Quantitative characterization and simulation of soil moisture distribution in heterogeneous vadose zone
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摘要:
包气带作为连接植被与地下水的重要纽带,其岩性结构是影响地下水生态功能的主要因子之一,而包气带结构中的弱透水夹层对来自地面的入渗水流具有延迟滞后、局部蓄水作用(上层滞水),尽管该部分水量很小,但对于极端干旱区植被生态的维持具有一定供水意义。为了探究包气带岩性结构如何影响土壤含水率分布,设计了均质土柱(O),单薄夹层土柱(A),单厚夹层土柱(B)以及双夹层土柱(C)4个土柱,开展了室内层状非均质土柱释水实验和入渗实验;依据物理实验开展了变饱和水流数值模拟。实验过程中,细粒土夹层结构导致水分在夹层内部和夹层上下界面处滞留,形成了水分聚集区;相较于O柱,A柱、B柱和C柱释水过程持续时长分别增加了290,500和780 h,持水量分别增加了6.20,7.90和7.83 cm;数值模拟结果表明采用修正后的van Genuchten模型能更好地模拟层状土含水率剖面。细粒土夹层对入渗水下移特征会产生明显阻滞效应,水分主要滞留在夹层内部和夹层上下界面处,夹层厚度、层数增多会导致夹层内和夹层界面处均滞留更多水分,通过引入虚拟变量和毛细高度修正van Genuchten模型,并将其对应的相对渗透率方程分为三段式,可有效提升层状土壤含水率剖面模拟精度。
Abstract:Objective The vadose zone serves as a critical link between vegetation and groundwater, with its lithological structure being one of the main factors influencing groundwater ecological functions. However, the weak permeable interlayer in the structure of the venous zone has a delayed and delayed effect on the infiltration flow from the ground and local storage (water retention in the upper layer). Although the water volume of this part is very small, it has certain water supply significance for the maintenance of vegetation ecology in extreme arid areas. In order to explore how lithologic structure affects soil water content distribution,
Methods Four soil columns were designed: homogeneous soil column (O), thin interbedded soil column (A), single thick interbedded soil column (B) and double interbedded soil column (C). The indoor drainage and infiltration experiments are conducted on these layered heterogeneous soil columns. The numerical simulation of variable saturation flow is carried out based on physical experiments.
Results During the experiment, the fine-grained soil interlayer structure causes water retention in the interlayer and at the upper and lower interfaces of the interlayer, forming a water accumulation area. Compared with column O, the water release duration of column A, column B and column C increased by 290 h, 500 h and 780 h, respectively, and the water holding capacity increased by 6.20 cm, 7.90 cm and 7.83 cm, respectively. The numerical simulation results show that the modified van Genuchten model can better simulate the layered soil moisture profile.
Conclusion Fine-grained soil interlayer has a significant retarding effect on the underwater migration characteristics of infiltration, and water is mainly retained inside the interlayer and at the upper and lower interfaces of the interlayer. The increase in the thickness and number of interlayers will lead to more water retention inside the interlayer and at the interlayer interface. The corresponding relative permeability equation is divided into three stages, which can effectively improve the simulation accuracy of layered soil water content profile.
