离心机法测定土壤水分特征曲线中的收缩特性

Soil shrinkage property during soil water characteristic curves being measured by centrifugal method

为了探明土壤在离心力作用下的收缩规律,开展了离心机法测定土壤水分特征曲线试验.砂壤土和黏壤土分别设定3个初始容重,以离心机法测定的数据为基础,研究了离心力变化下的土壤收缩规律,并通过van Genuchten-Mualem(VG-M)模型对2种情景模式(考虑容重变化和未考虑容重变化)下所测定的土壤水分特征曲线进行拟合,并以此估算所得的土壤水力特性参数对沟灌二维水分运动特性进行了数值模拟,同时结合室内试验对比分析了参数的合理性.结果表明,离心机转速增大,土壤含水率降低,容重随之增大,当吸力为7000 cm时,砂壤土和黏壤土的容重分别近于1.81和1.79 g/cm 3;基于土壤收缩特征曲线,供试土壤收缩过程可采用三直线模型进行表征,但各收缩段的吸力范围存在差异;与未考虑容重变化所得VG-M模型中的参数值相比,考虑土壤容重变化所得的滞留含水率θr和进气吸力值倒数a均增大,但形状系数n均减小;以考虑土壤容重变化所得VG-M参数为基础进行沟灌二维水分运动数值模拟,其入渗水量、湿润锋运移距离(垂直和水平)与实测值的误差绝对值均值分别为5.8%,3.0%和2.6%,较未考虑容重变化时精度分别提高了39.2%,57.2%和52.9%.因此离心机法测定土壤水分特征曲线的过程中需考虑土壤容重的变化,且以此获得的参数能够较为显著地提高数值模拟精度.

Soil water characteristic curves(SWCC)were measured by employing centrifugal method based on the sandy and clay loams in Yangling District of Shaanxi Province.The aim was to study soil shrinking property during the measurement.Three initial bulk densities were set up for the two kinds of soils,and soil shrinking characteristics were clarified in terms of soil hydraulic parameters measured.The test data were best fitted by making use of van Genuchten-Mualem(VG-M)model in two scena-rios,i.e.bulk density variation is considered or not,to obtain SWCCs.Two-dimensional numerical simulations were performed to obtain water transport features under furrow irrigation based on the soil hydraulic parameters determined,and the rationality of these parameters was argued according to the indoor experiments conducted.The results show that soil moisture content decreases,but soil bulk density increases gradually with increasing centrifugal speed(soil water suction).As the soil water suction is 7000 cm,the soil bulk densities of the sandy and clay loams are 1.81 and 1.79 g/cm 3,respectively.Based on the soil shrinking characteristics curve,the shrinking process of the sampled soils can be represented by a three-piecewise linear model for different ranges of soil water suction.The residual soil moistureθr and parameter a are larger,but the shape factor n is smaller in the VG-M model when the bulk density variation is considered in comparison with those obtained without variation in the bulk density.The parameters obtained by considering the variation in bulk density were input into software HYDRUS to simulate a two-dimensional furrow irrigation.The simulated results indicate that the ave-raged errors in the cumulative infiltration and the displacements of wetting front in the horizontal and vertical directions are 5.8%,3.0%and 2.6%,respectively,compared with the experimental data.Meanwhile,the accuracy of predictions of the cumulative infiltration and wetting front displacements in terms of the parameters determined by considering the variation in bulk density is better by 39.2%,57.2%and 52.9%,respectively,than that without variation in the bulk density.Therefore,the varia-tion of soil bulk density should be considered when a SWCC is measured by using centrifugal method.As a result,the soil hydraulic parameters estimated in this way can significantly improve the accuracy of soil water transport process simulation.