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Division S-5—Pedology

Do Soil Surveys and Terrain Analyses Identify Similar Priority Sites for Conservation?

USDA-ARS, National Soil Tilth Lab., 2150 Pammel Dr., Ames, IA 50011

^* Corresponding author (tomer{at}nstl.gov)

Terrain analyses may help target conservation practices, but soil survey is the traditional basis for conservation planning. This study's objective was to identify correspondence between terrain and soil survey attributes. Iowa's South Fork (78000 ha) and West Nishnabotna (64000 ha) watersheds provided example datasets. Major resource concerns are soil wetness in the South Fork's glacial terrain, and soil erosion in the West Nishnabotna's loess hills. Slope, contributing area, curvature, and wetness (W) and erosion (E) indices, calculated from National Elevation Database (30-m) data, were each divided into two groups according to highly erodible land (HEL), hydric-soil, drainage-class, topsoil-thickness, clay, and organic matter content attributes from soil survey delineations. All groupings had different means (p < 0.001), but proportions of captured variance ranged from <0.1 to 22%. Critical values of slope and W segregated hydric, drainage, and topsoil-thickness groups with 66 to 74% accuracy in both watersheds, and slope and E segregated HEL groupings with 71 to 76% accuracy. Finer detail in terrain data appeared realistic in dissected terrain, but artificial in near-level terrain. The largest 20% of W values were about 80% hydric soils in the South Fork, and the largest 20% of E values were about 80% HEL in the West Nishnabotna. However, similar statements on E and HEL in the South Fork, and W and hydric soils in the West Nishnabotna, applied to few cells (<2%) with <70% accuracy. Therefore, targeting of conservation practices based on 30-m grid terrain analysis can be consistent with soil survey information, but to a degree that varies by landscape and resource concern.

Abbreviations: A_s, specific upslope contributing area • ANOVA, analysis of variance • ß, slope (degrees) • C_s, surface curvature • DEM, digital elevation model • E, erosion index • HEL, highly erodible land • S, slope (%) • W, wetness index