Volumetric Water Content Values Research Articles

Evaluation of soil water content and bulk electrical conductivity across the U.S. Climate Reference Network using two electromagnetic sensors

AbstractSoil bulk electrical conductivity (BEC) was evaluated alongside soil volumetric water content (VWC) and soil temperature measurements using the HydraProbe (model HydraProbe, Stevens Water Monitoring Systems, Inc.) (hereafter called HP) with accuracy range of BEC ≤ 0.3 S m−1, and the time domain reflectometry (TDR)‐315L Probe (model TDR‐315L, Acclima, Inc.) (hereafter called AP) suitable for BEC up to 0.6 S m−1, at 23 stations of the U.S. Climate Reference Network. Previous evaluations revealed inconsistent performance of both sensors in some clay soils using manufacturer‐recommended calibrations in converting dielectric permittivity measurements to VWC. Here, we found that hourly values of BEC reached 0.6 S m−1 in high clay content soils and exceeded 2 S m−1 in high saline soils, and these high values of BEC were associated with poor performance and failures of both HP and AP sensors. Large values of BEC occurred in predominantly saturated soils where VWC values reached about 0.5 m3 m−3 for saline soils and about 0.7 m3 m−3 for clay soils, while low magnitudes of BEC were associated with low soil water content and seldomly saturated soils. Low hourly BEC values of less than 0.1 S m−1 were observed in wide variety of soil types, where sensor performance was typically excellent. The most influential factor on BEC was high soil water content conditions. Although dielectric permittivity measurements in estimating the soil water content were sensitive to BEC as some high clay content and high salinity soils increased BEC, the impact of large BEC on dielectric permittivity measurements was smaller in the well‐drained top soil layers than in deep soil layers that remained near saturation. Soil temperature had only a small impact on BEC. With high clay content and high salinity, the specific area of clay minerals was also associated with the magnitude of BEC.

The Effect of Ornamental Groundcover Habit and Irrigation Delivery on Dynamic Soil Conditions

Sustainable landscapes provide environmental, social, and financial benefits, with interest and adoption increasing due to environmental awareness. Ornamental ground-cover systems have garnered interest in the landscape due to the reduced need for water, fertilizers, pesticides, and maintenance compared to typical landscapes; however, limited research on groundcover ability to modulate soil conditions or suppress weeds exists. This study explored how ornamental groundcover systems impact the sustainability of landscapes. The effects of ground-cover growth habit (matting; bunching) and irrigation delivery method (micro spray; overhead) on soil temperature, volumetric water content (VWC), and electric conductivity (EC), along with impacts on weed growth, soil microbial communities, and plant coverage, were measured. Soil temperatures were generally lower under groundcover species with a matting growth habit, and to a lesser extent, bunching growth habits, in comparison to the warmer fallow systems. Groundcovers with a matting form led to lower VWC values compared to taxa with other growth habits, particularly when micro-irrigated. Plant form did not significantly influence EC values; however, micro spray irrigated plots had significantly higher EC values, likely attributed to irrigation spray patterns. Micro spray irrigation in tandem with matting growth habit taxa decreased weed density more effectively than taxa with bunching growth habits or groundcovers maintained under overhead irrigation. Selection of groundcover species with greater foliar coverage along with implementing more efficient irrigation practices can decrease soil temperatures, soil moisture, and weed density. Incorporating groundcovers in the landscape can decrease maintenance requirements and water/chemical use, thus increasing sustainability and decreasing environmental consequences.

Temporal response of urban soil water content in relation to the rainfall and throughfall dynamics in the open and below the trees

Abstract Rainfall interception process is an important part of the biohydrological cycle, in which vegetation plays an important role by regulating the amount and dynamics of rainfall reaching the ground. In this paper, an event-based analysis is performed to discuss the influence of vegetation on dynamic of temporal response of soil volumetric water content (VWC) in the upper soil layer during rainfall events. More specifically, six events that occurred between 19 November 2021 and 30 June 2022, characterized by different hydro-meteorological and vegetation conditions, are analyzed based on continuous measurements of VWC in the open and below groups of two deciduous (Betula pendula Roth.) and two coniferous trees (Pinus nigra Arnold), as well as rainfall in the open and throughfall on an urban experimental plot in Ljubljana, Slovenia. VWC values at the upper depth (16 cm) were the highest under the birch tree, followed by the location in the open and under the pine tree. However, in the lowest depth (74 cm) VWC values were the lowest under the birch tree. VWC responses to rainfall and throughfall showed seasonal patterns related to the pre-event wetness conditions, with a faster occurrence of maximum VWC values in the leafless period. Additionally, rainfall amount and its dynamics during the event significantly affect the response, as VWC in general reaches its peak after the occurrence of more intense rainfall. Such an event-based analysis, offering an insight into the dynamics of the event development, is crucial and very beneficial for understanding of the biohydrological processes.

