Tracer and hydrometric study of preferential ̄ow in large undisturbed soil cores from the Georgia Piedmont, USA

We studied the temporal patterns of tracer throughput in the out ̄ow of large (30 cm diameter by 38 cm long) undisturbed cores from the Panola Mountain Research Watershed, Georgia. Tracer breakthrough was a€ected by soil structure and rainfall intensity. Two rainfall intensities (20 and 40 mm hr71) for separate Cl7 and Br7 amended solutions were applied to two cores (one extracted from a hillslope soil and one extracted from a residual clay soil on the ridge). For both low and high rainfall intensity experiments, preferential ̄ow occurred in the clay core, but not in the hillslope core. The preferential ̄ow is attributed to well-developed interpedal macrochannels that are commonly found in structured clay soils, characteristic of the ridge site. However, each rainfall intensity exceeded the matrix in®ltration capacity at the top of the hillslope core, but did not exceed the matrix in®ltration capacity at the middle and bottom of the hillslope core and at all levels in the clay core. Localized zones of saturation created when rainfall intensity exceeds the matrix in®ltration capacity may cause water and tracer to over ̄ow from the matrix into macrochannels, where preferential ̄ow occurs to depth in otherwise unsaturated soil. Copyright # 1999 John Wiley & Sons, Ltd.