Tracing sediment sources in small watersheds during an extreme rainstorm using fallout radionuclides in the Wuding...

Following the large-scale implementation of the Grain for Green Program across the Loess Plateau, watershed sediment yield has declined substantially. Nevertheless, the potential shifts in sediment source composition remain poorly understood. Characterizing sediment sources at the small-watershed scale is a fundamental scientific basis for watershed management practices. Based on field investigations of sediment trapped by check dams after the extreme rainstorm on July 26, 2017, this study selected fourteen dam-controlled small watersheds without drainage facilities, which are located in the storm center of the Wuding River Watershed in the Yellow River Basin. The activities of ¹³⁷Cs and ²¹⁰Pb_ex (excess ²¹⁰Pb, refers to the atmospherically derived unsupported fraction of total ²¹⁰Pb in soils and sediments originating from atmospheric fallout) in topsoil samples from hillslopes, gully sides and dam lands were analyzed, the relative sediment contribution proportions of the two sediment sources (hillslopes and gully sides) were quantified for each individual watershed. The results indicate that the activities of ¹³⁷Cs and ²¹⁰Pb_ex in dam-deposited sediments are markedly lower than those in hillslope topsoils but highly consistent with those in gully side topsoils. During this extreme rainstorm, hillslopes only produced a moderate volume of sediment in most study watersheds. In contrast, gully side sediment contributions exceeded 69% of the total sediment yield across all investigated catchments. Although the spatial variability of fallout radionuclides and the inherent limitations of simple mixing models introduce uncertainties to the estimation results, the findings can still broadly reflect the dominant sediment source characteristics of small watersheds. Compared with the pre-conversion land-use conditions in the Loess Hilly Area, gully side sediment production has shown an increasing trend. Despite the dramatic changes in hillslope land-use patterns induced by ecological restoration, gully sides remain the predominant sediment source during extreme rainstorms. Accordingly, priority should be given to gully side erosion control in future watershed management strategies.