Variations in soil profile thickness, surface soil properties, erosion rates, runoff, and sediment properties within similar soil types and watersheds can generally be explained by slope factors that influence soil erodibility. This study was conducted to determine the effects of surface morphometry on the distribution of watershed soil properties that control erodibility and sediment properties. Each major soil type in six sub-watersheds in Walnut Gulch Experimental Watershed was sampled intensively along transects positioned to represent the normal landscape features associated with a particular mapping unit. At each sampling point, data were recorded for latitude-longitude, slope gradient, slope position, and slope aspect. Suspended and bedload sediment samples were collected from flumes located at the mouth of each sub-watershed. Clay contents of the soils and sediments ranged from 125.0 to 152.7 g kg-1 with averages of 136.8 and 178.1 g kg-1, respectively. Enrichment ratios (ER) calculated for each watershed indicated that suspended sediments were enriched in clay, relative to the soils, by a factor that ranged from 1.02 to 1.68. The aggregation index (AI), a measure of relative erodibility, ranged from 18.0 to 31.9. The correlation coefficient (r) determined for ER vs AI was - 0.927 (P < 0.05). The data indicate that watersheds with the lowest AI are producing the greatest amount of suspended sediment. The data also indicate that the highest soil AI values occur on summit, shoulder, footslope and toeslope positions, on slopes steeper than 13%, and on NW-, N-, and NE-facing slopes. These results indicate that this approach could be used to improve our understanding of hillslope erosion processes, and the accuracy of erosion prediction models.
