A new study led by Prof. Chen Hongsong from the Institute of Subtropical Agriculture, Chinese Academy of Sciences, has unveiled the mechanistic influence of soil thickness (as a representative litho-structural factor) on water movement dynamics. Their new groundbreaking study findings were published in Journal of Hydrology (2024) and Geophysical Research Letters (2025).

    Karst landforms, which are shaped by the dissolution of carbonate rocks, cover approximately 15% of the Earth's land surface and provide stable water resources for a quarter of the global population. The southwestern China hosts the largest exposed karst area globally, where intense dissolution processes have created a distinct surface-underground dual three-dimensional hydrological structure. This unique system exhibits rapid soil water infiltration, prominent preferential flow development, and extreme spatial heterogeneity in hydrological patterns. Under climate change, the dual attributes of rapid infiltration and weak water retention amplify risks of karst drought and flooding in these regions.

    Based at the Huanjiang Karst Ecosystem Observation and Research Station of the Chinese Academy of Sciences, they focused on hillslope plots with integrated 3D monitoring systems for surface-subsurface hydrology processes (Fig. 1). Through long-term, high-density, and high-frequency (5-minute interval) soil moisture observations, time-series analysis was employed to quantify rainfall response characteristics of both conventional matrix infiltration and preferential flow in slope soils.

    Based on the response of soil moisture to rainfall, the researchers quantified the response time, absolute variation, and wetting front migration velocity. Results revealed that rainwater penetrates the soil profile more rapidly in karst areas, with soil moisture wetting front velocities (1373 mm h⁻¹) significantly exceeding those in non-karst regions (17–610 mm h⁻¹). Influenced by microtopographic spatial heterogeneity, soil moisture response time and wetting front velocity increased from upslope to downslope and from surface to deeper soil layers (Fig. 2). Compared to thick-soil-covered slopes, infiltration parameters exhibited greater variability and more pronounced positional differences in thin-soil-covered slopes, attributed to weakened hydraulic gradient regulation caused by soil accumulation. Rainfall amount and intensity contributed substantially more to infiltration (30.9–63.9%) than antecedent moisture (12.7–26.1%). Additionally, increased soil thickness may diminish the topographic and rainfall controls on infiltration processes. These findings underscore the critical role of high-resolution monitoring in characterizing spatial heterogeneity of soil moisture infiltration and quantitatively defining the rapidity of hydrological processes in karst hillslopes.

    Based on the sequential response of layered soil moisture to rainfall events, two types of preferential flow were distinguished: macropore flow and soil-bedrock interface lateral preferential flow. The results revealed that macropore flow dominates in karst hillslopes (>63%), while interface lateral flow triggered by short-duration intense rainfall events is another critical preferential flow type. The frequency of preferential flow occurrence is higher in downslope areas than in upslope regions, which is linked to the spatial distribution pattern of soil hydraulic properties. On slopes with thicker average soil depth, soil depth exhibits a significant negative correlation with preferential flow frequency (R²=0.54; Fig. 3). In contrast, no correlation exists between preferential flow frequency and soil depth in shallow-soil-covered slopes, where rainfall intensity and antecedent soil moisture exert stronger controls. These findings highlight the influence of spatial heterogeneity in soil thickness on preferential flow characteristics, further enriching the complex infiltration-runoff theory in karst hillslopes.

    “Existing research has extensively explored fundamental characteristics, types, and pathways of soil water infiltration.” Dr. Zhang Jun, the postdoctoral fellow said, “Our new findings will advance the understanding of rainfall-infiltration-runoff mechanisms in karst systems and provide more valuable water resource management strategies in response to climate variability”.

Contact: Chen Hongsong 

E-mail: hbchs@isa.ac.cn

Figure 1 Monitoring plot and meteorological station in the catchment (a), planar photographs of the monitoring plots (b), burial depth of the soil moisture probe (c), excavated profile of the ditches (d) and (e), and arrangement of the soil moisture probes (Δ) in the two plots and the spatial distribution of the soil thickness (f) and (g). (Image by Zhang Jun)

Figure 2 Soil moisture response timing (Tp2p) across hillslopes, the DSP (a) and SSP (b) along with rainfall events (blue bars) over the monitoring period. (Image by Zhang Jun)

Figure 3 Linear relationship between the surface elevation and soil thickness with the preferential flow frequency on the DSH (a) and (c) and SSH (b) and (d). (Image by Zhang Jun)