Changes and influencing factors of the sediment load in the Xiliugou basin of the upper Yellow River, China

Based on the MK test and RUSLE model, changes in the soil erosion and sediment yield of the Xiliugou basin in the upper Yellow River during the period 1960-2013 were studied. The results indicate that hyperconcentrated floods are very typical in the Xiliugou basin. Hyperconcentrated floods occurred in 87.1% of the years during the period 1960-1990. Since the 2000s, the number of such floods was reduced considerably with the application of soil and water conservation, and only one hyperconcentrated flood was recorded from 2006 to 2013. Hyperconcentrated flows transfer most of the sediment yield in the Xiliugou basin. The sediment load carried by hyperconcentrated floods accounted for 84.7% of the total sediment yield in that basin during the period 1964-1990. Because hyperconcentrated flows may cause high and intensive erosion, the sediment eroded in gullies may be the main sediment source in the basin. Our results show that the percentages of gully erosion and slope erosion in this basin in the period 1964-1990 were 76.1% and 23.9%, respectively. A daily rainfall larger than 12 mm can cause hyperconcentrated flows, and the frequency of hyperconcentrated floods increases with the rainfall intensity. As a result of vegetation recovery and check-dam building, the frequency of hyperconcentrated flows and the annual sediment concentration and sediment yield for the same intensity of rainstorms were noticeably lower from 2006 to 2013 than those from 1964 to 1990. From 1960 to 2013, the annual sediment yield of the basin showed a significant downward trend, whereas no significant trend was detected in the annual precipitation. The annual sediment yield and precipitation from 1999 to 2013 decreased by 82.1% and 1.7%, respectively, compared with those in the period 1960-1998. The implementation of water and soil conservation measures and the reduced frequency of hyperconcentrated floods were the main reasons for the significant reduction in sediment yield in the period 2006-2013.