Investigating preferential flow in a large intact soil block under pasture

Abstract. A large soil block was constructed to determine the importance of preferential flow routes compared with matric flow pathways at a pasture site in mid-Devon. The sandy loam soil was well structured and uniform. The soil block measured 5 m×3 m×1 m and was instrumented with an array of 54 tensiometers, TDR wave guides and suction samplers connected to an in situ chloride analysis system. Four steady state irrigation experiments were conducted with a range of rainfall intensities. During each experiment chloride and nitrate tracers were applied and the patterns of movement were observed. Although the application of tracer was uniform and the soil was relatively homogeneous, there was large variability across the block in terms of time taken to reach the peak concentration (TPC) and the peak concentration itself. About 44 samplers operated at the greatest intensities (10–2 mm h⁻¹) and only 35 at the smallest (1 mm h⁻¹). No relationship was found between TPC and depth. The fastest TPC and largest concentrations were associated with the greatest rainfall intensities. Relative importance of the individual water pathways was a function of soil heterogeneity: parts of the soil block were highly active with several pathways having short TPCs and conductivities in excess of 4 m day⁻¹ whereas other areas had longer TPCs and conductivities of 1–2 m day⁻¹. The pattern was also dynamic, with conductivities of the pathways changing through time, though most of the faster pathways maintained their greater conductivities for more than one year.