Preferential flow patterns in forested hillslopes of east Tibetan Plateau revealed by dye tracing and soil moisture network

Abstract

Preferential flow (PF) through soil and regolith results in a rapid vertical and lateral water movement within the profile. This study focused on quantifying PF in contrasting forested hillslopes of the Hailuo Valley, located on China's east Tibetan Plateau. Quantifying PF in this region is challenging since the underlying matrix is complex, with shallow soils, thick underlying saprolite, and the combined effect of moraines and weathered residual material. We developed new methods that integrated dye tracing with a high-frequency soil moisture monitoring network to characterise PF pathways and quantity PF frequency on contrasting forested hillslopes (coniferous and broadleaved). Dye tracing experiments showed that soil-root interfaces in the upper soil layer (10–50 cm) and soil-rock interfaces in the deeper soil layer (50–100 cm) were the primary PF pathways and that large rocks strongly influence the percolation depth of water. The high-frequency soil moisture network revealed that the mean PF frequency was 67.8% and 71.7% in the coniferous and broadleaved forests, respectively. The frequency of PF in the deeper soil layer increased from upslope to the downslope locations on both forested hillslopes, highlighting the tendency for PF to occur downslope. In addition to matrix conditions (e.g., stony saprolite soil), the total amount and maximum intensity of precipitation (as throughfall in this study) were identified as factors that control PF occurrence. In contrast, the initial soil water conditions (at 10, 50 and 100 cm depth) were insignificant in predicting PF occurrence. Results of this field-based study highlight that PF is a ubiquitous and critical subsurface flow mechanism that regulates rainfall-runoff relationships in forests of the Tibetan Plateau, underscoring the need to consider PF in hillslope hydrological modelling.