CHARACTERIZING PREFERENTIAL FLOW OF NITRATE AND PHOSPHATE IN SOIL USING TIME DOMAIN REFLECTOMETRY

A series of infiltration and tracer experiments was conducted in unsaturated sand and gravel deposits on Cape Cod, Massachusetts. A network of 112 porous cup lysimeters and 168 time domain reflectometry (TDR) probes was deployed at depths from 0.25 to 2.0 m below ground surface along the centerline of a 2‐m by 10‐m test plot. The test plot was irrigated at rates ranging from 7.9 to 37.0 cm h−1 through a sprinkler system. Transient and steady state water content distributions were monitored with the TDR probes and spatial properties of water content distributions were determined from the TDR data. The spatial variance of the water content tended to increase as the average water content increased. In addition, estimated horizontal correlation length scales for water content were significantly smaller than those estimated by previous investigators for saturated hydraulic conductivity. Under steady state flow conditions at each irrigation rate, a sodium chloride solution was released as a tracer at ground surface and tracked with both the lysimeter and TDR networks. Transect‐averaged breakthrough curves at each monitoring depth were constructed both from solute concentrations measured in the water samples and flux concentrations inferred from the TDR measurements. Transport properties, including apparent solute velocities, dispersion coefficients, and total mass balances, were determined independently from both sets of breakthrough curves. The dispersion coefficients tended to increase with depth, reaching a constant value with the lysimeter data and appearing to increase continually with the TDR data. The variations with depth of the solute transport parameters, along with observations of water and solute mass balance and spatial distributions of water content, provide evidence of significant three‐dimensional flow during the irrigation experiments. The TDR methods are shown to efficiently provide dense spatial and temporal data sets for both flow and solute transport in unsaturated sediments with minimal sediment and flow field disturbance. Combined implementation of lysimeters and TDR probes can enhance data interpretation particularly when three‐dimensional flow conditions are anticipated.

Modeling the transport of chemicals is challenging due to the presence of preferential flow paths and the input data needed to describe these paths. We propose a simple equation that can predict the breakthrough of solutes without excessive data requirements under steady state flow conditions. In our generalized preferential flow model (GPFM), the soil is conceptually divided into a distribution zone and a conveyance zone. The distribution zone acts as a linear reservoir resulting in an exponential loss of solutes through preferential flow paths from this zone. In the conveyance zone, the transport of solutes is described with the convective‐dispersive (CD) equation. Input data required are the apparent water content of the distribution zone, and solute velocities and dispersivities in the conveyance zone. The model is tested with chloride breakthrough curves (BTCs) resulting from three sets of experiments performed with steady state artificial acid rainfall at different intensities applied sequentially using duplicate columns filled with undisturbed soil or sand both exhibiting preferential flow. The model is able to describe the breakthrough of solutes with physically meaningful parameters through coarse sand with fingered flow and through undisturbed soil cores. In coarse sand, water and solutes flow only through the fingered flow paths in the conveyance zone. In undisturbed columns there is both flow through the matrix and preferential flow paths, and two velocities in the conveyance zone are needed to simulate the breakthrough curves.

Time domain reflectometry (TDR) and the heat pulse method are both used to measure soil water content. Changes in ambient temperature have been shown to affect TDR measurements, but less is known about the behavior of heat pulse sensors in response to changes in temperature. This study directly measured and compared the temperature sensitivity of the TDR and heat pulse methods. Both methods were used to estimate water content in silt loam and sand at two fixed water contents across a wide temperature range. An increase in temperature led to an increase in measured water content in most cases. Across the 40°C temperature range, changes in measured water content were generally 0.04 m 3 m −3 or less for both methods. Weighted linear regression showed that in these soils the heat pulse method exhibited greater temperature sensitivity than the TDR method, although the differences were not statistically significant. A previously proposed correction for the temperature sensitivity of the TDR method produced mixed results. The temperature sensitivity of the heat pulse method was attributed to the changes in the density and specific heat of water and specific heat of soil with respect to temperature. When the changes in these parameters were accounted for, the temperature sensitivity was eliminated in three out of four cases.

