Study The Characteristics Of Preferential Flow In Granite Red Soil Base On Soil Column Simulation

Preferential flow is soil moisture penetrate into the deep quickly bypassing the soil matrix through the " Preferential path",is a common form of soil water movement caused by soil heterogeneity. Due to the special nature of the granite in recent years has emerged a series study of preferential flow in the Three Gorges granite region, many of this researches are about the characteristics of rainfall factors and irrigation influence on preferential flow. Currently, few research take into account the physical characteristics of the soil, a different perspective from the degree of development about the priority channel. Preferential flow is a non-equilibrium infiltration, based on high frequency monitoring combined preferential flow type and initial water content analysis of the response to the relationship between the preferential flow and rainfall factors on preferential flow study has important innovative and scientific value.This research use four kinds of granite parent material soil in Tongcheng County, “stable the upper loam layer and mutative lower sand layer” is the principle to making three different soil column boxes Ⅰ,Ⅱ,Ⅲ, then complete six times simulated rainfall every box in different initial moisture content(12.2%-23.6%) and rain intensity(0.7mm / min, 1.2mm / min). Time Domain Reflectometer(TDR) to monitor the entire soil moisture dynamics(cyclic frequency 6min / times). Then determine preferential flow generation, analysis of preferential flow characteristics(type, scope, quantity, degree) and the impact on water transport processes(infiltration, the flow, distribution),find out new method of qualitative analysis and judge the type of preferential flow. Study the basic preferential flow characteristics, occurrence and impact on water transport under different conditions(channel development degree, the initial moisture content, rainfall intensity) in granite parent material soil, the main conclusions are as follows:1. With the same upper laterite,Comparison three soil box lower sand content is soil column Ⅰ>Ⅱ ≈ Ⅲ. Biggest ability to permeate moisture exist in Ⅰ, soil column Ⅰ ratio of saturated hydraulic conductivity and porosity is Ⅱ and Ⅲ ’s 1.6 times. Column Ⅱ’s top priority channel development is strong,but its connectivity is weak. Column Ⅲ’s priority channel development is the most average degree, with strongest connectivity in all layers. Based on soil porosity, bulk density, saturated hydraulic conductivity and other physical parameters guide comprehensive assessment of the three soil column boxes, level the degree of priority channel advantages in development,result is Column Ⅰ(★★★), Ⅱ(★★)and Ⅲ(★★ ☆).2.Changes in frequency with soil moisture to determine preferential flow occurs,in Column Ⅰ which have strongest capacity of hydraulic conductivity Preferential flow occurrence area is the largest, accounting for 75% of the cross sectional area of the soil box. Water conductivity of the upper layer of soil is the main factor affecting the area of preferential flow, preferential flow area of soil column Ⅱ is greater than Ⅲ. Column Ⅲ with strong connectivity that preferential flow distribution deepest, Instantaneous recharge(64mm) of soil column Ⅰ is minimum. Penetration and passage ability of soil is define "fast" characteristics in preferential flow, while the the total amount of rainfall is to determine the reaction amount of preferential flow and soil, that is the main factors affect the extent of the exchange interaction with the surrounding matrix. rainfall intensity 1.2mm / min, the degree of preferential flow increase while initial water content decreases. When 0.7mm / min intensity, degree variation become bidirectional when initial moisture content changes. the magnitude of preferential flow degree increase caused by rain intensity increased is due to soil column priority channel development situation.3. The trends of incremental contour of soil moisture indicate that exist three forms of preferential flow: Macropore flow(M), finger flow / funnel flow(F) and bypass flow(B). Preferential flow occurs reduction with the increase of soil depth,.Macropore flow(M) in largest proportion of occurrence probability., which distribute in the range of 0-40 cm of soil collectively(Probability of occurrence is 72%). Advantages in laterally extending is larger than(F) and(B),which occurrence probability small distribute in 20-60 cm with 71 % and 64% occurrence probability.(F) and(B) determine trends of Macropore flows in the depth extension.(M)’s area is 2.0-2.7 times than(F) and(B) is the largest, which increased while rainfall intensity increases and increase more in strong connectivity column. While rain intensity decrease, the proportion of(M) decreased and the amplitude related to water conductivity;(B) proportion is risen and the amplitude related to channel connectivity.(F) and(B) occured strength is greater than(M), and in case of rain intensity increased intensity differences have increased.4. According to the time difference whit the different probe depths sensitive to moisture changes, The fastest rate of wetting front can be estimated. Rainfall intensity 1.2mm / min compare the wetting front rate in estimated value and actual measured value is the same: Ⅰ>Ⅲ> Ⅱ. Actual measured value raise with initial moisture content increase, but the estimated value is abnormal in lower intense of rainfall with lower initial moisture content. The rank of the influence level to rate of wetting front in three types Preferential flow is(B)>(F)>(M). Preferential flow can affect the absorption and utilization of water, the greater the degree of preferential flow, the smaller utilization amount by soil in water infiltration.5. The outflow perform different characteristics reactions by changes of the channel development status, the initial water content and the rainfall total amount. The beginning time of the outflow be faster while the rainfall intensity increase. Outflow time in soil column Ⅰ with excellent development in Channel is the fastest in average, the second is Ⅲ and the third is Ⅱ. Within a certain range, the outflow time will be fast due to increase of initial moisture content, but no matter too high or too low the initial moisture content is will delay the outflow. The rank of the maximum total amount of the cumulative outflow is Ⅰ>Ⅲ>Ⅱ. More development of the soil column channel, the faster and bigger amount of outflow. The initial water content is key influence factor on start time of outflow, while rainfall amount on outflow total amount. when rain intensity 1.2mm / min the preferential flow rate is 1.8-4.0 times when 0.7mm / min, the magnitude of the increase related to the development level of channel. The ratio of outflow rate and rainfall intensity indicate that preferential flow conductivity ability in column box Ⅱ was significantly weaker than Ⅰ and Ⅲ. 6. Preferential flow occurred affect the water distribution in soil and determine to the rainfall intensity, Macropore flow’s effect is the most obvious. Rainfall intensity 0.7mm / min, The D-value in moisture content(before and after rainfall 24h) decreases with increase in degree of preferential flow. But while the rainfall intensity become 1.2mm / min, the result is opposite that the D-value will increase. Statistics the degree of preferential flow in top three treatment, the D-value of moisture content is 1.8-2.1 times in preferential flow occurred area than unoccured area. Macropore flow occurred level the d-value is 4.1-20.8%,bigger than finger flow and bypass flow.