Water seepage and retention in purple soil bunds on sloping farmland in the Three Gorges Reservoir area, China

Soil bunds are a common land reconstruction practice in the Three Gorges Reservoir area of China, and play a key role in runoff regulation and soil conservation on cultivated hillslopes. This study investigated water movement pathways and retaining effects of soil bunds using water transport trajectory observations inferred from field dye tracing experiments. It was found that water infiltration in the bund was characterised by spatial heterogeneity in three dimensions. The stained area ratio (SAR) of the bund was much lower than that of the adjacent field at all depths. Dye patterns resembled inclined strips in the bund, and lateral seepage occurred mainly on the top of hard pan. SAR, stained path width (SPW) <20 mm, and stained path number (SPN) all decreased with increasing horizontal distance. The SPW <20 mm proportion and SAR both decreased according to a logarithmic function, while the changing trend of SPN followed a first-order exponential decay. Three pathways of water movement in the bund could be distinguished, namely, vertical infiltration, lateral seepage and flow along the bund-bedrock interface. The primary water seepage channel in the bund was likely to be the bund-bedrock interface (accounting for around 80% of the total flow). These results provide preliminary realisation of water movement mechanisms in the purple soil bunds on sloping farmland and inform management options for bund stability maintenance.