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PEDOLOGY

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

^a USDA-ARS National Sedimentation Lab., Oxford, MS 38655
^b USDA-ARS Southwest Watershed Research Center, Tucson, AZ 85719
^c USDA-ARS Hydrology and Remote Sensing Lab., Beltsville, MD 20705

^* Corresponding author (fred.rhoton{at}ars.usda.gov).

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 ¹³C 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.

Abbreviations: AI, aggregation index • ER, enrichment ratio • Fe_d and Mn_d, citrate–bicarbonate–dithionite extractable Fe and Mn • Fe_o and Mn_o, acid ammonium oxalate extractable Fe and Mn • Fe_p and Mn_p, sodium pyrophosphate extractable Fe and Mn • OC, organic carbon • SW, subwatershed • WDC, water-dispersible clay • WGEW, Walnut Gulch Experimental Watershed

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