Experimental study of the soil water characteristic curve and unsaturated permeability coefficient based on the evaporation method and combined measuring instrument
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摘要:
为确定以蒸发法为基础设计的土壤水分特征曲线和非饱和渗透系数联合测定仪的可靠性和适用范围, 选取粉黏壤土、壤土和壤砂土3种土样进行试验研究。在自然蒸发作用下, 利用联合测定仪获得蒸发通量、含水量和土柱表面下方5, 10 cm处的基质吸力, 以线性假设理论为基础处理实测数据, 利用HYDRUS-1D软件进行模拟和反演, 输入实测基质吸力和通量数据确定土壤水力参数, 利用RETC软件对已处理数据进行VG-Mualem模型拟合。结果表明, 含水率符合线性假设理论, 基质吸力线性变化不是很理想, 但除砂土之外的土样可以通过降低蒸发速率和缩短2次测量时间间隔来解决这个问题; 试验数据的拟合效果好, 与模拟结果的误差较小, 试验前期所测非饱和渗透系数与模拟值误差较大, 主要跟含水率接近饱和时水力梯度的精度不能得到保证有关, 可以忽略这些数据来改进。使用蒸发法和联合测定仪测定土壤水分特征曲线和非饱和渗透系数具有比较好的可靠性。
Abstract:Objective To determine the reliability and applicability of the soil water characteristic curve and unsaturated permeability coefficient combined measuring instrument designed on the evaporation method, three soil samples were selected: silty clay loam, loam and loamy sand.
Methods Under natural evaporation, the evaporation flux, water content, and matric suction at 5 and 10 cm below the surface of the soil column were obtained by using the combined measuring instrument. The measured data were processed based on the linear hypothesis. HYDRUS-1D was used for simulation and inversion, and measured matric suction and flux data were input to determine soil hydraulic parameters. VG-Mualem model was fitted to the processed data by using RETC program.
Results The results show that the moisture content accords with the theory of the linear hypothesis, and the linear variation in matric suction is not ideal. However, soil samples other than sandy soil can solve this problem by reducing the evaporation rate and shortening the interval between two measurements. The fitting effect of the test data is good, and the error is small with the simulation result. The unsaturated permeability coefficient measured in the early stage of the test has a large error with the simulated value, which is mainly due to the inability to ensure the accuracy of the hydraulic gradient when the water content is close to saturation. These data can be ignored for improvement.
Conclusion It is reliable to use the evaporation method and combined measuring instrument to measure the soil water characteristic curve and unsaturated permeability coefficient.
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图 1 土壤水分特征曲线和非饱和渗透系数联合测定仪示意图
1.土样放置容器; 2.纱网; 3.电子计数秤; 4.张力计; 5.PVC透明水管; 6.传感器; 7.负压实时显示器
Figure 1. Schematic diagram of the soil water characteristic curve and unsaturated permeability coefficient combined measuring instrument
图 2 空间线性分布分析图(RMSE.均方根误差; RE.相对误差，下同)
Figure 2. Analysis diagram of spatial linear distribution
图 5 SWCC拟合情况(a1~a3)和非饱和渗透系数拟合情况(b1~b3)
Figure 5. Fitting situation of SWCC (a1-a3) and unsaturated permeability coefficient (b1-b3)
表 1 试验土的基本参数
Table 1. Basic parameters of the test soil
编号 饱和含水率 干密度/(g·cm-3) 颗粒组成/% 美国制定名 砂粒 粉粒 黏粒 1 0.491 0 1.315 0 65.60 34.40 粉黏壤土 2 0.482 3 1.444 49.86 36.00 14.14 壤土 3 0.512 4 1.381 83.54 7.97 8.49 壤砂土 
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表 2 土壤水力参数反演结果
Table 2. Inversion results of soil hydraulic parameters
编号 θr α/cm-1 n Ks/(cm·d-1) 1 0.065 0.009 1.488 12.640 2 0.090 0.005 1.663 6.776 3 0.158 0.012 1.288 4.593 注：Qr为残余土壤体积含水率；α为进气值的倒数；n为土壤孔隙尺寸分布参数；Ks为饱和导水率
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表 3 土壤水力参数拟合结果
Table 3. Fitting results of soil hydraulic parameters
编号 θr α/cm-1 n Ks/(cm·d-1) 1 0.167 0.024 1.397 281.184 2 0.280 0.007 2.312 10.507 3 0.248 0.005 1.983 1.046
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