Spatial variation of event-based suspended sediment dynamics in the middle Yellow River basin, China

Abstract

Event-based suspended sediment dynamics (ESSD) provides insights into soil erosion and sediment transport and signals of changing environments within catchments, but it is still poorly understood in the middle Yellow River basin (MYRB). This study focuses on the hysteresis relationship between suspended sediment concentration (SSC) and flow discharge (Q), interpreting the ESSD and its spatial variation. The datasets in the 1960s from 73 hydrologic gauging stations were analyzed as the baseline of the MYRB without experiencing significant climate change and human disturbances. The Q-SSC hysteresis loops were first analyzed in terms of their direction and shape, and the Lloyd et al.'s (2016a) hysteresis index (HI) was adopted to quantify the hysteresis characteristics. Two types of catchments have been identified across the MYRB in terms of the dominant hysteresis pattern and the sign of the HI value. The dominant loops of the Type A catchments are clockwise with positive HI values and located in the southernmost rock mountain regions; Type B appears elsewhere, i.e., in the softstone, aeolian, and loess regions, where most of the events present anticlockwise or complex loops with generally negative HI values. The HI follows an upward-concave parabolic curve with either mean annual precipitation (Pm) or vegetation cover (NDVI). Two sets of thresholds are determined as Pm or NDVI increases. At Pm = 670 mm or NDVI = 0.74, a transition from Type B to Type A occurs as the HI value changes from negative to positive because the vegetation cover is too high to change the catchment from non-limited to limited supply. At Pm = 490 mm or NDVI = 0.40, the negative HI value reaches its minimum, suggesting the maximum availability of sediment in Type B due to the change in the dominant factor of soil erosion from precipitation erosivity to erosion resistance of vegetation. This study highlights the overall dynamics of soil erosion and sediment transport in the MYRB, which helps prioritize watershed management measures on the Loess Plateau under a changing environment.