Comparison of TDR and X-ray method for determining moisture transport parameters

Sorption is a well-known phenomenon that may cause the retardation effect of zinc in the subsurface environment. In this study, the governing process for zinc sorption during transport was investigated by conducting 2-D plume tests in a laboratory scale sand tank model using the time domain reflectometry (TDR) method. Tracer solutions of NaNO3 and ZnSO4 were applied at a constant flow rate as a pulse type to capture the plumes of both solutes based on TDR-measured resistance. It was revealed that the observed zinc sulfate plume showed no retardation relative to sodium nitrate with a retardation factor of R ≈ 1. Instead of retardation, a prominent reduction of zinc sulfate mass occurred during transport through the tank model due to the irreversible sorption as well as longitudinal dispersion. This indicates that the controlling factor for the sorption process of zinc sulfate in the sand tank model is kinetic rather than equilibrium. These hydrogeological parameters would provide valuable information on the prediction of the fate of zinc in sandy aquifer materials.

<p>Preferential flow pathways (PFPs) are key contributors for the ecological status of the hydrosphere in high mountain environments, as the precipitation will transfer to PFPs with rapid solute transport from soil to groundwater. This particularly refers to nutrient allocation from soils to groundwater and surface waters.</p><p>To understand the effects of the pedogenesis and forest types on the soil PFPs, the soil preferential flow was studied by <em>in situ</em> dye tracing image analysis and elemental chemical analysis at the Hailuogou glacier chronosequence, Gongga Mountain on the eastern Tibetan Plateau. A soil chronosequence and a vegetation primary succession following the retreat of the Hailuogou glacier has been forming since ~1890. Three sites representing different exposure age (45, 85 and 125 years) in the Hailuogou glacier retreat area chronosequence and two sites typical forest lands (deciduous broadleaf forest and coniferous forest) were selected to carry out a brilliant blue dyeing experiment to visualize the distribution of water infiltration in soil.</p><p>The tracer-infiltration patterns were parameterized by dye coverage (DC), preferential flow fraction (PF), length index (L<sub>i</sub>) and the variation coefficient of DC in the PFPs (C<sub>V</sub>). Furthermore, the distribution of PFPs, transported solute of soil PFPs was analyzed including Hailuogou glacier chronosequence and vegetation succession.</p><p>According to the comparison of PFPs parameters, soil PFPs at the 125-year-old site extremely more developed than that at the younger site due to the fracture development between rock and soil on the process of soil development. The soil PFPs under broadleaf forest is more pronounced than that in coniferous forest soils, largely depending on the different root system.</p><p>In general, PFPs in Gongga Mountain were important contributors to the potential translocation of bioavailable inorganic P (PBPi) and organic P translocation to the hydrosphere. The elements transported with PFPs could be divided into three categories, reactive, conservative, and both reacted and conservative elements for the concentration of the elements remain in the PFPs. The results indicated that Mg and Al are the reactive elements, while Na, K, Ca and Mn are the conservative elements in the PFPs. Iron is both reacted and conservative element in the PFPs. Zn, Na, K, Mg, PBPi, had a significant correlation with the variation coefficient of DC in the PFPs (C<sub>V</sub>).</p><p>The results highlight the effects of the pedogenesis and forest types on the distribution of PFPs and solute transfer. Preferential flow contributes largely to elements flow at the Hailuogou glacier chronosequence and vegetation succession, Gongga Mountain.</p><p>The financial support of this work was obtained from National Natural Science Foundation of China (Grant No. 41272220) and Natural Scientifc Foundation for Young Scientists of Guangxi Zhuang Autonomous Region of China (Grant No. 2017GXNSFBA198162). The first author was financially supported by the Sino-German (CSC-DAAD) Postdoc Scholarship Program funded by China Scholarship Council (CSC) and Deutscher Akademischer Austausch Dienst (DAAD).</p>

