ROLE OF LEAF LITTER IN INITIATING TRACER TRANSPORT PATHWAYS IN A WOODLAND HILLSLOPE SOIL

It has been suggested that the leaf litter layer influences soil infiltration processes in hillslopes. This paper examines the effect of the presence of the leaf litter layer in an oak woodland on conservative tracer transport measured in situ using time domain reflectometry (TDR) probes. This was done using repeated tracer miscible displacement studies, first with the litter layer intact and then again after removal of the litter layer. Tracer transport, as measured breakthrough curves (BTCs), was characterized using temporal moment analyses. These plot scale studies, performed at a field location in northern California, did not identify consistent changes in tracer transport patterns in the vertical transport plots before and after litter layer removal. In contrast, downslope lateral tracer displacement in the upper 20 cm of the soil did change after litter removal. However, no differences were evident deeper in the soil (30–50 cm) in temporal moments before and after litter removal during lateral tracer transport. This result demonstrates that the litter layer had a localized (near soil surface) influence on lateral transport. This influence may be explained by vertical anisotropy in effective soil hydraulic conductivity on the direction of lateral flow relative to the soil surface in the hillslope plots. It was demonstrated that the litter presence promotes a more vertical direction for the lateral flow in the first 20 cm of the soil. Despite the small changes in the first 20 cm observed in tracer transport after litter removal, the physical presence of the leaf litter layer was not found to be a major influence on tracer transport at the 30- to 50-cm depth in this hillslope soil where most of the lateral flow occurs.