Perched Water Tables on Hillsides in Western Oregon: I. Some Factors Affecting Their Development and Longevity

Abstract

Perched water tables on hillsides located on the western border of the Willamette Valley in Oregon in some cases have the potential to transport pollutants from either domestic or agricultural sources downslope to streams, ponds, or reservoirs, resulting in the deterioration of the quality of these waters. In this paper, some factors responsible for the development and longevity of these potentially problem‐causing perched water tables on three hillsides were examined. Analyses of hydraulic conductivity data and the relationships between rainfall, soil water pressure potential, and time suggest that permeable rock below 110 cm rather than clayey B horizons is mainly responsible for the development of perched water tables on upper convex slope positions on two of these hillsides. The data also suggest that subsurface flow from the upper convex regions contributed significant amounts of water to the lower convex slope positions. Further, perched water tables developed more rapidly, to a greater extent, and lasted longer in upper horizons of a lower concave region of slope where a shallow perched water table was already present at the onset of rainfall. Finally, perched water tables did not develop in the upper horizons on a third hillside that had no impermeable regions in the soil or upper rock mantle.

Geologic and hydrologic controls on the movement of water through a thick, heterogeneous unsaturated zone underlying an intermittent stream in the western Mojave Desert, southern California

“…Although clearly present at the field scale [ Getzen , 1983; Garabedian et al , 1988; LeBlanc et al , 1988; Killey and Moltyaner , 1988], a high degree of anisotropy at the laboratory scale has not been demonstrated on measurements from individual core samples. Laboratory measurements of vertical and horizontal hydraulic conductivity on porous media from a wide range of geologic materials and soils show a low degree of anisotropy with a ratio near 1 [ Pryor , 1973; Hammermeister et al , 1982; Yasuike et al , 1987; Burger and Belitz , 1997].…”

The influence of groundwater on surface flow erosion processes during a rainstorm

“…Agricultural plough pans at depths between 10 cm and 20 cm are common (Burwell and Larson, 1969;China et al, 1985;Michiels et al, 1989), and studies have found perched water tables on agricultural soils from Europe (Poesen, 1981) to Australia (George and Conacher, 1993). Perched water tables have also been found within 15 cm of the surface in Typic Torriorthent soils in semi-arid SE Spain (Puigdefabregas et al, 1998), above impermeable bedrock in Oregon, USA (Hammermeister et al, 1982), within 8-23 cm of the surface on a forested clayey hillslope in Luxembourg (Cammeraat, 1992), and above silt-clay deposits in the UK (Brown and Bradley, 1995).…”

“…While this study provides an analogue of agricultural soils above a plough pan, thin soils above impermeable rock or other layers are also likely to develop water tables during rainstorms (Hammermeister et al, 1982). At larger scales this may be especially common where antecedent moisture is high, such as near the base of slope or in cross-slope hollows.…”

A classification of drainage and macropore flow in an agricultural catchment

“…Beven and Germann (1981) summarized the effect of these factors in the field by describing the initiation of macropore flow as the result of one of two conditions: either when rainfall intensity exceeds the maximum infiltration rate of the soil matrix; or when the soil matrix becomes saturated. Thus macropore flow can occur from the soil surface under saturated (McDonnell, 1990;Wilson et al, 1997) or unsaturated matrix conditions (Hammermeister et al, 1982;Jardine et al, 1989;Wilson et al, 1997) or be initiated at depth in the soil profile at a site of discontinuity in the soil structure (Heppell et al, 2000a, b). The horizontal and lateral connectivity of the macropore system itself is also an important factor controlling the extent of macropore flow in a soil.…”