Effects of rock outcropping on soil water infiltration in karst cropland slope

Abstract: Soil erosion has posed a serious threat to the ecological environment in the karst hill-slope of southwest China. Among them, soil water can dominate the redistribution of rainfall through runoff, surface evaporation, infiltration, and soil erosion in the circulation of the terrestrial surface system water and heat resources. Particularly, the thin soil and bare rock outcropping can significantly determine surface runoff and sediment production. The abundant exposed rock has also been a typical geomorphic landscape on the karst cropland slope. However, only a few studies were focused on the infiltration characteristics of soil water under different slope gradients and various shapes of the rock outcropping. Thus, this study aims to explore the influence of rock outcropping on the soil water infiltration on the karst cropland slope. A series of artificial rainfall experiments were performed on the five shapes of rocks under three slope gradients in three areas in the field. The simulation results showed that: 1) There was a significant decrease in the infiltration rate for the rock of various outcropping slopes and bare slopes with the increase of slope gradient. In general, the more stable change with the higher infiltration rate was observed in the soil infiltration process under the smaller rock area and the lower slope gradient. In the larger rock area and the steeper slope gradient, the infiltration process fluctuated greater with the lower infiltration rate. 2) The maximum initial, stable and average infiltration rates all appeared on the slope gradient of 15°, and the minimum on the slope gradient of 25°. On the whole, the greater the slope gradient and rock area were, the greater the stable infiltration rate of soil water was. 3) The 15°, 20°, and 25° slope gradients of rock outcropping contributed 2.63%, 20.88%, and 76.49% to the infiltration increment, respectively, compared with the bare slope. Small, moderate, and large areas of rock contributed 46.39%, 32.88%, and 20.73% to the infiltration increment, respectively. The contribution rate of the shape to the infiltration increment was ranked in the descending order of the oblique strip rock (31.50%), horizontal strip rock (27.66%), circular rock (27.08%), triangle rock (10.81%) and vertical strip rock (4.36%.) 4) The bare slope presented the highest fitting degree (> 0.89), while the oblique strip rock was the lowest (about 0.60) after fitting. The best fitting degree was achieved for the Horton model, followed by the Kostiakov and Philip model. Therefore, the Horton model was more suitable for the fitting of soil water infiltration under different rock outcroppings on the karst cropland slope. The finding can greatly contribute to the soil water infiltration performance under the rock outcropping on the karst cropland slope, particularly for the characteristics of runoff and sediment yield subjected to the rainfall. In addition, this work can also provide a strong reference for the control of soil erosion and rocky desertification in karst areas, further promoting the management, protection, and utilization of agricultural water and soil resources.