Dating reservoir deposits to assess the recently changing sediment yields from a medium-sized agricultural catchment

Medium-sized catchments exhibit different patterns of sediment yield than small catchments and large river basins (taking scale dependency into account) and play in soil loss from upstream areas with downstream sediment transportation over larger spatial scales than small catchments. Few studies have used reservoir depositional records to assess historical sediment yields at the scale of a medium-sized catchment. By analyzing six cores obtained from reservoir deposits, this investigation reconstructed the recent sediment yield from a medium-sized agricultural catchment (the Xujiaya catchment in Northern China) and examined the changing pattern of sediment yield over the past 57 years. The cores were dated using the Cs-137 and Pb-210(ex) methods. The historical specific sediment yields (SSYs) were estimated based on the original reservoir capacity curve and the depositional chronology modeled using a composite chronological model based on the constant rate of supply (CRS) approach (i.e., the C-CRS model). The mean SSYs estimated from the six cores over each of seven short stages (i.e., 1959-1963, 1964-1972, 1973-1980, 1981-1989, 1990-1997, 1998-2007, and 2008-2016) defined over the 57-year study period ranged between 4.6 +/- 1.8 and 16.2 +/- 6.3 t ha(-1) yr(-1), with a mean value of 8.1 +/- 3.2 ha(-1) yr(-1) for all seven stages, and this mean value can be used to broadly represent the sediment yield from the catchment since dam construction. These SSY values are relatively high, reflecting intense agricultural disturbance and the underlying lithology. The values were highest for the 1959-1963 stage and lowest in the following stage of 1964-1972 and then generally increased over the successive stages of 1973-1980, 1981-1989, and 1990-1997 before decreasing slightly during the last two stages after 1997. This change was driven mainly by human activity over the past 57 years. The results indicate that the environmental factors (e.g., human activity and lithology) influencing sediment yield in a medium-sized catchment are more complicated than those affecting a small catchment and reveal the characteristic pattern of sediment yield in medium-sized catchments.