James Thompson

Digital elevation model resolution: effects on terrain attribute calculation and quantitative soil-landscape modeling

The accuracy of digital elevation models (DEM) and DEM-derived products depends on several factors, including the horizontal resolution and vertical precision at which the elevation data are represented, and the source of the elevation data. This accuracy becomes increasingly important as we extend the use of DEM data for spatial prediction of soil attributes. Our objective was to compare terrain attributes and quantitative soil-landscape models derived from grid-based DEM represented at different horizontal resolutions (10 and 30 m), represented at different vertical precisions (0.1 and 1 m), and acquired from different sources. Decreasing the horizontal resolution of the field survey DEM produced lower slope gradients on steeper slopes, steeper slope gradients on flatter slopes, narrower ranges in curvatures, larger specific catchment areas in upper landscape positions, and lower specific catchment areas values in lower landscape positions. Overall, certain landscape features were less discernible on the 30-m DEM than on the 10-m DEM. Decreased vertical precision produced a large proportion of points with zero slope gradient and zero slope curvature, and a large number of steeply sloping and more highly curved areas. Differences among DEM from different sources were more significant, with less accurate representation of depressions and drainage pathways with the USGS DEM as compared to the field survey DEM. Empirical models developed from different DEM included similar predictive terrain attributes, and were equally successful in predicting A-horizon depth (AHD) in the validation data set.

Laboratory comparison of soil redox conditions between red soils and brown soils in Minnesota, USA

Certain soil morphological characteristics create significant problems for hydric soil identification due to lack of commonly observable Fe-based redoximorphic features that indicate seasonally saturated and reduced conditions. Examples include soils with high levels of organic C and red soils (7.5YR or redder) containing high amounts of hematite iron. The objectives of this research were to examine the effects of organic C content and Fe mineralogy on (i) soil redox response and (ii) the development of Fe-based redoximorphic features. Redox reactions related to the fundamental biochemical processes that occur in saturated soils were studied in a controlled laboratory experiment. The experimental design included six columns of red soils (5YR) from northeastern Minnesota and six columns of brown soils (10YR), possessing varying organic carbon contents, from southeastern Minnesota. Redox measurements taken throughout the experiment suggest different soil redox environments between the brown and red soils. The brown soils had redox potentials that decreased gradually and then remained at a constant decreased state around 0.0 mV. Redox potentials for the red soils decreased rapidly and equilibrated in the range of 100 to 300 mV. Variations between these soil responses may be due to different Fe-oxide mineralogies and/or the amount of bioavailable organic carbon.