Research On Characteristics Of Soil Macropore And Its Preferential Flow Effect In Three Gorges Of Yangtze River

Preferential flow comes into being in soil macropore for the sake of rain, which is the main method for forest to regulate runoff and reserve water and soil. The study of soil macropore and its preferential flow effect in mountains and forests of vertical zonality, will increase the knowledge of the mechanism of forest to reserve water. It helps to further understand the process of forest to regulate hydrology theoretically on one hand and predict the influence on water circle by mountainous land utilization practically on the other hand. Such a practice also provides theoretical and political references for vegetation recovery and restoration of reforestation program and natural forest protection project, and water and soil reservation in mountainous areas as well.Located in the transition areas between the first and the second step, Three Gorges of Yangtze River have complicated natural and geographical elements in weather, terrain, soil and hydrology, especially in mountainous areas with distinct vertical zonality. Traditionally, the reservoir area of Three Gorges is one of the most inclined areas where soil erosion and flood occur, and it has become one of the regions of ecological and environmental protection. The hydrological process of forest soil in Three Gorges directly influences the ecological security of the middle and lower reaches of Three Gorges and the hydrological function of forest soil largely rests on the process of soil macropore and preferential flow. The influencing elements of soil macropore and preferential flow in different vertical zones of Three Gorges are different and so is the capacity for reserving water and regulating runoff. Thus, to conduct the study of soil macropore and preferential flow in the mountainous areas of Three Gorges will help to make clear the runoff mechanism of forest land and improve the precision of runoff. Also, it is of great practical significance to the reduction of water and soil loss, underground erosion, landslide and mud-rock flow, etc.This research selects four typical sites with different elevations as pilot areas in Dengcun Village—Dalaolin Mountainous Area in Yiling District, Yichang City in the beginning part of the reservoir area of Three Gorges. Three are forest-soil sites, namely Dalaoling site covered with warm-temperate deciduous broad-leaved forest and mountain brown earth at subalpine zone, Baishuitou site covered with evergreen-deciduous broad-leaf mixed forest and mountain yellow brown earth at mid-mountain zone, and Zhulinwan site covered with warm coniferous forest and mountain yellow earth at low-mountain zone; one is abandoned agricultural land on slopes, Luojiayan deserted farmland dominated with Artemisia at low-mountain zone. Dye tracing method is applied to observe and simulate the soil macropore and preferential flow in pilot areas; also, Photoshop7.0is exploited to analyze dyeing representation of soil profile. Accordingly, it is available to study distribution profile of soil macropore, morphological characteristics and genetic types of preferential flow in each vertical zone of reservoir area of Three Gorges, and macropore genetic differences of soil macropore of different forest soil and deserted farmland soil in different zones. It is found in the study that, there exist certain differences regarding the soil macropore characteristics in different vertical zones; the vertical variations of preferential flow in forest soil are distinct; preferential flow exerts great influence on the filtration of water in soil and certain influence on hydrological process within the drainage basin. The study results are as follows:(1) The distinction of vegetation-soil type of different vertical zones in Three Gorges, contributes to different profile features of soil macropore. Root soil macropore is extensively distributed in warm-temperate deciduous broad-leaved forest and mountain brown earth at subalpine zone and evergreen-deciduous broad-leaf mixed forest and mountain yellow brown earth at mid-mountain zone and the proportion of dyeing area is large. Cracks are comparatively broader in illuvium and eluvial horizon and the dyeing filtration is the deepest in warm coniferous forest and mountain yellow earth at low-mountain zone. However, Surface humus layer has a strong adsorbability with regard to filtration solution and dyeing filtration is relatively shallow in forest and mountain brown earth at subalpine zone. As of deserted farmlands on slopes covered with herbage vegetation, the soil structure is poor, root macropore and cracks are few, and the proportion and depth of dyeing is lower than forest soil.(2) Preferential flow in forest soil of each genetic horizon represents different variations in different vertical zones of Three Gorges. Preferential flow within humus layer of forest-soil sites of different elevation is cave flow. As arbor root system is developed in at subalpine zone, cave flow is most overt; shallow rooted plants are the main vegetation at mid-mountain zone and low-mountain zone, flow rate and flow velocity is comparatively small.There are cave flow and crack flow regarding preferential flow in eluvial horizon and illuvium of forest-soil sites. As for mountainous areas with relatively low elevation, the soil development degree is low; slant and vertical cracks are widely distributed in illuvium and eluvial horizon; the distinction of humus layer in preferential flow is large. As for the soil of deserted farmlands, there are few macropores with small caliber; water content on the interface of different soil layer greatly and differently influenced preferential flow and the representation is finger flow.(3) Vegetation types, soil development degree and agricultural cultivation jointly and primarily contribute to the formation of macropore in forest-soil zones with different elevations in the mountainous areas of Three Gorges. There are rich organic matters in forest soil and the macropore in humus layer with high weathering degree majors in root pores. The development degree of soil in illuvium and eluvial horizon is relatively low at mid-mountain zone and low-mountain zone with coarse soil and a majority of cracks regarding macropore. In addition, agricultural work disturbs soil and damages the structure of macropores. Thus, compared with forest soil, there are fewer soil macropores in deserted farmlands.