Identification of Suspended Sediment Sources Using Soil Characteristics in a Semiarid Watershed

Abstract

Identification of primary sediment source areas in watersheds is necessary to ensure that best management practices are installed in areas that maximize reductions in sediment and chemical loadings of receiving waters. Our objectives were to use a soil geomorphology–erodibility approach to locate sediment sources in the Walnut Gulch Experimental Watershed (WGEW). Major soil mapping units were sampled along transects in six subwatersheds (SWs). At each sampling point, latitude–longitude, slope gradient, slope aspect, and hillslope position were recorded. Samples collected from the surface 5.0 cm were characterized for a range of basic soil characterization properties. Additionally, 137Cs, 40K, 226Ra, and stable C isotope distributions were quantified as potential source area indicators. Suspended sediment samples collected from WGEW and SW flumes were characterized for the same properties. Relative to the SW soils, the suspended sediments were generally enriched in silt, clay, organic C, inorganic C, total N, extractable cations, extractable Fe and Mn, 13C from C3 plants, 40K, and 226Ra. The suspended sediment from three SWs was enriched in 137Cs. Eleven characterization parameters were used in a multivariate mixing model to identify the SWs contributing the greatest sediment loads in the WGEW. The mixing model results indicated that three SWs were contributing approximately 86% of the sediment, and that the greatest amount originated in the three SWs with the lowest soil aggregation index (highest erodibility).These results were supported by the δ13C data, which indicated that approximately 65% of the stable C isotopes leaving the WGEW during this period were derived from C3 plants (shrubs), the dominant vegetation on the three SWs.