林青(1981—),女,山东烟台人,博士,副教授,主要从事地下环境中水流和溶质运移及模拟研究。E-mail:
土壤水是地表水与地下水联系的纽带,是物质输送和运移的载体,在土壤-植物-大气(SPAC)系统中起着关键性的作用。土壤水分运动是一个非常复杂的过程,为充分了解水分在土壤中的运动过程,应用微扰动的高密度电阻率仪(ERT)监测了一层状结构的土壤剖面在注水入渗试验前、后不同时刻的电阻率的变化过程;同时,用时域反射仪(TDR)测量了点尺度上土壤体积含水量,建立了土壤电阻率和含水量的定量关系。结果表明,高密度电阻率法能够较为准确地监测土壤水分入渗深度和剖面含水量,土壤水分运动以向下的垂向运动为主并伴有微弱的水平流动;当土壤含水量低于0.15 cm3·cm–3时,随着含水量的增大,电阻率变化较大;当土壤含水量较高时,电阻率随含水量的变化不明显;根据建立的电阻率和含水量的定量关系公式,估算出在土壤界面处当上层土壤质量含水量达到0.136 g·g–1时水分开始向下层入渗,相关研究成果为定量分析土壤水在分层界面处的变化情况提供了一种新的方法。
Soil water is the link between surface water and groundwater, and the carrier of solute transport in soil, which plays a key role in the Soil-Plant-Atmosphere Continuum (SPAC) system. The movement of soil water is a very complex process, especially in the layered structure. Heterogeneity of texture and pore of layered soil changes hydraulic characteristics at the interface of the soil layer, and thus the soil water movement and solute transport differ significantly from that in homogeneous soil.
To fully understand the process of soil water movement, infiltration was evaluated using electrical resistivity tomography (ERT) in soil with a stratified profile. A field infiltration test was performed on an 8.7-m-long transect and successive measurements using ERT allowed determining resistivity changes as infiltration progressed. In the meantime, the soil water content was measured by time domain reflectometry (TDR) at the point scale, and the quantitative relationship was established between resistivity and water content. In addition, the soil water contents derived by ERT were validated with the soil water content derived by the drying method.
Results showed that the Multi-electrode resistivity method can fairly monitor the process of soil water movement, identify the depth of water infiltration, 90 cm in our study, and quantitatively retrieve the profile water content. The movement of soil water is mainly vertical downward with a weak horizontal flow. A good correlation between resistivity and soil moisture measurements revealed the capability of resistivity measurements to infer soil moisture spatial and temporal variability with root mean square error (RMSE) equal to 0.042 cm3·cm–3 for loam and 0.041 cm3·cm–3 for clay loam. However, when the soil water content was lower than 0.15 cm3 cm–3, the electrical resistivity changed greatly with the increase in water content, while the soil water content was higher, the resistivity did not change significantly with water content. When the soil water reached the interface (~30 cm) between loam and clay loam, the soil water did not immediately move to the lower layer. According to the established relationship between resistivity and water content, it is estimated that at the soil interface, when the mass moisture content reached 0.136 g·g–1 at the upper layer of the soil, the water infiltrated the lower layer. Unexpectedly, abnormally increased resistivity appeared under the area of the soil water infiltration, which was presumably caused by the significant difference in resistivity of soil adjacent layers during the infiltration process.
In comparison with TDR, the resistivity method gives information integrated on a greater volume of soil and the measurements are easier and quicker to be carried out without disturbing the soil. Therefore, this method can be considered as an alternative tool to be employed for qualitative and quantitative soil moisture monitoring in the field. Also, this study provides a new method for quantitative analysis of the movement of soil water at a layered interface.
