Investigating preferential flow in a large intact soil block under pasture

A large soil block was constructed to determine the importance of preferent ial 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 x 3 m x 1 m and was instrumented with an array Of 54 tensiome ters, TDR wave guides and suction samplers connected to an is situ chloride analysis system. Four steady state irrigation experiments were conducted w ith a range of rainfall intensities. During each experiment chloride and ni trate tracers were applied and the patterns of movement were observed, alth ough the application of tracer was uniform and the soil was relatively homo geneous, there was large variability across the block in terms of time take n 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 w ater pathways was a function of soil heterogeneity: parts of the soil block : were highly. active with several pathways having short TPCs and conductiv ities in excess of 4 m day(-1) whereas other areas had longer TPCs and cond uctivities of 1-2 m day(-1). The pattern was also dynamic, with conductivit ies of the pathways changing through time, though most of the faster pathwa ys maintained their greater conductivities far more than one year.