There is increasing agreement that preferential flow in soils is fast, limited to infiltration, and occupies but small portions of porosity. However, how fast is it, how much water is involved, what is its flow rate, and how far does it carry? [Jarvis et al. (2016)][1] hardly touched an approach

Time and frequency domain reflectometry for the measurement of tree stem water content: A review, evaluation, and future perspectives

아스팔트 콘크리트 포장의 하부구조는 보조기층, 동상방지층 및 노상층으로 구성되어 있으며, 노상층의 경우 겨울철을 지나 봄이되면 동상효과에 따른 포장 파손과 여름철 장마 기간 동안의 수분 침투로 인한 지지력 감소와 같은 문제점이 발생할 수 있다. 따라서 도로하부의 함수비를 계측하기 위하여 시간이력 반사방식(Time Domain Reflectometry, TDR)의 함수센서를 통하여 실시간 함수비를 계측하는 기술이 국내에 도입되었다. TDR 방식의 함수센서는 흙과 물의 유전율을 통하여 함수비를 측정하기 때문에 온도의 영향을 받을 수 있다. 특히 국내는 계절 영향을 받아서 겨울철 대기 온도가 지역적으로 영하 <TEX>$20^{\circ}C$</TEX> 보다 낮은 온도를 보여줄 수 있기 때문에 영하의 측정 온도 범위에 대한 함수센서의 정확도를 파악할 필요가 있다. 따라서 본 연구에서는 흙의 온도 범위를 영하 <TEX>$2^{\circ}C$</TEX> 미만까지 확장하여 TDR 방식의 함수비 센서에 대한 온도 민감도 시험을 수행하였다. 체적함수비 20%와 30%에 대한 시험 결과 공시체 표면 온도가 영하로 떨어지면서 센서부근의 수분이 감소하고, 공시체 상부의 함수비가 증가하는 경향을 보여주었다. 그러나 함수센서에 대한 온도의 영향은 함수비가 낮을 경우 미소한 것으로 나타났다. The infrastructure of flexible pavement is composed of aggregate subbase, anti-frost layer, and subgrade. In particular, the subgrade performance is affected by climates such as frost action and precipitation. The method of TDR(Time Domain Reflectometry) sensors to measure moisture contents in subgrade layer has been used in the research. Due to the TDR method using dielectric permitivity of soil and water, the sensors can be affected by the low subgrade temperatures. The air temperatures frequently drops below <TEX>$-20^{\circ}C$</TEX> in the winter in Korea. As a result, it is necessary to estimate the accuracy of the TDR moisture sensors in the range of below zero temperatures. In this study, the subgrade temperatures of lower than <TEX>$-2^{\circ}C$</TEX> were extended to evaluate temperature sensitivity of the TDR moisture sensors. The test results revealed that the moisture contents around the sensors were reduced while those of the upper part of specimen showed a tendency to increase as the specimen surface temperature drops below zero under the volumetric moisture contents(VMC) of 20% and 30%. However, the impact of temperature on the function of the sensor at lower water contents was found to be negligible if any.

