Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/19/2006
Publication Date: 5/16/2007
Citation: Rhoton, F.E., Emmerich, W.E., Goodrich, D.C., Miller, S.N., Mcchesney, D.S. 2007. An Aggregation/Erodibility Index for Soils in a Semiarid Watershed, Southeastern Arizona. Soil Science Society of America Journal, 71:984-992.
Interpretive Summary: 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. The soil properties that influence sediment production can vary according to slope position, slope class, and slope aspect. We determined the effect of these slope factors on soil aggregation index, the determining factor in soil erodibility. Based on the data obtained for the relationship between soil erodibility and the slope factors, we developed a sediment production index which indicated that the greatest potential for sediment production was on the 13-20% slope class, and on the south facing slopes. This ability to identify primary sediment source areas in watersheds will improve the design of best management practices to affect the greatest reduction in sediment and chemical contaminant loadings at watershed scales.
Technical Abstract: The identification of primary sediment source areas in watersheds is necessary for the design of best management practices to reduce sediment and chemical loadings in receiving streams. This study was conducted to determine the effects of soil geomorphology on watershed soil properties that influence erodibility and sediment properties within the context of locating primary sediment sources at watershed scales. The research was conducted on the Walnut Gulch Experimental Watershed at Tombstone, AZ. Each major soil type in six sub-watersheds (SW) was sampled along transects positioned to represent the normal soil geomorphological features associated with a given mapping unit. At each sampling point, latitude-longitude, slope gradient, slope position and slope aspect were recorded. Soil samples collected from the surface 2.5 cm were characterized for particle size distribution, water dispersible clay (WDC), total and organic C, magnetic susceptibility, pH, and quantitative color. Suspended sediment samples collected from supercritical flumes at the mouth of each SW were characterized for similar parameters. Clay contents of the soils ans sediments averaged 141.3 and 179.3 g kg-1, respectively. An aggregation index (AI) was calculated for each SW as follows: 100 (1-WDC/total clay). Enrichment ratios (ER) for clay contents ranged from 1.03 to 1.67. A correlation coefficient (r) of -0.946 (P < 0.01) was obtained for AI versus ER indicating that AI can be used to infer watershed soil erodibility and suspended sediment properties. A potential sediment production index (SPI) calculated by multiplying the percent relative area in a slope class or aspect times (100-AI) indicated that the E class (13-20%) and south facing slopes are primary sources of sediment in these SW.