scholarly journals Transect Scale Solute Transport Measured by Time Domain Reflectometry

2002 ◽  
Vol 33 (2-3) ◽  
pp. 145-164 ◽  
Author(s):  
Magnus Persson ◽  
Ronny Berndtsson
Keyword(s):  
Solute Transport ◽  
Mass Balance ◽  
Water Content ◽  
Time Domain ◽  
Field Experiments ◽  
Breakthrough Curves ◽  
Time Domain Reflectometry ◽  
Transport Parameters ◽  
Entire Experiment

Two quasi steady-state solute transport experiments were carried out in a loamy sand under field conditions. The flux was 40 mm/d in experiment 1 and 18.7 mm/d in experiment 2. Both water content (θ) and resident concentration (Cr) measurements were taken using 64 time domain reflectometry probes at depths ranging from 0.05 to 0.90 m. The Cr measurement was calibrated in situ for each probe location in the field. The convective dispersive equation (CDE) and convective lognormal transfer function (CLT) models were fitted to the breakthrough curves (BTCs). The results indicated fingered flow, which has been shown to exist in previous studies of this soil. The finger width was larger in experiment 1 leading to smaller horizontal heterogeneity and a relatively smaller solute transport velocity. The location of the fingers was consistent between the two experiments resulting in a high correlation between the velocity and mass balance fields. Mass balance calculations showed that the solute mass integrated over depth one day after the solute application was larger than the mass balance for the entire experiment (integrated over time). The probable reason being that solutes were transported out of the measurement volume by horizontal flow across the Ap/E horizon boundary. The investigation of the transport parameters revealed that both the CDE and CLT models could be successfully used to predict most individual BTCs. Horizontally averaged global CDE and CLT models were also fitted to the data. Global solute transport was better modeled with the CDE model in experiment 1, while in experiment 2, the CLT model was better. This study clearly shows the applicability of using TDR with the in situ calibration technique in field experiments with varying water content.

Measuring transient solute transport through the vadoze zone using time domain reflectometry

Soil Research ◽  
2001 ◽  
Vol 39 (6) ◽  
pp. 1359 ◽  
Author(s):  
I. Vogeler ◽  
S. Green ◽  
A. Nadler ◽  
C. Duwig
Keyword(s):  
Electrical Conductivity ◽  
Solute Transport ◽  
Time Domain ◽  
Deterministic Model ◽  
Soil Surface ◽  
Time Domain Reflectometry ◽  
Richards Equation ◽  
Electrical Conductivities ◽  
Destructive Measurement

Time domain reflectometry (TDR) was used to monitor the transport of conservative tracers in the field under transient water flow in a controlled experiment under a kiwifruit vine. A mixed pulse of chloride and bromide was applied to the soil surface of a 16 m2 plot that had been isolated from the surrounding orchard soil. The movement of this solute pulse was monitored by TDR. A total of 63 TDR probes were installed into the plot for daily measurements of both the volumetric water content (θ) and the bulk soil electrical conductivity (σa). These TDR-measured σa were converted into pore water electrical conductivities (σw) and solute concentrations using various θ–σa–σw relationships that were established in the laboratory on repacked soil. The depth-wise field TDR measurements were compared with destructive measurement of the solute concentrations at the end of the experiment. These results were also compared with predictions using a deterministic model of water and solute transport based on Richards’ equation, and the convection–dispersion equation. TDR was found to give a good indication of the shape of the solute profile with depth, but the concentration of solute was under- or over-estimated by up to 50%, depending on the θ–σa–σw relationships used. Thus TDR can be used to monitor in situ transport of contaminants. However, only rough estimates of the electrical conductivity of the soil solution can so far be obtained by TDR.

Time domain reflectometry for water content and density of soils: study of soil-dependent calibration constants

2005 ◽  
Vol 42 (4) ◽  
pp. 1053-1065 ◽  
Author(s):  
V P Drnevich ◽  
A K Ashmawy ◽  
X Yu ◽  
A M Sallam
Keyword(s):  
Dielectric Constant ◽  
Water Content ◽  
Time Domain ◽  
Field Experiments ◽  
Engineering Application ◽  
Time Domain Reflectometry ◽  
Sensitivity Analyses ◽  
Dry Density ◽  
Theoretical Predictions ◽  
Apparent Dielectric Constant

The paper studies the soil-dependent calibration constants used for determining water content and density of soil using time domain reflectometry (TDR), specifically, to establish the typical soil calibration values and study the extent of the uncertainty in calibration factors on measurement accuracy. The TDR method described here makes use of a calibration equation normalized by soil dry density, which involves two soil-dependent constants, a and b. Both a and b have physical significance, with the value of a related to the apparent dielectric constant of the dry density – normalized dry soil solids and the value of b related to the apparent dielectric constant of the pore fluid. From theoretical predictions, typical values of a are around 1.0, and typical values of b are around 9. Practically, the constants a and b are obtained through calibration tests performed in conjunction with standard compaction tests. Experimental study shows that calibration constants fall within the ranges from theoretical predictions. Tests on five soil mixtures provided average values of a = 0.945 and b = 8.76, while 11 clean sands resulted in average values of a = 1.0 and b = 8.5. The study also shows that there are no significant effects of compaction energy on the measured values of a and b. Sensitivity analyses indicate that variations in a and b both cause variations in TDR-determined water content and density, but the variations are typically within acceptable limits for engineering application purpose. Results from TDR tests on simulated field experiments are consistent with the sensitivity analyses.Key words: time domain reflectometry, TDR, calibration constants, water content, dry density, sensitivity.

scholarly journals Soil moisture monitoring in the Toce valley (Italy)

