Model-based exploration of hydrological connectivity and solute transport in a forested hillslope

Preferential flowpaths control hydrological connectivity in forested hillslopes. However, current understanding on the nature, extent, and impacts of hydrological connectivity within hillslopes is still limited. Distributed 2-pore domain models of subsurface water flow and solute transport in and between the preferential flowpaths and the soil matrix provide applicable means to investigate the connectivity. We identified possible (dis)connections of preferential flowpaths in a forested hillslope section from dye tracer data and incorporated them in a 3D 2-pore domain model; the model was run and evaluated against ion tracer data. The objectives were to explore hydrological connectivity within the hillslope and quantify its impacts on solute transport. The main control for capturing the internal concentration dynamics of the ion-tracer plume with the model was an explicit description of local disconnections of preferential flowpaths in the form of local reductions in their hydraulic conductivity. Continuous preferential flowpaths were formed by rooting activities, erosion caused by subsurface flow, freezing-thawing cycles, and soil fauna, but they were locally disconnected by, for example, cemented soil material. As a result, the dominant lateral flowpath was tortuous, and tracer transport within this pathway was highly preferential: Compared to earlier models without flowpath disconnections, increase in the average lateral tracer load was 1.7-fold and in the maximum load 7.9-fold. The results indicate that the volume and hydraulic conductivity of the actual, connected preferential flowpaths control, in combination with linkages between preferential flowpaths and soil matrix, pollutant transport in forested hillslopes.