Variations in the hydrology of an underdrained clay hillslope

This paper presents evidence for the existence of the hydrological routes by which rainfall moves to the drainage system through clay soil at Wytham tin Oxfordshire, UK), focusing particularly on the hydrology of the A horizon. The drain discharge was continuously monitored at the site, and piezometer and tensiometer arrays were installed over the hillslope in order to study the spatial variations in hydrological response to rainfall. A combination of dye tracing, ring and tension infiltrometry was used to determine the variations in infiltration capacity over the hillslope and to characterise macropore flow. The study established that both lateral and vertical macropore flow through inter-pedal cracks are an important component of the hydrology of an underdrained site. Two regions of macropore discontinuity were identified in the soil profile; at the base of the rotovated layer and at the base of the A horizon. Saturated layers developed at both these locations during rainfall. Over the winter, a surface seal developed at the base of the hillslope. Tension infiltrometer measurements suggested that once the soil surface was sealed, earthworm burrows of >6 mm diameter were responsible for up to 86% of the vertical infiltration into the A horizon. Temporal variations in hydrological characteristics were noted due to the swelling of the clay soil as it wetted up during the autumn and winter period. Lateral flow took place by a combination of macropore and preferential routes in saturated regions within the A horizon. During periods of winter drainflow lateral macropore flow in the top 10 cm of the soil was the most important lateral route for water and was initiated by a critical pare water pressure of +0.3 KPa. Deeper lateral flow was limited and occurred predominantly through the soil matrix. During autumn drainflow, however, inter-pedal cracks were well developed through the A horizon and lateral Row at the base of the A horizon was dominant. (C) 2000 Elsevier Science B.V. All rights reserved.