The Sierra Foothill Region in California represents an important interface between the agricultural and urban areas of the Great Valley and the wildlands of the Sierra Nevada. Although rangelands of the Sierra Foothills have been mapped for most of California, the spatial detail and map unit complexity do not always adequately describe these highly variable soil landscapes. Considerable progress has been made in numerical approaches to soil mapping, however, little has been directly integrated into current soil survey operations in California. Our primary objective was to identify dominant soil-forming factors and processes that affect soil variability, at the landscape to watershed scale, within the Sierra Foothill Region. Specifically, we were interested in determining how well commonly used (DEM-derived) terrain-shape and microclimate indices accounted for spatial variation in soil moisture and soil properties. Two benchmark catenas were established on representative lithologies (metavolcanic and granitic) of the Sierra Foothill Region. Indices of terrain shape, geomorphic elements and microclimate gradients were extracted from a 1-m elevation model built from real-time kinematic (RTK) GPS measurements within each catena. Soils were described and sampled by genetic horizon, and standard laboratory methods were used to measure particle size distribution, color, pH cation exchange capacity, total carbon, total nitrogen, exchangeable cations, acid-oxalate extractable Fe and Mn, and citrate-bicarbonate-dithionite (CBD) extractable Fe and Mn. Elemental concentrations data were obtained for each sample by X-ray fluorescence in order to assess variation in lithology. In-situ monitoring of volumetric water content was used to evaluate typical interpretations of commonly used terrain-shape and microclimate indices, with respect to soil-forming processes. Our findings suggest that predictive power of modeled solar radiation (a proxy for microclimate) and the compound topographic index (a proxy for effective precipitation) are highly variable within the Sierra Foothill Region. These indices do not always account for expected differences in measured soil moisture; thus care should be taken when interpreting the coefficients associated with soil-landscape models built from these indices. We also found that landscape scale variability in lithology contributed more to soil variation than terrain-shape indices in some cases. As part of this research effort, we have developed an R package that supports the interpretation of massive soil databases through numerical extensions to traditional methods of visualizing, aggregating, and classifying soils information. Further development of these numerical analogues will provide a new set of quantitative tools that soil scientists and soil survey can use in conjunction with well-established, qualitative methods.
|Advisor:||O'Geen, Anthony T.|
|Commitee:||Dahlgren, Randy A., Laca, Emilio A.|
|School:||University of California, Davis|
|Department:||Soils and Biogeochemistry (formerly Soil Science)|
|School Location:||United States -- California|
|Source:||DAI-B 73/01, Dissertation Abstracts International|
|Subjects:||Physical geography, Biogeochemistry, Environmental science|
|Keywords:||Near-surface processes, Pedology, Sierra foothills, Soil mapping, Soil variability, Soil-landscape modeling|
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