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Key words:
- layered heterogeneity /
- soil moisture distribution /
- numerical simulation
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表 1 数值模拟实验模型设置表
Table 1. Setting table of numerical simulation experiment model
土柱 O A B C 柱高/cm 80 夹层分布高度/cm 无 50~60 45~60 20~30、50~60 初始条件 初始时刻土柱均饱和 持续时长/h 释水阶段 63 627 713 1268 入渗阶段 187 73 87 132 上边界 释水阶段 零通量边界 入渗阶段 在实验初期0.1 h内,顶部为已知流量边界,入渗流量为 10590 mL/h,在0.1 h后,顶部边界转换为零通量边界下边界 释水阶段 底部为已知水头边界条件,水头变化依照实测值设置 入渗阶段 已知水头边界条件,水头为0 m 表 2 不同SWCC模型及其对应的相对渗透系数模型表达式
Table 2. Different SWCC models and their corresponding relative permeability model expressions
模型名称 模型表达式 渗透系数表达式 van Genuchten(VG)模型 $\theta =\left\{ {\begin{array}{*{20}{l}} {{\theta _{\mathrm{r}}} + \dfrac{{({\theta _{\mathrm{s}}} - {\theta _{\mathrm{r}}})}}{{{{\left[1 + {{\left| {\alpha{H_{\mathrm{p}}}} \right|}^n}\right]}^m}}}}&{{H_{\mathrm{p}}} < 0} \\ {{\theta _{\mathrm{s}}}}&{{H_{\mathrm{p}}} \geqslant 0} \end{array}} \right.$ $K = \left\{ {\begin{array}{*{20}{l}} {{K_{\mathrm{s}}}{S_{\mathrm{e}}}^l{{[1 - {{(1 - {S_{\mathrm{e}}}^{\frac{1}{m}})}^m}]}^2}}&{{H_{\mathrm{p}}} < 0} \\ {{K_{\mathrm{s}}}}&{{H_{\mathrm{p}}} \geqslant 0} \end{array}} \right.$ Brooks-Corey(BC)模型 $\theta = \left\{ {\begin{array}{*{20}{c}} {{\theta _{\mathrm{r}}} + \dfrac{{({\theta _{\mathrm{s}}} - {\theta _{\mathrm{r}}})}}{{{{\left| {\alpha {H_{\mathrm{p}}}} \right|}^n}}}}&{{H_{\mathrm{p}}} < - \dfrac{1}{\alpha }} \\ {{\theta _{\mathrm{s}}}}&{{H_{\mathrm{p}}} \geqslant - \dfrac{1}{\alpha }} \end{array}} \right.$ $K = \left\{ {\begin{array}{*{20}{l}} {{K_{\mathrm{s}}}{S_{\mathrm{e}}}^{\frac{2}{n} + l + 2}}&{{H_{\mathrm{p}}} < - \dfrac{1}{\alpha }} \\ {{K_{\mathrm{s}}}}&{{H_{\mathrm{p}}} \geqslant - \dfrac{1}{\alpha }} \end{array}} \right.$ Log-Normal Distribution(LN)模型 $\begin{aligned}&\theta = {\theta _{\text{r}}} + Q\left[\dfrac{{\ln ({H_{\mathrm{p}}}/{h_{\mathrm{m}}})}}{\sigma }\right]({\theta _{\text{s}}} - {\theta _{\text{r}}}) \\&Q(x) = \int_x^\infty {\dfrac{1}{{{{(2{\text π})}^{{1 \mathord{\left/ {\vphantom {1 2}} \right. } 2}}}}}{\text{exp}}( - \dfrac{{{x^2}}}{2})} {\mathrm{d}}x \end{aligned}$ $K = {K_{\mathrm{s}}}{S_{\mathrm{e}}}^{1/2}{(Q[{\mathrm{ln}}({H_{\mathrm{p}}}/{h_{\mathrm{m}}})/\sigma + \sigma ])^2}$ 注:θ为体积含水率,cm3/cm3;θr为残余含水率,cm3/cm3;θs为饱和含水率,cm3/cm3;Hp表示压力水头,cm;α为拟合参数,cm−1;n、m为曲线形状参数,无量纲;Se表示土壤水饱和度;K为非饱和渗透系数,cm/h;Ks为饱和渗透系数,cm/h;l为拟合参数,通常取0.5;Q(x)为互补累计正态分布函数;x为自变量的统称;hm为孔隙毛细管压力分布的峰值,cm;σ为孔隙毛细管压力分布的标准差;hm和σ均为参数拟合值;下同 表 3 不同SWCC模型参数表
