UNDERSTANDING AND PREDICTING THE IMPACT OF AGRICULTURE ON THE ENVIRONMENTAL INTEGRITY OF MANAGED WATERSHEDS
Location: Water Quality and Ecology Research
Title: Sediment source identification in a semiarid watershed at soil mapping unit scales
Submitted to: Catena
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 1, 2011
Publication Date: July 30, 2011
Citation: Rhoton, F.E., Emmerich, W.E., McChesney, D.S., Nearing, M.A., Ritchie, J.C. 2011. Sediment source identification in a semiarid watershed at soil mapping unit scales. Catena. 87:172-181.
Interpretive Summary: Sediment source identification is a scientific approach used to locate the primary sources of sediment and chemical pollutants in watersheds. Given the generally strong association of pollutants with the fine sediment sizes eroded from upland fields, identification of sediment source areas within watersheds is important for targeting areas of the watershed that are the greatest contributors of the pollutants and may require a change in management systems. We developed an accurate soil geomorphology-pedology approach to quantifying soil erodibility in terms of a soil aggregation index which was used to calculate a potential sediment yield index for each individual soil type mapped in a watershed. This ability to identify primary sediment source areas at soil mapping unit scales will permit site specific location of best management practices, and more efficiently reduce sediment and chemical contaminant loadings in watersheds.
Selective erosion and transport of silt and clay-particles from watershed soil surfaces leads to enrichment of suspended sediments by size fractions that are the most effective scavengers of chemical pollutants. Thus, preferential transport of highly reactive size fractions represents a major problem relative to sediment/chemical transport in watersheds, and offsite water quality. The objective of this research was to develop an approach to identify sediment sources at a soil mapping unit scale for the purpose of designing site specific best management practices which affect greater reductions in runoff and erosion losses. Surface soil samples were collected along transects from each of the major 25 mapping units in six subwatersheds of the Walnut Gulch Experimental Watershed. Suspended sediments were collected from supercritical flumes at the mouth of each subwatershed. Laboratory analyses included basic soil/sediment physical and chemical properties, radioisotopes, and stable carbon isotopes, all by standard methods. Aggregation index (AI) values [100 (1-water dispersible clay/total clay)] were taken as an indicator of relative soil erodibility. Potential sediment yield index (PSYI) values were calculated by multiplying percent relative area for individual soil mapping units times (100-AI). The results indicated that suspended sediment properties were generally enriched relative to the watershed soils. Clay contents of the soils and sediments averaged 141 and 178 g kg-1, respectively. Clay enrichment ratios (ER), which averaged 1.26, were significantly (P = 0.01) correlated with AI, an indication that both factors can be equated with erodibility and sediment yield. The PSYI values for subwatersheds 3, 7, 9, 10, 11, and 15 were 81.7, 77.4, 68.0, 71.6, 76.0, and 72.9, respectively. The stable carbon isotope data for the sediment had a C3 plant (shrubs) to C4 plant (grasses) ratio of 2.03, 2.25, 1.06, 1.08, 1.14, and 1.33 for the same watersheds, indicating greater erosion from the more highly erodible, shrub-dominated subwatersheds (3, 7, and 15). Correlation coefficients for PSYI versus clay ER, C3/C4 plant ratios, and multivariate mixing model results were: 0.962 (P = 0.01), 0.783 (P = 0.05), and 0.816 (P = 0.05), respectively. These results strongly support the accuracy of a potential sediment yield index approach for identifying suspended sediment sources at soil mapping unit scales.