Characterising preferential flow and its interaction with the soil matrix using dye tracing in the Three Gorges Reservoir Area of China

Dye tracing experiments provide direct visual evidence of preferential flow in the soil. In this study, we applied the Brilliant Blue tracer across three forest sites (high-mountain forest, HF; middle-mountain forest, MF; and low-mountain forest, LF) and one cultivated field (CL) in the Three Gorges Reservoir Area of China to visualise preferential flow and characterise its interaction with the surrounding soil matrix. A set of parameters was extracted from photographs of dye-stained soil profiles to measure preferential flow, including (1) the ratio of the stained area to the total area of a soil section (SAR), (2) the degree of lateral water mixing of preferential flow into the soil matrix (LWM), (3) the greatest stained depth (SD), and (4) the stained path width (SPW). The highest SAR of all of the stained areas (i.e. a measure of the degree of preferential flow) was for MF (80%), followed by LF (68%), CL (48%), and HF (30%). The higher SAR in MF and LF was likely associated with more abundant and interconnected void spaces created by roots and soil fissures. The shallower rooting depth together with the higher content of clay and soil organic matter might lead to the lowest SAR in HF, suggesting a higher likelihood of soil erosion due to surface runoff. The relatively lower SAR in CL could be a result of soil compaction after tillage destroyed soil macropores. Moreover, the spatial distribution of preferential flow with soil depth varied among slope positions. In HF and MF, macropore flow dominated the A horizon with limited lateral diffusion. However, in the subsoil, although the SAR of all of the stained areas declined, the LWM (quantified as the SAR of yellow and green patches that have a lower concentration of the dye tracer) intensified. In the sandy soils at the LF site, macropore flow via soil fissures was the major type of preferential flow that showed a limited lateral diffusion. In CL, the degree of preferential flow (mainly as finger flows) decreased with soil depth. Based on the SPW profile, flow patterns were classified along soil depth at each site. The lower degree of preferential flow and the reduced SD in agricultural soils demonstrated the substantial impact of soil management on preferential flow and thereby infiltration. Therefore, current agricultural management exacerbates surface runoff and soil erosion and causes ecological degradation and sediment deposition in the Three Gorges Reservoir Area of China.