Table 3. Different SWCC model parameters table
土壤类型 饱和渗透系数
Ks/(cm·h−1)模型 模型参数 α/(cm−1) n m l hm/cm σ θs/(cm3·cm−3) θr/(cm3·cm−3) 中砂 86.88 BC 0.619 0.755 − 0.500 − − 0.400 0 VG 0.028 6.956 0.856 0.500 − − 0.369 0.045 LN − − − − 36.360 0.2439 0.364 0.037 粉土 2.16 BC 0.001 1.200 − 0.500 − − 0.452 0.100 VG 0.002 1.230 0.187 0.500 − − 0.450 0.160 LN − − − − 501.000 2.751 0.460 0.034 表 4 A、B、C柱中夹层及夹层上下方5 cm持水量汇总
Table 4. Summary table of water holding capacity of 5 cm above and below the interlayers in columns A, B and C
土柱 A B C 上 夹层 下 上 夹层 下 上a 夹层a 下a 上b 夹层b 下b 持水量/cm 0.19 4.22 0.36 0.18 6.26 0.77 0.30 4.24 0.51 1.30 4.25 1.85 占比/% 1.10 24.50 2.09 0.95 33.09 4.06 1.62 22.86 2.75 7.01 22.91 9.97 总持水量/cm 17.22 18.92 18.55 注:a为C柱中50~60 cm处的夹层,b为C柱中20~30 cm处的夹层 表 5 土柱实验中中砂和粉土的修正VG模型参数
Table 5. Modified VG model parameters of medium sand and silt in soil column experiment
土壤类型 θs θr α/(cm−1) n Ks/(cm·h−1) l θm/(cm3·cm−3) θk/(cm3·cm−3) 中砂 0.3585 0.0580 0.0319 3.1803 86.88 0.5 0.2863 0.35 粉土 0.4500 0.0465 0.0031 1.0166 2.16 0.5 0.4400 0.45 表 6 不同模型对3种土柱实验模拟含水率的RMSE和NSE
Table 6. RMSE and NSE of three soil columns simulated by different models
土柱 S-VG模型 S-BC模型 S-LN模型 S-mVG模型 NSE RMSE NSE RMSE NSE RMSE NSE RMSE A 0.8071 0.0869 0.6849 0.1013 0.9128 0.0483 0.9424 0.0461 B 0.8439 0.0629 0.8696 0.0706 0.9077 0.0581 0.9402 0.0572 C 0.7699 0.0874 0.7377 0.0950 0.8345 0.0675 0.8361 0.0638 表 7 夹层上下方5 cm范围内的持水量实测值与模拟值
Table 7. Comparison of measured and simulated water holding values in the range of 5cm above and below the interlayer
土柱类型 S-VG模型 S-BC模型 S-LN模型 S-mVG模型 高度/cm 实测值/cm 模拟值/cm 误差/% 模拟值/cm 误差/% 模拟值/cm 误差/% 模拟值/cm 误差/% A 60~65 0.1891 0.0771 −59.22 0.1274 −32.63 0.0993 −47.49 0.2127 +12.48 45~50 0.3566 0.2270 −36.34 0.2627 −26.33 0.2205 −38.17 0.3048 −14.53 B 60~65 0.1758 0.1012 −42.43 0.0567 −67.75 0.1412 −19.68 0.2127 +20.99 40~45 0.7713 0.3747 −51.42 0.5022 −34.89 0.6776 −12.15 0.6853 −11.15 C 60~65 0.3020 0.2532 −16.16 0.2518 −16.62 0.2885 −4.47 0.2914 −3.51 45~50 0.5147 0.2245 −56.38 0.3510 −31.80 0.4717 −8.35 0.5286 +2.70 30~35 1.3036 0.7548 −42.10 0.6268 −51.92 1.2136 −6.90 1.3080 +0.34 15~20 1.7946 1.8479 +2.97 1.9064 +6.23 1.8390 +2.47 1.6870 −5.60 -
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