Soils derived from different lithologies and their controls on preferential flow remain underexplored in forested landscapes. In the same lithology, the propensity for preferential flow occurrence at different hillslope positions also remains largely elusive. By utilizing a soil moisture response time method, we compared preferential flow occurrence between a shale site (Shale Hills, silt loam soils) and a sandstone site (Garner Run, sandy loam soils) at four hillslope positions: ridge‐top, North‐ and South‐facing mid‐slopes and toe slope, for over 2 years. The catchments are neighbouring and covered by temperate forest. For the four hillslope positions, Shale Hills had higher preferential flow frequencies compared to Garner Run. Between these two catchments, the South‐facing mid‐slope sites showed the highest contrasts in preferential flow frequency (33.5% of events at Shale Hills vs. 8.8% at Garner Run) while the ridge‐top sites showed the lowest contrasts (18.7 vs. 13.2%). Additionally, over the unfrozen period, for seven out of eight monitoring sites, drier antecedent conditions tended to be more favourable for preferential flows to occur, with significant (p &lt; .01) relationships at two sites. Except for the South‐facing mid‐slope sites, both Shale Hills and Garner Run had two preferential flow pathways. The characteristic preferential flow pathways at Shale Hills were the Bwand C horizons, and for Garner Run, preferential flow moved from the E/AE horizon to the Bwhorizon. This study shows that shale‐derived soils tended to have higher preferential flow occurrence than sandstone soils, but hillslope positions exhibit different levels of contrasts. More effort should be paid to study the impact of lithology on preferential flows in the context of land surface modelling and biogeochemical reactions to improve ecosystem services of headwater catchments.

Time domain reflectometry (TDR) is a technique that allows simultaneous estimates of apparent permittivity (ε a ) and hence volumetric water content (θ) and bulk electrical conductivity (σ a ). Difficulties arise for θ and σ a determination, however, when uncoated TDR probes (UP) are used in highly conductive media. This work shows that σ a can be estimated in highly conductive media using a TDR probe coated with a high‐dielectric insulator (CP). To this end, the Dalton method for σ a estimations was applied to a 10‐cm‐long three‐rod TDR probe insulated with a 0.2‐mm‐thick epoxy‐ceramic composite coating with a relative permittivity, ε r , of 32.3. This method was calibrated on different NaCl–water solutions (0–15 dS m −1 ) and compared with the standard long‐time TDR method for σ a estimations using an UP. The method was subsequently used for determining σ a in four different soils with different values of θ and σ a (0–6 dS m −1 ) and again compared with the standard TDR procedure. The low error (RMSE = 1.5) for the comparison between the ε a measured with the CP and that calculated with the analytical solution for coaxial probes indicates that the CP is accurate enough for ε a estimations. For σ a values &lt;4 dS m −1 , the UP allows determinations of ε a and the most accurate estimations of σ a using the standard TDR method ( R 2 = 0.99). For higher σ a values, however, estimations of ε a were only possible using the CP, where σ a was satisfactorily determined ( R 2 = 0.99) using the Dalton method. To this end, a previous calibration between the real and the apparent bulk electrical conductivity estimated with the CP was required.

The concepts of preferential flow are reviewed. Five types of preferential flow are recognized and defined. These include macropore flow, gravity-driven unstable flow, heterogeneity-driven flow, oscillatory flow, and depression-focused recharge. Spatial scales at which each of these preferential flow processes occurs are suggested. An overview is presented of the relationship between preferential flow and water quality, both for surface water sources and ground water sources. Much of the direct knowledge about the impact of preferential flow on water quality has been derived within the last two decades. The evidence for the impact of preferential flow on surface water quality is much more plentiful, probably because it is easier to measure chemical breakthroughs to surface water sources. Some of the newer methods for experimentally quantifying preferential flow are outlined. These methods include the tension infiltrometer, time domain reflectometry, chemical tracing, and geophysical methods such as ground penetrating radar and electrical resistance tomography. Theoretical analyses of preferential flows are based on conservation of mass and momentum. It is explained that the use of Darcy's law (and therefore Richards' equation) may not be valid for some preferential flows.