土壤水作为陆地水循环系统的重要组成部分,是联系大气降水、地表水与地下水之间的纽带,影响着大气降水、灌溉水和地表水等对地下水的补给过程。土壤水分运动过程的监测,其传统方法,如烘干法、中子法和时域反射仪法(Time-domain reflectometry,TDR)等[
ERT在探测土壤水分运动方面已有较好的理论基础和应用实例。Garré等[
由于自然和人为因素的影响,田间土壤一般呈层状结构,使土壤水分运动过程进一步复杂化,特别是土壤剖面中含有渗透性较差的黏土层时更是如此。因此,本文通过田间注水入渗试验,利用ERT监测水分入渗和再分布过程中土壤电阻率的变化特征,研究土壤水分随时间与空间的变化过程;同时,用TDR监测土壤水分含量,建立电阻率与含水量的定量关系模型,并分析不同土层电阻率随含水量的变化特征;最后,结合质量含水量探讨ERT法反演土壤含水量及水分入渗深度的准确性。
试验场地位于青岛即墨市西北部大沽河畔的移风店镇上泊村(36°33′ N,120°12′ E),土壤类型为棕壤,其为青岛市分布最广、面积最大的土壤类型,占土壤总面积的59.8%。土壤剖面分层结构明显,存在渗透性相对较差的黏壤土层,研究区土壤基本理化性质见
研究区土壤基本物理性质
Physical properties of soil in the study site
土壤类型 |
土层 |
土壤质地 |
砂粒 |
粉粒 |
黏粒 |
容重 |
棕壤 | 0~30 | 壤土 | 44.65 | 43.64 | 11.71 | 1.64 |
Brown soil | 30~60 | 黏壤土 | 24.45 | 47.03 | 28.52 | 1.75 |
60~100 | 粉壤土 | 28.08 | 52.12 | 19.80 | 1.63 |
ERT法数据采集采用中国地质仪器公司制造的DCX-1G高密度电阻率实时成像系统仪(北京),电压测量精度优于± 0.3%,分辨率为1 μV;电流测量精度优于± 0.5%,分辨率为1 μA。田间入渗试验前,按照间距为0.3 m布设电极,将电极缓慢插入土壤10 cm,尽量减少对原状土壤的破坏,电极数量为30个,样带长度为8.7 m。测量采用对垂直变异敏感的Wenner型电极排列方式,根据电极排列及间距计算获得的最大理论测量深度为2.7 m,由于电流分布对电阻率对比度的依赖,有效测量深度受土壤分层的影响,故反演的实际深度为1.37 m。
电极布设完成后,在距离测线约5 cm处、10号与21号电极之间挖一条长3 m,深5 cm,宽5 cm的沟槽进行注水入渗试验。注水前,首先利用ERT测量剖面的初始电阻率分布。注水过程保持沟槽积水高度为2 cm,注水时间为130 min,总入渗水量为200 L,以开始注水为0时刻,分别在25、45、60、80、130、160、200、230、290和310 min进行电阻率的数据采集。同时,在沟槽另一侧约5 cm处,设三个观测点,用TDR(TRIME管式)测量0~30 cm与30~60 cm的土壤体积含水量。入渗试验结束后,用土钻分别取表层、30、60、90、120和150 cm附近处土样带回实验室,采用烘干法测定土壤质量含水量。电极布设与样点分布情况如
电极布设与样点分布图
The distribution of electrodes and soil sampling points
采用目前国际上应用较多的Res2dinv反演软件,用有限差分法求解计算地下介质的理论视电阻率的分布,采用时滞反演方法,使模型计算电阻率与观测的视电阻率差值最小,得到土壤真实电阻率的分布,用均方根误差(RMSE)来描述反演的好坏。Archie经验公式是描述土壤电阻率与含水量关系的代表性公式,被用于多种不同质地土壤的电阻率和含水量关系的描述[
式中,
采用时滞反演法对ERT测量数据进行反演,迭代次数选择为5次,RMSE均小于5.0 %,反演的真实性较好。
注水前土壤剖面电阻率分布
The electrical resistivity of the soil profile before water infitration
注水后土壤剖面电阻率随时间的变化情况如
注水后土壤剖面电阻率随时间变化
The electrical resistivity of the soil profile at different times after water infiltration
注水后土壤剖面电阻率变化率随时间的变化
The rate of change in electrical resistivity of the soil profile at different times after water infiltration
注水后表层土壤电阻率迅速降低,这是由于表层土壤比较干燥,土壤孔隙较大,孔隙水连通性较好,而且初始土壤含水量也比较低,基质势梯度值较大,水分向下运动速度较快。随着入渗时间的延续,表层土壤含水量不断增加并趋于饱和,此时土壤电阻率的变化减小,并且逐渐趋于稳定。入渗60 min后,30 cm以下电阻率基本未变化,这是因为该深度处往下黏粒含量增大,土壤孔隙较小,土壤透水性减弱,水分下渗缓慢。注水阶段(0~130 min),土壤中的水基本上以活塞流的形式下渗;注水结束,土壤水再分布阶段随着水分的持续下渗,表层电阻率开始降低,在50 cm深度处出现了侧向电阻率降低的现象,由于此时处于水分再分布阶段,排除水平侧向流的影响,推测是异常值有所缓解所致。总体而言,在整个水分入渗过程中,土壤电阻率的变化呈层状向下推移,同时伴随着微弱水平方向的电阻率变化,说明水分的运动以向下的垂直运动为主,并伴随着微弱的水平流动。
