Changes in maximum daily runoff depth and suspended sediment yield on the Loess Plateau, China

Abstract

The maximum river flow is of important reference significance to flood disaster prevention on the Loess Plateau of central China. However, due to the lack of hydrological data, previous studies have rarely explored spatiotemporal changes in the annual maximum daily runoff depth (Hmax) and annual maximum daily suspended sediment yield (SSYmax) of rivers in this region. This study uses daily discharge data and daily sediment concentration data from 104 hydrological stations on the plateau to compare the Hmax and the SSYmax during period P1 (1971–1987) and period P2 (2008–2016). The results show that 66% of stations exhibited a significant decline in multi-year mean Hmax from P1 to P2 (p < 0.05). However, the multi-year mean SSYmax showed more widespread changes, with 77% of stations seeing a significant decline (p < 0.05). And on average, the runoff on the day of Hmax contributed 6.6% and 5.8% of the annual runoff during P1 and P2, respectively; by contrast, SSYmax contributed 27.9% and 33.5%, respectively, to the total annual suspended sediment yield in P1 and P2. Analyses using Spearman’s rank correlation coefficient indicate that the spatial distribution of Hmax has larger variability than SSYmax during both P1 and P2 and that the spatial distribution pattern of SSYmax during P2 is more unstable than during P1. We found that the suspended sediment concentration (SSC) on the day that Hmax occurs (SSChmax) remained relatively stable for most stations studied within the loess gullied-hilly area across the Loess Plateau, but the stable SSChmax value during P1 differed from the value during P2. We conclude that human activities (e.g., afforestation, terracing, and construction of check dams) may be the cause of the differing SSC values during peak discharge between the two periods. We also found that the average percentage of years in which Hmax and SSYmax occurred on the same day on the plateau was 55%. Many factors can lead to Hmax and SSYmax happening asynchronously, such as heavy runoff events lasting several consecutive days, the characteristics of a particular basin, and the influence of human activities. These results provide information that will be useful for understanding how to manage extreme hydrological events on the Loess Plateau.