Spatial variability of soil hydraulic conductivity and runoff generation types in a small mountainous catchment

As an important soil property, saturated hydraulic conductivity (Ks) controls many hydrological processes, such as runoff generation types, soil moisture storage and water movement. Because of the extremely harsh natural environmental conditions and soil containing a significant fraction of gravel fragments in high-elevation mountainous catchments, the measurement data of Ks and other soil properties are seriously lacking, which leads to poor understanding on its hydrological processes and water cycle. In this study, the vertical variation (0–150 cm) of Ks and other soil properties from 38 soil profiles were measured under five different land cover types (alpine barren, forest, marshy meadow, alpine shrub and alpine meadow) in a small catchment in Qilian Mountains, northwestern China. A typical characteristic of soil in mountainous areas is widespread presence of rock and gravel, and the results showed that the more rock and gravel in the soil, the higher Ks and bulk density and the lower the soil capillary porosity, field water capacity and total porosity. The Ks of the lower layer with rock and gravel (18.49 ± 10.22 mm·min−1) was significantly higher than that of the upper layer with relatively fine textured soil (0.18 ± 0.18 mm-min−1). The order of values of the Ks in different land cover types was alpine barren, forest, alpine shrub, marshy meadow and alpine meadow, and the values of the Ks in the alpine barren were significantly higher than those of other land covers. Most rainfall events in the research catchment had low rain intensity (<0.04 mm·min−1), and deep percolation (DP) was the dominant runoff generation type. When the rainfall intensity increased (0.11 mm·min−1), subsurface stormflow (SSF) appeared in the alpine meadow. Infiltration excess overland flow (IOF), SSF and DP existed simultaneously only when the rainfall intensity was extremely high (1.91 mm·min−1). IOF and SSF were almost never appeared in the alpine barren because of high Ks. The alpine barren was the main runoff-contributed area in the mountainous catchment because of high Ks and low water-holding capacity, and the alpine shrub and meadow showed more ecological functions such as natural water storage and replenishment pool than contribution of runoff.