Quantifying sediment source contributions in an agricultural catchment with ephemeral and classic gullies using 137Cs technique

Soil loss caused gully erosion can be substantial in agricultural catchments in the Mollisol region of northeast China. However, their contributions to total soil loss have never been quantified systematically at various spatial scales. Here we selected an agricultural catchment with ephemeral and classic gullies in Keshan county, Heilongjiang province. The fallout Cs-137 activity adjusted to fine fraction of < 63 mu m was employed as tracer to quantify the contribution of fine sediment eroded from topsoil and subsoil (gully bank) in the whole catchment and each sub-catchment by using a linear mixing model. The results showed that the coefficient of variations (CVs) of Cs-137 activity for subsoil, ranging from 86.4 to 166.1%, were greater than those of topsoil varying from 18.2 to 39.9%. In topsoil, cropland had the greatest Cs-137 concentration in both < 1 mm and < 63 mu m fractions, followed by forestland and road. Although the Cs-137 activity varied greatly in sources, the contrasting concentrations between sources render Cs-137 an effective tracer for distinguishing topsoil and subsoil contributions to sediment discharge. The allocated proportions of both deposited (bed) and suspended (trap) sediment with different particle sizes showed the eroded fine sediment (<63 mu m fraction) of the catchment mainly came from gully bank, accounting for about 90%, but the significant difference of apportionment between deposited and suspended sediment was obtained in some catchments, indicating that caution must be exercised when taking deposited sediment out of bed materials. The result for suspended sediment showed contributions of subsoil in catchment with classic gullies were greater than those with ephemeral gullies, and the gully head and bank were the primary sediment sources. This study provided valuable quantitative information of sediment sources for agricultural catchments with both ephemeral and classic gullies, and suggested soil conservation measures be concentrated on stabilizing gullies, especially controlling gully head advance.