An Aggregation/Erodibility Index for Soils in a Semiarid Watershed, Southeastern Arizona

Abstract

Variations in soil profi le thickness, surface soil properties, erosion rates, runoff, and sediment properties within similar soils and watersheds are controlled by slope factors such as steepness, length, and position through their infl uence on soil water regimes, and thus soil erodibility. This study was conducted to determine the effects of slope on the variation of soil erodibility at watershed scales using an aggregation index (AI) approach and soil attributes that infl uence erodibility and suspended sediment properties. Each major soil type in six subwatersheds (SWs) was sampled along transects positioned to represent the normal slope factors within a given mapping unit. At each sampling point, latitude‐longitude, slope steepness, position, and aspect were recorded. Soil samples collected from the surface 5.0 cm were characterized for particle-size distribution, water dispersible clay (WDC), total and organic C, pH, and quantitative color. Suspended sediment samples collected from each SW were characterized for similar parameters. Clay contents of the soils and suspended sediments averaged 141.3 and 179.3 g kg −1 , respectively. An AI, was used as an indicator of soil erodibility. Enrichment ratios (ER) for clay contents in the sediment ranged from 1.03 to 1.67. The correlation coeffi cient (r) for AI versus ER was −0.946 (p ≤ 0.01) indicating a strong relationship between watershed soil erodibility and suspended sediment properties. The data show that AI was greatest on the steeper slope classes, toeslope and backslope positions, and on the more northern aspects. These results suggest that AI can be used to determine the erodibility over a range of soil and slope conditions. Abbreviations: AI, aggregation index; ER, enrichment ratio; MLRA, Major Land Resource Area; SWs, subwatersheds; WDC, water dispersible clay; WGEW, Walnut Gulch Experiment Watershed .