Size Distribution of Sediment Transported Within and from Small Agricultural Watersheds

Abstract

ABSTRACT THE aggregate size distributions of transported sediment for severe and less-severe rainfall-runoff-erosion events during 1976-1980 were investigated for five nested watersheds in a heavily row-cropped region in east-central Iowa. For a given site and event, larger runoff rates were associated with higher suspended-sediment concentrations and more coarse particle size distributions. During the less-severe events of 1976-1978, the intrabasin station had a higher percentage of fine sediment (on a load-weighted basis) in transport than the smaller field or larger stream-channel sites, possibly due to an effective grassed waterway. However, for severe events, the percentage of fine-sized particles in the suspended sediment increased as sediment was transported, with the greatest percentages of fine sediment transported in the downstream channel. Comparison of the two adjacent field watersheds showed that, during less-severe events, the watershed with the greater runoff volume also had a greater sediment load and a larger percentage of coarse material in transport. During the severe events, there were generally no differences in particle size distributions between the field watersheds and the particle size distributions were comparable to the size distributions of soil surface samples. On an annual basis, chisel plowing resulted in sediment with a larger percentage of fine material than disking, with size distributions for moldboard plowing and disking treatments being similar.