Research On The Effects Of Macropores On Rainfall Infiltration In Vadose Zone Soils Of Well Vegetated Slope

Since the mid-twentieth century, because of the ever-increasing global warming, the frequency of extreme intense rainfall events and well vegetated slope failure events following the heavy rains in mountains are getting higher and higher. In-depth researches of this problem have great practical significance for landslide susceptibility assessment. Up to now, the existing research achievements of landslides induced by rainfall mainly focused on the interrelation between rainfall events and landslide events which no direct relation, as well as the aspects of water-rock interaction in rock-soil mass of slope. But the response process and response mechanism of groundwater to rainfall has received much less attention. The response of soil water to rainfall is the first step of the response of groundwater to rainfall. It is widely recognized that this response process in soils are strongly determined by soil structure. There are various macropores in vadose zone soils of well vegetated natural slope. However, a lot of research work has not been considered the influence of macropores on rainfall infiltration in slope. So, the research of rainfall infiltration in well vegetated slope is the bottleneck of systematically understanding the response of groundwater to rainfall. The effects of macropores on rainfall infiltration in vadose zone soils of well vegetated slope are studied by methylene blue dye tracer infiltration experiment, iodine-starch visual method, theoretical analysis and HYDRUS-2D finite element software numerical simulation method. The main research achievements are summarized as the following:1、The physical-chemical-hydraulical properties of unsaturated soil in well vegetated slope were investigated by laboratory test and field test in Chenggong experimental plots and in Touzhai experimental plots. The test results show that the water content, total porosity, organic matter, storativity of free water and the saturated hydraulic conductivity of soil in well vegetated slope are larger than the bare slope and the cultivated soils:on the contrary, the soil specific gravity and the bulk density are smaller in well vegetated slope. The storativity of free water and the saturated hydraulic conductivity decreased with increasing soil depth.2、In order to distinguish the types of macropores, the distribution patterns of macropore flow paths and the effects of root systems, organic matter, gravel and soil fauna on the distribution patterns of macropore flow paths. Methylene blue was chosen as the dye tracer. Dye tracer infiltration experiments were conducted in well vegetated slope. Results show that:Root channels (including with bark, without bark and root-soil interstice), animal burrows, inter aggregate porosity and gravel-soil interstice are common macropores in well vegetated slope. Root systems have significant effects on the quantity, the scale and the distribution patterns of macropore flow paths, especially, those which had developed vertically or in a downslope direction. The organic matter was favorable for the formation of the inter-aggregate pore. Gravel is favorable for the formation of macropore flow paths, especially in the topsoils. The effects of soil fauna on the morphology of animal burrows network are related to the life inbit of soil animals, soil texture and root distribution.3、Iodine-starch visual method and cluster analysis were used to study the distribution pattern of water flow paths in well vegetated slope. Research indicated that the heterogeneity of the distribution pattern of water flow paths increased with decreasing soil depth under intense rainfall condition. The heterogeneity of water flow paths in the direction of downslope is more significant than in the direction of vertical.4、The theory of soil water movement studies indicate that the research of unsaturated soil water movement has gone through two leaps. The existing research achievements of macropore flow model mainly focused on the agricultural soil, which are unfit for researching on the macropore flow in well vegetated slope. The conception mode of root-soil interstice macropore flow in well vegetated slope are presented.5、Base on the results of dye tracer infiltration experiments, water flow paths tracer experiments and the relative existing research outcomes, the effects of macropores on rainfall infiltration in well vegetated slope were discussed. We come to the following conclusions: Macropore flow in vadose zone of well vegetated slope is initiated through macropores intercept subsurface storm flow. Macropore flow lead to the fast rise in the infiltration and speed up rainfall infiltration. Stem flow can reach the deep soil quickly. The saturated hydraulic conductivity of the animal burrows and the soil are correlated positively with the biomass, total area, length and volume of animal burrows. Gravel-soil interstice intercept subsurface storm flow and other macropore flow, and increase the infiltration capacity of soils. The effect of root channel on rainfall infiltration is far greater than from animal burrows. The heterogeneity of macropore network system leads to ununiformity of the distribution of soil water content.6、Making use of the finite element program of HYDRUS-2D, the influence of macropore and the diameter, the length, the tortuosity, the density and the orientation of macropores were simulated under same rainfall process. The result indicates:macropore accelerate the rainfall infiltration and weaken the subsurface flow, and delay the time of appearance of transient state saturated zone, and then lend to the shape of wetting front is irregular. The influence of the diameter of single macropore to the distribution of the soil water content and the pressure head, the shape of wetting front are not obvious. The increasing of macropore diameter lead to the velocity of preferential flow decreases, the decay of the time of ponding in macropores and the ponding fast discharge. The increasing of the length of macropore lead to the shape of wetting front is more irregular, the depth of infiltration increases, the decay of the time of ponding in macropores and the ponding depth of macropore increases. The velocity of preferential flow is less influenced by the length of macropore. The decay of the time of ponding in macropores and the velocity of preferential flow begin to increase and then decease with the increasing of the tortuosity of macropore. The depth of rainfall infiltration decreases and the ponding depth of macropore increases with the increasing of the tortuosity of macropore. The tortuosity of macropore effects on the shape of wetting front are more important at the start of the infiltration than at later times. The irregularitiy of the shape of wetting front decreases with the increasing of the tortuosity of macropore. Because of the increasing of the density of macropore, the complex of shape of wetting front and getting more and more. The penetration rate of rainfall increases with the increasing of the density of macropore. The increasing of density of macropore lead to the interaction between differential macropore getting more and more pronounced. The irregularitiy of the shape of wetting front decreases with the decreasing of the angle between the orientation of macropore and the trend of the slope. The velocity of preferential flow increases with the decreasing of the angle. Macropore that in the topsoil and parallel to slope surface leads to an even lower infiltration rate of precipitation.