Submitted to: Interagency Conference on Research in the Watersheds
Publication Type: Proceedings
Publication Acceptance Date: 10/27/2003
Publication Date: 1/27/2004
Citation: Rhoton, F.E., Emmerich, W.E., Goodrich, D.C., Mcchesney, D.S., Miller, S. 2004. Soil contributions to sediment properties in walnut gulch experimental watershed: influence of slope factors. Interagency Conference on Research in the Watersheds.415-421. Interpretive Summary: The severity of erosion is largely determined by factors such as rainfall characteristics, topography, and vegetative cover. If these factors remain constant and the soil resource changes, variations in soil losses can be attributed to differences in soil properties that influence soil erodibility which in turn is determined by soil aggregate stability. The distribution of soil properties that influence aggregate stability in the landscape can vary as a function of slope position, slope class, and slope aspect. These factors must be accounted for before we can improve our understanding of the relationship between soil characteristics and sediment properties. We characterized the distribution of aggregate stability/soil erodibility relative to differences in slope factors and its impact on soil erosion and sediment properties in a semiarid watershed. Our research has demonstrated that soil erodibility zones can be delineated in watersheds and may be used to improve the prediction capabilities of soil erosion and sediment transport models at the watershed scale.
Technical Abstract: 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.