The International Atomic Energy Agency (IAEA) continues to expand its use of unattended measurement systems. An increasing number of systems and an expanding family of instruments create challenges in terms of deployment efficiency and the implementation of data authentication measures. In collaboration with the IAEA, tamper-indicating measures to address data-transmission authentication challenges with unattended safeguards systems are under investigation. Pacific Northwest National Laboratory (PNNL) is studying the viability of active time-domain reflectometry (TDR) along two parallel but interconnected paths: (1) swept-frequency TDR as the highly flexible, laboratory gold standard to which field-deployable options can be compared, and (2) a low-cost commercially available spread-spectrum TDR technology as one option for field implementation. This report describes PNNL’s FY15 progress in the viability study including: an overview of the TDR methods under investigation; description of the testing configurations and mock tampering scenarios; results from a preliminary sensitivity comparison of the two TDR methods; demonstration of a quantitative metric for estimating field performance that acknowledges the need for high detection probability while minimizing false alarms. FY15 progress reported here sets the stage for a rigorous comparison of the candidate TDR methods, over a range of deployment scenarios and perturbing effects typical of IAEA unattended monitoring systems.

The freezing temperature of water in soil is not constant but varies over a range determined by soil texture. Consequently, the amounts of unfrozen water and ice change with temperature in frozen soil, which in turn affects hydraulic, thermal, and mechanical properties of frozen soil. In this paper, an Am‐241 gamma ray source and time‐domain reflectometry (TDR) were combined to measure unfrozen water content and ice content in frozen soil simultaneously. The gamma ray attenuation was used to determine total water content. The TDR was used to determine the dielectric constant of the frozen soil. Based on a four‐phase mixing model, the amount of unfrozen water content in the frozen soil could be determined. The ice content was inferred by the difference between total water content and unfrozen water content. The gamma ray attenuation and the TDR were both calibrated by a gravimetric method. Water contents measured by gamma ray attenuation and TDR in an unfrozen silt column under infiltration were compared and showed that the two methods have the same accuracy and response to changes of water content. Unidirectional column freezing experiments were performed to apply the combined method of gamma ray attenuation and TDR for measuring unfrozen water content and ice content. The measurement error of the gamma ray attenuation and TDR was around 0.02 and 0.01 m3/m3, respectively. The overestimation of unfrozen water in frozen soil by TDR alone was quantified and found to depend on the amount of ice content. The higher the ice content, the larger the overestimation. The study confirmed that the combined method could accurately determine unfrozen water content and ice content in frozen soil. The results of soil column freezing experiments indicate that total water content distribution is affected by available pore space and the freezing front advance rate. It was found that there is similarity between the soil water characteristic and the soil freezing characteristic of variably saturated soil. Unfrozen water content is independent of total water content and affected only by temperature when the freezing point is reached.

Most solute transport measurement techniques are tedious and require extensive soil excavation. A field experiment was conducted to evaluate whether surface transport properties determined by a nondestructive time domain reflectometry (TDR) technique could be used to accurately predict tile flux concentrations. A 14 by 14 m field plot selected above a 1.1‐m deep tile drain was studied. Low electrical conductivity (EC) water was sprinkled on the plot surface, and after reaching a steady‐state condition, a pulse of calcium chloride solution (16.3 cm) with an EC of 23 dS m −1 was applied through the same sprinklers. Time domain reflectometry equipment was used to record the change in EC of surface (∼ top 2 cm) soil at 45 locations. The EC of the tile drainage flow was measured continuously with an EC probe. The surface convective lognormal transfer (CLT) function parameters, log mean irrigation depth, μ I , and its standard deviation, σ I , were found to be 3.44 and 0.94 [ln(cm)], respectively, for a reference depth of 110 cm. These surface parameters were used in a one‐dimensional (1‐D) CLT model and in a two‐dimensional (2‐D) model (CLT vertical function combined with exponential horizontal transfer function) to predict the tile flux concentrations. The 1‐D CLT model predicted an earlier arrival time of chemicals to the tile drain than observed values. The root mean square error, RMSE, of the 1‐D CLT predictions was 0.123, and the coefficient of efficiency, E, was −0.47. The 2‐D model predictions of tile flux concentrations were similar to the observed values. The root mean squared errors (RMSE) and E were 0.023 and 0.94, respectively. The findings suggest that in this field soil, the surface solute transport properties determined by TDR could be combined with a 2‐D transport model to make reasonable predictions of tile flux concentrations.