Research On The Preferential Flow Path And Root-Soil Annulus Flow In Vadose Zone Of Well Vegetated Slopes

Since the middle of last century, intensively driven by the global warming trend, the frequencies of abnormal extreme weather events and natural vegetation slopes stability disaster events induced by it are getting higher and higher, and they also threat the safety of people's life and property and destroy the ecological environment, so it has great practical significance to have further study of this problem. The existing research achievements of landslides induced by rainfall mainly focused on the interrelation between rainfall and landslide which have no direct relation, as well as the aspects of groundwater and landslide which directly linked, but research on the process of rainfall transforming to groundwater occupies the fundamental position have been marginalized. Although people have begun to notice the preferential flow's influence on the rainfall infiltrating to natural slopes, but the research level is not high, and its mechanism needs to have a further and perfectly study. Therefore, we had studied the preferential flow path and root-soil annulus flow in vadose zone soils of well vegetated slopes, and the indoor and outdoor experiments, theoretical analysis and analytical solution were included. The main work and results of the paper are as follows:1. The physical-chemical-hydraulical properties of unsaturated soils in well vegetated slope were investigated by laboratory test and field test in five plots of Touzhaigou and Duanjiaying experimental sites. It turned out that Touzhaigou's soil volumetric water content, void ratio, permeability coefficient, water capacity and specific yield were greater than that of Duanjiayin, but the natural density, dry density, specific gravity, specific retention, gravel and clay particle content were opposite; both of the trial sites soils were highly compressible and acidic; the former belonged to the expansive soils, the shrinkage had largely spatial variability, and it was more likely to be organic soil. The weathering degree of soil on Maka mountain was lower than that of Liao mountain.2. From the perspective of effective macropore, making macropores physical model in room, simulating the dyeing tracer test that we did in field, and based on particle package theory and pythagorean theorem, we had a test to get the macropores infimum.It proposed a simple method to test the macropores infimum of soil—Anti-inference Method of Particle Size, and the value obtained was 32.46?m. Macropore function, physical and chemical properties of methylene blue, organic matter, minerals were all factors to the define process of macropores. Because of restrictions of research methods, purposes and test instruments, the define controversy of macropore still exist.3. Collecting the soils of basalt slope covered by Pinus yunnanensis and Cynodon dactylon in Duanjiaying sites, based on the water breakthrough experiment, using analysis of variance, regression analysis method, we had studied the characteristics of preferential flow path that had a significant impact on landslide, the analysis concluded that:Vegetation types were unable to control the whole development of soil macropore moisture movement, but affected the amplitude and settling time of water breakthrough curve. The number of soil macropores in each layer about Pinus yunnanensis was greater than that of Cynodon dactylon, both the two soils showed that the quantity of large diameter macropore was small, but the quantity of small diameter macropore was large. Macropore size range was controlled by vegetation types, but the size distribution did not. Macropore's characteristics had different influences on water flow rate of different vegetations soils. Content of organic matter and root had different degree of influences on macropore characteristics of different vegetations soils. Particle size distribution had no significant effect on parameters in each layer of Pinus yunnanensis and Cynodon dactylon soils. Another value of macropores infimum was 600 ?m.4. Maintaining the primitive state of slope, simulating the process of rainfall, carrying out dyeing tracer experiment with methylene blue, combining with the moisture penetration method, capturing high precision dyeing section image, based on image processing, variance analysis and regression analysis method, we had studied the soils preferential flow distribution of different vegetations communities slopes in rainfall process and its affecting factors. The analysis arrived at those conclusions that the profile distribution range of macropore flow in woody vegetation community soil was bigger, and with lower continuity degree, higher differentiation degree and more complicated dyeing pattern. High coverage slope tended to have the rainfall infiltration down the slope. The dyeing coverage and the mean number of path about the two vegetation communities soil decreased with depth increasing, and were characterized by the polarization of soil surface and bottom. Root channel could existed in aggregate particles. The pore system formed by living roots was not stable, dead roots could form different root channels in different phases. Roots had the stronger effect on macropore flow distribution in woody soils, but organic matter had the stronger effect on macropore flow distribution in herbal soils. The root size was not the only factor to control macropore size. Root system and organic matter related to the vegetations had a positive contribution to the formation and scale of macropore. Macropore distribution significantly affected the distribution of macropore flow.5. Based on a series of assumptions, combining with the physical and mathematical model of sleeve, under the condition of small Reynolds number (laminar state), the diversion features of root-soil annulus in soils of different vegetation communities slopes were researched, and the root's effect on rainfall infiltration in soils were also analyzed. Draw a conclusion: The size and volume of root-soil annulus in woody and herbaceous vegetation communities soils decreased with soil depth increasing; root-soil annulus's size of the former in all layers were smaller than that of the latter, but the volume of annular were on the contrary. Because of the spacial differences of root and annular sizes, spatial variation of the maximum of water flow velocity happened. Under the condition of the same annular number and Reynolds number, average flow velocity, maximum flow velocity and annular contribution rate increased with depth increasing, but the flux were on the contrary. The flow velocity, flux and contribution rate of the former in all layers were greater than that of the latter. Vegetation types, plant morphology and root length, size, number, volume, ecological niche were the important factors that affected the diversion features of root channel. Macropores formed by root system had the characteristic of small content with large water transfer quantity.