Variations in the hydrology of an underdrained clay hillslope

This paper presents evidence for the existence of the hydrological routes b y which rainfall moves to the drainage system through clay soil at Wytham t in Oxfordshire, UK), focusing particularly on the hydrology of the A horizo n. Tile drain discharge was continuously monitored at the site, and piezome ter and tensiometer arrays were installed over the hillslope in order to st udy the spatial variations in hydrological response to rainfall. A combinat ion of dye tracing, ring and tension infiltrometry was used to determine th e variations in infiltration capacity over the hillslope and to characteris e macropore flow. The study established that both lateral and vertical macr opore flow through inter-pedal cracks are an important component of the hyd rology 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 locatio ns during rainfall. Over the winter, a surface seal developed at the base o f the hillslope. Tension infiltrometer measurements suggested that once the soil surface was sealed, earthworm burrows of >6 mm diameter were responsi ble for up to 86% of the vertical infiltration into the A horizon. Temporal variations in hydrological characteristics were noted due to the s welling of the clay soil as it wetted up during the autumn and winter perio d. Lateral flow took place by a combination of macropore and preferential r outes in saturated regions within the A horizon. During periods of winter d rainflow lateral macropore flow in the top 10 cm of the soil was the most i mportant lateral route for water and was initiated by a critical pare water pressure of +0.3 KPa. Deeper lateral flow was limited and occurred predomi nantly through the soil matrix. During autumn drainflow, however, inter-ped al 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 righ ts reserved.