2003 ◽  
Vol 7 (6) ◽  
pp. 890-902 ◽  
Author(s):  
M. Menziani ◽  
S. Pugnaghi ◽  
S. Vincenzi ◽  
R. Santangelo
Keyword(s):  
Mass Balance ◽  
Water Content ◽  
Soil Water ◽  
Soil Water Content ◽  
Time Domain ◽  
Heavy Precipitation ◽  
Time Domain Reflectometry ◽  
Balance Method ◽  
Richards Equation ◽  
Mass Balance Method

Abstract. In the framework of the Mesoscale Alpine Programme (MAP), soil water content profiles were collected at a point station in the Toce Valley (Lago Maggiore MAP Target Area). The data are for the first 70 cm depth of soil for the period April–November, 1999. All measurements were made by a Time Domain Reflectometry device. The time variation of the water stored in a column of soil was estimated by a mass balance method. Evaporation was estimated from the data collected in the summer period. Likewise, by applying the mass balance method to the data collected during and after heavy precipitation events, the water infiltrated into the soil was also estimated. A qualitative evaluation of ponding and/or runoff as the difference between the precipitated and the drained water was obtained under suitable assumptions. Furthermore, the time evolution of the soil water content profile was studied by solving the Richards equation both analytically and numerically for two particular cases: the driest period and a period following a heavy precipitation event. Finally, during the MAP Special Observing Period, two intensive campaigns were performed, together with measurements using an airborne passive microwave radiometer, to assess the spatial distribution of the surface (0–30 cm depth) soil water content in fields with different physical and agricultural characteristics. Keywords: soil water content, Time Domain Reflectometry, TDR temperature-dependence, evaporation, infiltration, runoff, linearised Richards equation.

Analysis of partial breakthrough data by a transfer-function method

Soil Research ◽  
2006 ◽  
Vol 44 (2) ◽  
pp. 175 ◽  
Author(s):  
M. A. Mojid ◽  
D. A. Rose ◽  
G. C. L. Wyseure
Keyword(s):  
Transfer Function ◽  
Solute Transport ◽  
Calcium Chloride ◽  
Unsaturated Soils ◽  
Function Method ◽  
Sandy Loam ◽  
Breakthrough Curves ◽  
Time Domain Reflectometry ◽  
Transport Parameters ◽  
Transfer Function Method

A transfer-function method has been applied to determine solute-transport parameters from earlier sections of complete breakthrough data. Time-domain reflectometry allows the measurement of breakthrough data in unsaturated soil. In fine-textured soils, the flow of water must be kept low to maintain unsaturated conditions, and so experiments for a complete breakthrough of solute may last a very long time. Substantial savings of time and computer memory might be achieved if data could be analysed from an earlier section of breakthrough data. Data at 2 vertical positions (input at upper and response at lower position) from a complete breakthrough of calcium chloride applied as a pulse input to 4 unsaturated soils (coarse sand, sandy loam, clay loam, clay) were divided into 4 sets of increasing duration. Transport parameters of calcium chloride were determined by a transfer function, which results in similar values of the parameters from the last 3 datasets in all 4 soils. In the clay soil, however, because of erroneous breakthrough data the fit between the measured and estimated breakthrough curves (BTCs) was poor, but the transport parameters were consistent among different segments of data. We show that it is possible to determine successfully solute-transport parameters from partial breakthrough data, which include the peak of the response BTC. This transfer-function method is thus a powerful tool to shorten breakthrough experiments.

Determining Moisture Content of Soil Layers with Time Domain Reflectometry and Micromechanics

2008 ◽  
Vol 2053 (1) ◽  
pp. 30-38 ◽  
Author(s):  
Sang Ick Lee ◽  
Dan G. Zollinger ◽  
Robert L. Lytton
Keyword(s):  
Dielectric Constant ◽  
Soil Moisture ◽  
Moisture Content ◽  
Water Content ◽  
Time Domain ◽  
Soil Moisture Content ◽  
Time Domain Reflectometry ◽  
Volumetric Water Content ◽  
Dry Density

Although the moisture condition of pavement sublayers can significantly affect pavement performance, accurate interpretation of in situ soil moisture measurements has been difficult to achieve because of the limitations of existing methods. Time domain reflectometry (TDR), originally developed to detect breaks or shorts in electrical conductors, has been used for measuring parameters related to the in situ soil moisture content. However, the apparent length method currently used to determine dielectric constant ignores other electrical properties of the conducting medium that may affect the interpretation of TDR trace to determine soil moisture. Furthermore, the existing methods for computing volumetric water content ignore the variations of dry density and determine the model parameters with assumption or regression analysis. These deficiencies can, in many cases, create a significant systematic error in the final determination of volumetric water content. To minimize these errors and improve the accuracy of moisture content estimate, a new three-step approach was proposed. The approach uses the transmission line equation to calculate the dielectric constant, conductivity, and reflectivity of a soil mixture. A micromechanics and self-consistent scheme was used to determine the volumetric moisture content and dry density on the basis of calibrated values of the solid and water dielectric constants. The system identification method was used iteratively to solve for dielectric parameters, soil moisture content, and dry density values. The validation of the new approach with ground-truth data indicated that the calculated errors were significantly less than those of existing method.

Determining local-scale solute transport parameters using time domain reflectometry (TDR)

1993 ◽  
Vol 148 (1-4) ◽  
pp. 93-107 ◽  
Author(s):  
M. Vanclooster ◽  
D. Mallants ◽  
J. Diels ◽  
J. Feyen
Keyword(s):  
Solute Transport ◽  
Time Domain ◽  
Local Scale ◽  
Time Domain Reflectometry ◽  
Transport Parameters
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