Effects of laminated rock fragments on soil infiltration processes in Karst regions

Laminated rock fragments (LRF) are widely existed in karst areas of Northwest Guangxi, China, and their effects on soil infiltration processes are very important for water cycle mechanisms and vegetation rehabilitation in karst region. In this paper, through laboratory soil-column experiments, the effects of LRF (gravitational soil-stone ratio=1:1) with different rock fragment sizes (5-20 and 20-40 mm in diameter) and positions (top(0-20 cm), middle(10-30 cm)and lower(20-40 cm)) on soil infiltration processes were analyzed. The results showed that the cumulative infiltration capacity was the highest when LRF was located in the middle position (10-30cm) with the size of 5-20 mm. However, when it was located in the lower position (20-40cm), the cumulative infiltration capacity reached the highest with the size of 20-40 mm. The relatively small size of rock fragments benefits soil infiltration for a certain position of LRF. When rock fragment size was 5-20 mm, stable infiltration rate was the highest and the time that reached the stable infiltration condition was the shortest. However, when LRF was located in the lower position with different sizes, the stable infiltration rates had no significant difference. The position and size of the rock fragments had no effect on the initial infiltration rate. When LRF was located in the upper position (0-20 cm), the time for wetting front to break through LRF reduced and similarly it was the longest when LRF was located in the middle position with the particle size of 20-40 mm. When LRF was located in the lower position, the time for wetting front to reach and break through LRF reduced. Both Kostiakov infiltration formula and Philip equation were suitable for simulating the change of cumulative infiltration capacity for stony soils, but the former had better simulation results.