Friday, 14 July 2006
107-3

Soil-Hydrology Interpretations Developed from Soil Surveys Based on Soil Morphology.

Doug Wysocki, Jim Richardson, David Hammer, and Steve Peasley. NRCS-USDA, 100 Centennial Mall, Lincoln, NE 68508

Water creates a distinctive soil morphology specific to water activity as it travels through landscapes. The morphology differs as a function of the frequency, duration, and intensity of water passage through structurally, texturally and biologically created soil pores. We can predict hydro-periodicity and water dynamics in open erosional landscapes using soil morphology combined with landscape position. To demonstrate the basic principles of water-created soil-landscape relationships we selected three landscapes based on their abilities to retain, focus, or shed water. Redoximorphic features and kinds of soil horizons yield evidence on length of wetness (hydro-periodicity), anisotropy, and flow dynamics. In Iowa, a wide nearly level summit restricts runoff and promotes infiltration. The only water source on this landform is precipitation because this is the highest place in the landscape. Soil wetting and drying creates strongly developed Btg horizons on the summits. When precipitation exceeds evaporation a temporary (perched) water table develops over the Btg horizon because water infiltrates the silty surfaces faster than it can percolate through the clay-enriched argillic horizons. The perched water then seeps slowly, laterally toward and through a narrow hillslope shoulder. A paleosol at the base of the shoulder and upper backslope acts as an aquitard. The aquitard (paleosol) creates a hillslope seep. The volume of upland water storage is potentially large because of the thickness of the loess cap and the width of the summits. The strongly anisotropic conditions causes lateral water flow by creating differential head in the soil landscape. A similar stratigraphic setting (loess/paleosol/till) exists in the Nebraska landscape. The landscape differs from Iowa in that the summits are narrow remnants and the shoulders are much larger portions of the hillslopes with more areal extent in the landscape. The Bt-horizon is less wet and has less clay than the argillic horizons in the Iowa soils. The redoximorphic features are below depths of 0.5 m. The backslopes are small with little or no footslope components, and descend sharply to ephemeral drainage. Summit water storage is limited because of the narrow, convex summits, and terracing is used both to increase soil water storage and to reduce surface soil erosion. Water returns to the surface through the shoulder and discharges in small seeps from the glacial till that acts as an aquitard. The third landscape, in ND, lacks a hillslope summit. A few, small isolated hill crests (rounded shoulders) descend to broad, coalesced footslopes. The soil at the crest lacks a B-horizon and classifies as an Entisol. In this landscape, the remarkably well-developed footslope landforms dominate areally and store water in thick A-horizons. At the footslope terminus, a soil with a Btn horizon results from wetting, drying, and evaporative discharge that concentrates salts. The salts are leached periodically during pluvial cycles or major precipitation events. The result is clay and organic matter dispersion (slick spots). Soil morphology linked to a landscape (geomorphology) setting reliably predicts both directions and magnitudes of transient flow in all three soilscapes we examined.


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