where a combination of land cover change (reforestation, as well as soil and water conservation measures) and river channel engineering (sand mining and check dam construction) interact with a changing climate. Records (1971–2010) of river flow and suspended sediment loads were combined with five land-use maps from 1974, 1991, 2001, 2006 and 2009. Average annual sediment yield decreased from 13.7 t ha−1 yr−1 to 8.3 t ha−1 yr−1 between the period 1971–1985 and the period 1986–2010. A distributed hydrological model (Soil and Water Assessment Tools, SWAT) was set up to simulate the sediment sourcing and transport process. By recombining land-use and climate data for the two periods in model scenarios, the contribution of these two factors could be assessed with engineering effects derived from residual measured minus modeled transport. Overall, we found that 47.8% of the decrease was due to land-use and land cover change, 19.8% to climate change, resulting in a milder rainfall regime, 26.1% to watershed engineering measures, and the remaining 6.3% was due to the simulation percent bias. Moreover, mean annual suspended sediment yield decreased drastically with the increase of forest cover, making diverse forest cover one of the most effective ecosystems to control erosion. For consideration of stakeholders and policymakers, we also discuss at length the modeling uncertainty and implications for future soil and water conservation initiatives in China.
Highlights
Sediment transport in rivers can be a symptom of systemic erosion problems, but it increases with landslides, riverbank instability, and human disturbances such as construction and mining activities (Verbist et al, 2010). Walling and Fang (2003) found that among 145 rivers in a global data set on annual sediment loads, 4.8 % (7 rivers) had an increased load, 49.3 % (70 rivers) were stable, and 46.9 % (68 rivers) had a decreased load, probably mostly due to reservoir construction. Liu et al (2008) classified the 10 major rivers in China, and found 7 with decreasing sediment and stable runoff, 1 with decreasing sediment and runoff, and 2 with significant decreases in sediment and runoff
A similar pattern of sediment yield change was observed at the Yichang station on the Yangtze River (Dai et al, 2009)
There was some uncertainty in the sediment simulation, the results indicate the modeling captured 63 % (58 %) of the measured suspended sediment yield (SSY) with 0.57 (0.52) of 95PPU band width in the calibration period
Summary
Sediment transport in rivers can be a symptom of systemic erosion problems, but it increases with landslides (natural or unnatural), riverbank instability, and human disturbances such as (road) construction and mining activities (Verbist et al, 2010). Walling and Fang (2003) found that among 145 rivers in a global data set on annual sediment loads, 4.8 % (7 rivers) had an increased load, 49.3 % (70 rivers) were stable, and 46.9 % (68 rivers) had a decreased load, probably mostly due to reservoir construction. Liu et al (2008) classified the 10 major rivers in China, and found 7 with decreasing sediment and stable runoff, 1 with decreasing sediment and runoff, and 2 with significant decreases in sediment and runoff. Walling and Fang (2003) found that among 145 rivers in a global data set on annual sediment loads, 4.8 % (7 rivers) had an increased load, 49.3 % (70 rivers) were stable, and 46.9 % (68 rivers) had a decreased load, probably mostly due to reservoir construction. Liu et al (2008) classified the 10 major rivers in China, and found 7 with decreasing sediment and stable runoff, 1 with decreasing sediment and runoff, and 2 with significant decreases in sediment and runoff. X. Ma et al.: Reduction of suspended sediment in the Kejie catchment reported that the decadal suspended sediment flux decreased by 70.2 % from 1.81 Gt yr−1 for 1954–1963 to 0.54 Gt yr−1 for the period 1996–2005 in nine major rivers in China
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