Characteristics And Formation Mechanism Of Preferential Flow In Forest Soils Of Purple Sandstone Regions, Three Gorges Reservoir Area

Preferential flow is a fast and non-equilibrium soil water movement process. Preferential flow in soils of forested watershed can increase soil water availability and reduce the genesis of surface runoff, which has an important influence on forest hydrological processes and is conducive to water resources conservation and regulation. Predicting formation mechanism of preferential flow and their interactions with the soil matrix under different environmental conditions has been identified as a key area where synergies between pedology and hydrology are urgently needed.Brilliant blue dye tracing and ponding infiltration methods were applied to observe preferential flow processes in four different forest soils (broadleaf forest, coniferous forest, mixed forest and shrub), which are widespread in purple sandstone regions of the Three Gorges Reservoir area. Based on the principles of stereology, spatial point pattern analysis and water breakthrough curves method, the morphological characteristics and occurrence types of preferential flow, the quantity and distribution characteristics of preferential flow paths were studied, respectively. The formation mechanism of preferential flow in forest soil of purple sandstone regions was revealed. The main results in this research were as follows:(1) Based on the principles of stereology, in purple sandstone forest soils of the Three Gorges Reservoir area, the morphological characteristics of interflow showed that preferential flow commonly occurred with soil matrix flow, and the water transport rates of preferential flow were 4 to 18 times faster than that of matrix flow. The preferential flow processes in different types of forest land were distinctive. The spatial variability of preferential flow in soils was gradually intensified with the soil depth. With stained path width<10 mm and> 100 mm as the criteria, and consulting water flow pattern changes, the forest interflow could be classified as five types:homogenous matrix flow, heterogeneous fingering flow, macropore flow with high interaction, macropore flow with mixed interaction and macropore flow with low interaction, which refers that fingering flow and macropore flow belong to preferential flow. Preferential flow in purple sandstone forest soils was mainly macropore flow, which generally occur at 10-20 cm depth of soil profiles and could reach to the top of parent material layers.(2) Point pattern analysis was used to reveal the spatial distribution of preferential flow paths in purple sandstone forest soils of the Three Gorges Reservoir area. The results showed that under different infiltration water volumes, with spatial scales in radius of 25 cm, the distribution of preferential flow paths was significant accumulation. Following the spatial scale expansion, preferential flow paths distribution trended to uniform. In the 0-20 cm depth soil surface layers, the spatial position of preferential flow paths and plant roots had a significant positive correlation. The formation of preferential flow paths in the surface soil layers was related to the growth of plant shallow roots. In the depth middle and deeper soil layers (20-50 cm), by purple sandstone debris gradually increased and amount of plant roots reduced, the spatial positive correlation of preferential flow paths and plant roots was decreases. In the bottom soil layers, the spatial distribution of preferential flow paths and plant roots showed a little negative correlation, which may be caused by the growth of tree vertical roots plugged original soil cracks.(3) Water breakthrough curves of soil samples in the dye stained area and the blank area were significantly different. The water effluent rate and its penetration time had a significant logarithmic relationship. The time of water effluent rate to stabilize in blank area was generally 5-10 s later than that in the dye stained area. The macropores in purple sandstone forest soils could be divided into four pore equivalent radius levels as 0.3-0.5 mm,0.5-0.7 mm,0.7-1.5 mm and 1.5-3.0 mm, respectively, which was calculated by water breakthrough curves. The quantities of soil macropores in different types of forest were distinctive, and macropores amounts in the dye stained area were 10 times bigger than that in the blank area. The correlation between macropore number and water stable effluent rates was dropped with the decreasing of macropores equivalent radius. More densely macropores distribution in the dye stained area promoted the formation of preferential flow, and generated its spatial heterogeneity.(4) Preferential flow occurrence in forest soil was attributed to external factors and internal factors. Water conditions were the external factors and environmental terms relevant to plant-soil were the internal factors. Thirteen environmental factors with higher Spearman correlation coefficient were selected for the principal component analysis, namely soil density, sand content, slit clay ration, organic matter content, root holes amount, root weight density, root length density, initial soil water content and infiltration water volume. The result showed that the soil pore structure factor, soil moisture factor, soil type factor, precipitation factor were the four principal components of formation mechanism of preferential flow in forest soils of purple sandstone regions, the Three Gorges Reservoir area. The soil pore structure was the most important factor with occurrence of preferential flow in forest soils, which accounted for 46.76% of the variance.