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Overland Flow from Sloping Land: Effects of Perched Water Tables and Subsurface Drains¹

ABSTRACT

Overland flow from fall-planted agricultural subwatersheds in the Willamette Valley was found to be associated with ephemeral perched water tables, high antecedent soil moisture levels, and seepage zones. The transient perched water tables developed because of restrictive layers commonly found near the ground surface in most soils in the area and due to storm characteristics. The water table was also closer to the ground surface in lower landscape positions, which produced higher moisture conditions and seepage zones. The average annual precipitation for this area is about 1,000 mm with approximately 90% being received during the 9-month period from September to May. This coincides with most of the growing season for fall-planted small grain and grass crops in the area.

Rainfall, runoff events, depth to water table, piezometric head, and matric suction data were collected from two subwatersheds for 2 years. Following this, subsurface drains were installed in a 1.4-ha subwatershed (E4) to determine the feasibility of this management practice for reducing runoff caused by saturated soil conditions. Subsurface drains eliminated seepage areas and reduced the occurrence of an ephemeral perched water table. They also eliminated base flow from the watershed hydrograph. Runoff resulting from a surfaced water table and capillary fringe at or near the ground surface was markedly decreased, although overland flow continued to occur following rare storm events with high rainfall intensity or extremely long duration.

Key Words: ephemeral perched water tables • surface sealing • subsurface drains • saturated soils • hydraulic conductivity

¹ Contribution from the Oregon Agric. Exp. Stn., Oregon State Univ., Corvallis, OR 97331. Technical Paper no. 5,904.

² Graduate Research Assistant, Assistant Professor, and Professor, respectively, Oregon State Univ., Corvallis. Present address of Senior Author is Dep. of Soil Science, Univ. of Wisconsin-Madison, Madison, WI 53706.

Received for publication April 1, 1981. Accepted for publication August 21, 1981.