准确确定土壤电阻率特性与含水量之间的关系是利用电阻率监测含水量的基础。在试验过程中对0~30 cm与30~60 cm处的土层利用TDR进行了现场体积含水量的监测。根据公式(1),将ERT反演获得的土壤电阻率数据和TDR所获得的土壤体积含水量数据建立定量的相关关系,在此过程中舍去了异常高电阻率数据及所对应的含水量数据。由于试验区土壤呈层状结构,电阻率与含水率定量关系受土壤质地的影响[
土壤电阻率与体积含水量的拟合曲线
Relationship between the electrical resistivity and volumetric water content from TDR
电阻率与含水量定量关系参数拟合结果
The fitted parameters for relationship between electrical resistivity and volumetric water content
土层 |
孔隙度 |
饱和电阻率 |
饱和度指数 |
RMSE |
|
0~30 | 0.38 | 17.27 | 1.05 | 0.702 | 0.042 |
30~60 | 0.34 | 22.66 | 1.12 | 0.627 | 0.041 |
由于本研究为注水入渗试验,从
30~60 cm土层拟合的确定性系数相对较低(
为了验证ERT方法确定土壤水入渗过程的准确性,入渗试验结束后,利用铝盒取回的土样测定土壤剖面不同深度土壤质量含水量,如
试验结束后土壤剖面质量含水量随深度的变化
The change of soil mass water content with depth at the end of the infiltration test
ERT反演的体积含水量与质量含水量估算的体积含水量之间的关系
Comparison of soil water content derived by ERT(
层状土壤是自然界中常见的土体结构,通过对土壤水入渗过程的分析发现,土壤水在到达表层壤土与下层黏壤土的界面(约30 cm处)时,土壤水并没有即刻向下层运移,而是在表层土壤含水量(水势)达到一定值时才会继续向下层入渗。这主要是因为层状土壤分层界面处存在毛管障碍[
本文利用ERT与TDR结合的方法,探讨了土壤水分入渗和再分布过程中电阻率与含水量的变化情况,分析了层状土壤中水分的运动特征,主要得到以下几点结论:
(1)ERT方法能够较准确监测土壤水分的运动过程,利用ERT反演得到的土壤水入渗深度(100 cm)和土壤含水量与开挖剖面测得的入渗深度(90 cm)和含水量相差不大,土壤水分的运动以向下的垂向运动为主并伴随有微弱的水平流动。(2)对于质地黏重的土壤,用Archie公式描述土壤电阻率与含水量的关系误差较大;当土壤含水量低于0.15 cm3·cm–3时,随着含水量的增大,电阻率变化较大;当土壤含水量较高时,电阻率随含水量的变化不明显,此时要通过ERT准确估计较高含水量是比较困难的。(3)利用电阻率和土壤含水量间的关系,推算在土壤界面处当上层土壤质量含水量达到0.136 g·g–1时,水分开始向下层入渗,为定量分析土壤水在分层界面处的变化情况提供了一种新的方法。(4)土壤水分入渗下方出现了电阻率异常增大的区域,推测是入渗过程中相邻土层土壤电阻率差异显著导致异常点出现,具体原因有待进一步研究。
Robinet J, von Hebel C, Govers G, et al. Spatial variability of soil water content and soil electrical conductivity across scales derived from Electromagnetic Induction and Time Domain Reflectometry[J]. Geoderma, 2018, 314: 160—174.
Sun H, Wang Y Q, Zhao Y L, et al. Assessing the value of electrical resistivity derived soil water content: Insights from a case study in the Critical Zone of the Chinese Loess Plateau[J]. Journal of Hydrology, 2020, 589: 125132.
Loke M H, Chambers J E, Rucker D F, et al. Recent developments in the direct-current geoelectrical imaging method[J]. Journal of Applied Geophysics, 2013, 95: 135—156.
Ma D H, Zhang J B, Wu Z D, et al. Application of electrical resistivity tomography to study on soil hydrology and its advance[J]. Acta Pedologica Sinica, 2014, 51(3): 439—447.
马东豪, 张佳宝, 吴忠东, 等. 电阻率成像法在土壤水文学研究中的应用及进展[J]. 土壤学报, 2014, 51(3): 439—447.
