Preferential Flow Variability in a Well-Structured Soil

Abstract

A series of preferential flow experiments were conducted to investigate the temporal and spatial variability in a large block (5.4 by 3.4 by 1.2 m) of undisturbed soil (Dystric Eutrocrept) situated in mid-Devon, UK. Chloride and nitrate tracers were applied using rainfall sprinklers and the soil water was sampled continuously using 54 ceramic suction samplers installed throughout the block. Two main types of breakthrough curve occurred, a rapid response with a high peak concentration and a slower response with a lower peak concentration. Analysis of the spatial patterns of these characteristics confirmed the importance of the horizontal and vertical heterogeneity of flow. Peak concentration and time to peak concentration were related to depth in one quadrant but no relationship was found when the entire block was considered. Preferential flow paths occurred at intensities above 2 mm h−1 and were less important for intensities of 1 mm h−1 A delayed leaching experiment was conducted in which a nitrate tracer remained in the soil for 10 d before leaching commenced. The pattern of response to the 5-mm h−1 rainfall application was similar to the earlier experiments. Analysis of the shape of the breakthrough curves and time to peak suggested that nitrate movement was minimal in the wet soil during the intervening period. The spatial variability, noted even at this limited block scale, suggests that simplified approaches to understanding water and chemical transport are unable adequately to describe field behavior.