Friday, 14 July 2006
107-2

Hydropedology Applied to Imperfectly Drained Landscapes in Closed Basins.

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

The basic premises of hydropedology are: 1) soil morphology is largely a function of hydro-periodicity and water movement in saturated and unsaturated flow conditions in the soil across the entire landscape, 2) oxidation and reduction combine with dissolution, transport and precipitation of soil constituents to cause volumes of concentrations and depletions in soils and landscape positions, and 3) soil morphology can be used to interpret pathways of water movement in landscapes. The objective of this paper is to illustrate an analytical methodology for areas with closed, young, non-integrated drainage networks with abundant very poorly-drained soils. We provide two contrasting examples of the hydropedologic approach that can readily be used to interpret the subsurface flow of water through soil. The analytical methods start with an examination of the soil survey mapping units in the area of concern. The mapping units are the keys to the morphology over the entire landscape, and allow spatial reconstruction of soil attributes in watersheds. The construction of a cross-section in a fence diagram that shows spatial changes in soil morphology within and among landforms is a critical step in analysis. From the fence diagram aquifer and aquitard zones can be identified; hydrologic gradient and saturated hydraulic conductivity can be estimated based on material and landscape properties. From these data, a flow net can be constructed showing flow directions and relative amounts of water. The flow net recharge, flowthrough and discharge areas are noted in cross-section. These features can then be marked on a catchment map. The soils are then organized into recharge, discharge and intermediate (flowthrough below surface) zones. Field verification can be conducted at any stage in the construction of the hydropedology map.In one example, the wide pocosin wetlands of North Carolina illustrate a stagnant, slow- moving large wetland system on the lower Atlantic coastal plain. The muck centers grade outward to the better drained edges. The well drained soils are recharge areas and the central area drains so slowly that the flow net suggests nearly no flow. We introduce the term “stagnant recharge” for this central zone with well-leached, acid organic soils. The discharge areas are at the base of the escarpments at the edges of the lower organic soils that are associated with the streams dissecting the pocosin upland. Frequently occurring small closed basins in glacial areas offer contrasting flow systems within very close proximity (less than a meter). Flow nets developed by Toth (1963) illustrate a depression-focused recharge zone at the leached center of the wetland and a calcareous edge occupying the higher ground. Similar wetland combinations of varying sizes cover large areas in prairie glacial areas of the US and Canada. The flow net reveals a mounded water table under the depression with a large amount of water loss by evapotranspiration at the edges. The soils at the edges have calcareous Bk horizons (resulting from accumulations of water-transported calcium) versus the leached Btg horizon in the center (from which the calcium has been leached. Hydropedologic analysis allows identification of soil landforms that occur in recharge, flowthrough, and discharge zones in the landscape, which are useful interpretations for soil survey users, and from which one can predict the impacts of long-term changes in soil water regime, such as those due to urbanization, land drainage or irrigation. We successfully have used these techniques in numerous landscapes throughout low and moderate relief areas in the United States. The techniques of the hydropedologic method yield knowledge of water flow in soils that is hidden from view and often would take many years to model using tensiometers or piezometers. The examples given in the text illustrate hummocky and smooth landscapes with nearly no surface flow (closed basins).

Back to 1.1A Hydropedology: Fundamental Issues and Practical Applications - Theater II
Back to WCSS

Back to The 18th World Congress of Soil Science (July 9-15, 2006)