Samouëlian A, Cousin I, Tabbagh A, et al. Electrical resistivity survey in soil science: A review[J]. Soil and Tillage Research, 2005, 83(2): 173—193.
Dumont G, Pilawski T, Dzaomuho-Lenieregue P, et al. Gravimetric water distribution assessment from geoelectrical methods(ERT and EMI)in municipal solid waste landfill[J]. Waste Management, 2016, 55: 129—140.
Archie G E. The electrical resistivity log as an aid in determining some reservoir characteristics[J]. Transactions of the AIME, 1942, 146(1): 54—62.
Mojid M A, Cho H. Wetting solution and electrical double layer contributions to bulk electrical conductivity of sand-clay mixtures[J]. Vadose Zone Journal, 2008, 7(3): 972—980.
Garré S, Koestel J, Günther T, et al. Comparison of heterogeneous transport processes observed with electrical resistivity tomography in two soils[J]. Vadose Zone Journal, 2010, 9(2): 336—349.
Kelly B F J, Acworth R I, Greve A K. Better placement of soil moisture point measurements guided by 2D resistivity tomography for improved irrigation scheduling[J]. Soil Research, 2011, 49(6): 504.
Garré S, Günther T, Diels J, et al. Evaluating experimental design of ERT for soil moisture monitoring in contour hedgerow intercropping systems[J]. Vadose Zone Journal, 2012, 11(4): 1—14.
Koestel J, Kemna A, Javaux M, et al. Quantitative imaging of solute transport in an unsaturated and undisturbed soil monolith with 3-D ERT and TDR[J]. Water Resources Research, 2008, 44(12): W12411.
Calamita G, Brocca L, Perrone A, et al. Electrical resistivity and TDR methods for soil moisture estimation in central Italy test-sites[J]. Journal of Hydrology, 2012, 454/455: 101—112.
Zhang Z X, Xu S H, Cui J L, et al. Preliminary exploration on resistivity method for determining soil water retention curve[J]. Soils, 2013, 45(6): 1127—1132.
张志祥, 徐绍辉, 崔峻岭, 等. 电阻率法确定土壤水分特征曲线初探[J]. 土壤, 2013, 45(6): 1127—1132.
Brillante L, Bois B, Mathieu O, et al. Monitoring soil volume wetness in heterogeneous soils by electrical resistivity. A field-based pedotransfer function[J]. Journal of Hydrology, 2014, 516: 56—66.
Fan J L, Scheuermann A, Guyot A, et al. Quantifying spatiotemporal dynamics of root-zone soil water in a mixed forest on subtropical coastal sand dune using surface ERT and spatial TDR[J]. Journal of Hydrology, 2015, 523: 475—488.
Sun J G. Archie's formula: Historical background and earlier debates[J]. Progress in Geophysics, 2007, 22(2): 472—486.
孙建国. 阿尔奇(Archie)公式: 提出背景与早期争论[J]. 地球物理学进展, 2007, 22(2): 472—486.
Friedman S P. Soil properties influencing apparent electrical conductivity: A review[J]. Computers and Electronics in Agriculture, 2005, 46(1/2/3): 45—70.
Besson A, Cousin I, Dorigny A, et al. The temperature correction for the electrical resistivity measurements in undisturbed soil samples[J]. Soil Science, 2008, 173(10): 707—720.
Clément R, Descloitres M, Günther T, et al. Influence of shallow infiltration on time-lapse ERT: Experience of advanced interpretation[J]. Comptes Rendus Geoscience, 2009, 341(10/11): 886—898.
Shanahan P W, Binley A, Whalley W R, et al. The use of electromagnetic induction to monitor changes in soil moisture profiles beneath different wheat genotypes[J]. Soil Science Society of America Journal, 2015, 79(2): 459—466.
Cho K W, Song K G, Cho J W, et al. Removal of nitrogen by a layered soil infiltration system during intermittent storm events[J]. Chemosphere, 2009, 76(5): 690—696.
Ma M M, Lin Q, Xu S H. Water infiltration characteristics of layered soil under influences of different factors and estimation of hydraulic parameters[J]. Acta Pedologica Sinica, 2020, 57(2): 347—358.
马蒙蒙, 林青, 徐绍辉. 不同因素影响下层状土壤水分入渗特征及水力学参数估计[J]. 土壤学报, 2020, 57(2): 347—358.