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. Highlights Combined dye tracing and soil moisture monitoring network were applied to explore PF on forested hillslopes. Root networks and rock fragments constituted the main PF paths. PF frequency increased from upslope to downslope in two forest types. Initial soil moisture conditions showed no controlling effect on PF occurrence.

Editorial for special issue on “Recent innovations in soil physical, chemical and biological analysis” (2nd National Symposium on soil analysis)

“…Lu et al (2018) developed a heat pulse‐based method that can simultaneously measure soil bulk density and water content, which can be used to monitor both soil structure and water dynamics in the fields. Wang, Wang, et al (2022) integrated dye tracing with a high‐frequency soil moisture monitoring network to study preferential flow in forested hillslope soils. Direct measurement of some soil physical properties is time‐consuming and impractical, and alternative methods have been developed.…”

Characterizing large‐scale preferential flow across Continental United States

“…Laboratory approaches involve studying PF behavior using soil samples (e.g., soil cores or columns) collected from the field. Dye tracer methods (Ghodrati & Jury, 1990; Wang et al., 2022) and time‐domain reflectometry have been employed in soil columns and lysimeter setups to investigate nonuniform flow and to understand PF behavior (Castiglione et al., 2003; Köhne et al., 2004; Köhne & Mohanty, 2005; Vanclooster et al., 1995). While these methods visually illustrate PF pathways, soil sampling is destructive and may alter the soil structure and boundary conditions, limiting the latent representation of PF pathways and incurring high costs and effort.…”