Soil hydraulic properties on the steep karst hillslopes in northwest Guangxi, China

Soil hydraulic properties such as soil infiltration rate and hydraulic conductivity are closely linked to runoff generation and infiltration processes but little is known about them on karst hillslopes. The objectives of this paper were to investigate the change in soil stable infiltration rate (q (s)) and near-saturated hydraulic conductivity (K (ns)) in different slope positions and to understand their relationship with rock fragment content and soil texture within the topsoil in subtropical karst regions of southwest China. Tension infiltrometers (20 cm in diameter) were used to measure q (s) and K (ns) at pressure head of -20 mm on hillslopes 1 (a disintegrated landslide failure) and 2 (an avalanche slope). The change of q (s) and K (ns) was great and they mostly had a moderate variability with coefficient of variations (CV) between 0.1 and 1.0 in the different slope positions. On average, q (s) ranged from 0.43 to 4.25 mm/min and K (ns) varied from 0.75 to 11.00 mm/min. These rates exceed those of most natural rainfall events, confirming that overland flow is rare on karst hillslopes. From bottom to top, q (s) and K (ns) had a decrease-increase-decrease trend due to the presence of large rock outcrops (> 2 m in height) on hillslope 1 but had an increasing trend on hillslope 2 with less complex landform. They tended to increase with increase in total rock fragment content (5-250 mm) within the topsoil as well as slope gradient on both hillslopes. Pearson correlation analysis suggested that higher coarse pebble (20-75 mm), cobble (75-250 mm), and sand (2-0.05 mm) contents as well as total rock fragment content could significantly facilitate water infiltration into soils, but higher clay (< 0.002 mm) content could restrict water movement. This result indicated that rock fragment, sand, and clay contents may remarkably affect water flow in the topsoil layers, and should be considered in hydrological modeling on karst hillslopes in subtropical regions.