No abstract

Soil cracks significantly affect the preferential flow, and various uncertainties are associated with the effect of soil cracks on preferential flow in karst areas in Southwest China. This study investigated soil crack properties (inclusions, width, and configuration) by applying ground‐penetrating RADAR (GPR) to pedons. Blue dye‐tracing experiments were designed based on the geophysical detection results to assess the influences of crack inclusions (sand grains and rock fragments), crack widths (1, 1.5, and 2 cm), and configurations (I‐shape, V‐shape, and Λ‐shape) on preferential flow. The following results were obtained: (1) the GPR envelope can describe the configuration of isolated soil cracks; (2) soil cracks can accelerate infiltration and increase the maximum dye‐penetration depth, cumulative infiltration, and wetting‐front depth by averages of at least 5.2%, 63.2% and 4.4%, respectively; and (3) the I‐ and Λ‐shaped soil crack configurations contributed to preferential flow, but the preferential flow was not observed along the V‐shaped‐configuration‐crack pore paths despite the retardation of brilliant blue FCF. The I‐shaped configurations, with a crack width of 1.5 cm, were filled with rock fragments and had higher preferential flow ratios (18.2%–52.3%) and length indices (4.0%–33.8%) compared to the other configurations. In summary, inclusions, crack width, and configuration significantly influence the preferential flow (p < 0.05), and the influence of these soil crack properties on preferential flow cannot be neglected during vegetation restoration and groundwater security operations in karst areas.

The ecological environment in the karst region of southwest China is fragile, and vegetation degradation is considered the main cause of this phenomenon. Vegetation growth is affected by preferential flow due to a large number of fissures and macropores in the soil. Therefore, the preferential flow and preferential paths in local typical vegetation need to be studied. The distribution and developmental characteristics of preferential flow and preferential paths in three typical local stands were obtained by conducting field staining tracer experiments and using image processing techniques. The results showed that there was preferential flow movement in the soil moisture of the local understorey and that there was a high degree of preferential flow development and high variability. The degree of preferential flow in several typical forests can be expressed, from highest to lowest, in the following order: secondary forests, Cupressus duclouxiana Hickel (Hickel., 1814) forest, and Pinus massoniana (Lamb., 1803) forest. The preferential paths in the soil were mainly distributed above 15 cm, and the number of preferential paths with a small influence radius was much greater than that with a large influence radius. The preferential paths with an influence radius of 2.5~5 mm accounted for the largest proportion. The total number of preferential paths in the P. massoniana forest was small. The preferential paths in the secondary and C. duclouxiana Hickel forests were similar in number, but the secondary forests had more preferential paths with a large influence radius. This study investigated the characteristics of preferential flow and preferential paths in the typical forests of the karst region in southwest China, and it can provide data and research directions for the study of hydrological processes in local forest vegetation.

Soil cracks significantly affect preferential flow; however, there are some uncertainties associated with the effect of soil cracks on preferential flow in karst areas in southwest China. In this study, ground-penetrating radar (GPR) was applied to pedons to investigate the soil crack properties (inclusion, width, and configuration). Blue dye tracing experiments were designed, based on geophysical detection results, to assess the influences of inclusions (sand grains and rock fragments), crack width (1, 1.5, and 2 cm), and configuration (I-shape, V-shape, and Λ-shape) on the preferential flow. Our results indicated that (1) GPR envelope can describe the configuration of isolated soil cracks; (2) the Brilliant Blue FCF (C.I. Food Blue 2) infiltration rate and depth were over 1.5 times slower and 1.2–3.8 times lower those of water, respectively, during infiltration; (3) soil cracks can accelerate infiltration and increase the maximum dye-penetration depth, cumulative infiltration, and wetting front depth by at least an average of 5.2% and 63.2%, respectively; and (4) the I- and Λ-shaped soil crack configurations contributed to preferential flow, while the flow was not observed along the V-shaped configuration crack pore paths. The I-shaped configurations, with a width of 1.5 cm, were filled with rock fragments and had higher preferential flow ratios (18.2%–52.3%) and length indexes (4.0%–33.8%) than those of other configurations. Inclusions, crack widths, and configurations had significant influences on preferential flow ( p < 0.05). The influence of soil crack properties on preferential flow cannot be neglected during vegetation restoration and groundwater security processes in karst areas.