Irrigation Zone Delineation and Management with a Field-Scale Variable Rate Irrigation System in Winter Wheat

Understanding spatial and temporal dynamics of soil water within fields is critical for effective variable rate irrigation (VRI) management. The objectives of this study were to develop VRI zones, manage irrigation rates within VRI zones, and examine temporal differences in soil volumetric water content (VWC) from irrigation events via soil sensors across zones. Five irrigation zones were delineated after two years (2016 and 2017) of yield and evapotranspiration (ET) data collection. Soil sensors were placed within each zone to give real time data of VWC values and assist in irrigation decisions within a 23 ha field of winter wheat (Triticum aestivum ‘UI Magic’) near Grace, Idaho, USA (2019). Cumulative irrigation rates among zones ranged from 236 to 298 mm. Although a statistical comparison could not be made, the irrigation rates were 0.6 to 21% less than an estimated uniform grower standard practice (GSP) irrigation approach. Based on soil sensor data, crop water stress was avoided with VRI management in all but Zone 3. Thus, this simple approach to VRI zone delineation and VWC monitoring has the potential to reduce irrigation, such as this study, on average by 12% and should be evaluated in other site-years to assess its viability.

Interseeded cover crops did not reduce silage corn performance in the sandy loam soils of South Carolina

AbstractA major concern with interseeding summer cover crops alongside cash crops is the possibility of cover crops competing with the main crop for water and nutrients and thus, reducing the main crop yields. We conducted on‐farm trials in the sandy loam soils of the upstate of South Carolina in 2020 and 2021 to evaluate the effect of white clover (Trifolium repens L.), buckwheat (Fagopyrum esculentum Moench), and pigeon pea (Cajanus cajan (L.) Millsp.) and their mixture interseeded at V4, V7, and V10 corn growth stages on silage corn performance and soil moisture content. We found that none of the cover crops affected silage corn height and aboveground biomass production regardless of the interseeding time. Further, the volumetric water content in the upper 20‐cm soil profile was not decreased by cover crops irrespective of their interseeding time. At physiological maturity, corn height, aboveground biomass, and volumetric water content ranged between 131 and 163 cm, 9 and 23 Mg ha−1, and 10% and 22%, respectively, in season‐1 and between 181 and 216 cm, 0.48 and 1.03 Mg ha−1, and 10% and 17%, respectively, in season‐2 when interseeded with cover crops. When cover crops were not interseeded, the average corn height, aboveground biomass, and volumetric water content values were 152 cm, 13 Mg ha−1, and 13%, respectively, in season‐1 and 204 cm, 0.7 Mg ha−1, and 14%, respectively, in season‐2. These results would encourage farmers who want to consider cover crop interseeding with corn in the regional production systems.

Field Evaluation of Conventional and Downhole TDR Soil Water Sensors for Irrigation Scheduling in a Clay Loam Soil

Highlights Soil profile water content derived from Acclima TDR-315™ sensors approximated those from NMM measurements. Soil profile water content from Campbell Scientific SoilVUE™10 sensors grossly underestimated those from the NMM. VWC values from SoilVUE10 sensors were consistently less than those reported by the TDR-315 sensors at all depths. These findings do not support SoilVUE10 use for irrigation scheduling in clay loam soils. Abstract. A field study was performed to evaluate the efficacy of two commercially available time domain reflectometry (TDR) soil water sensors for irrigation scheduling in a clay loam soil near Bushland, Texas. SoilVUE10 (Campbell Scientific Inc., Logan, Utah) and TDR-315 (Acclima Inc., Meridian, Idaho) sensors were installed within 30 cm of neutron moisture meter (NMM) access tubes in a research field planted to corn (Zea mays L) in 2020 and irrigated by a center pivot sprinkler system. Irrigation treatments included 50%, 75%, and 100% of evapotranspiration (ET) replacement with two access tubes installed in each plot, totaling six sensor evaluation sites. Semiweekly measurements with a field-calibrated NMM were used to monitor soil water status and schedule irrigation throughout the growing season. Soil profile water content values integrated over the surface to 1.1-m depth range were derived from SoilVUE10 and vertically distributed arrays of Acclima TDR-315 sensors installed at equivalent depths and were compared with those from NMM data. Average profile soil water contents from the TDR-315 sensors trended well with those from the NMM having mean bias difference (MBD) values of -9.8, -3.1, and 8.4 mm for the 50%, 75%, and 100% treatments, respectively. In contrast, soil profile water content values from the SoilVUE10 sensors grossly underestimated those from the NMM for all irrigation treatments with MBD values of -54.4, -70.5, and -89.8 mm for the 50%, 75%, and 100% treatments, respectively. Comparisons of volumetric water content (VWC) at each of the nine depths common to both electromagnetic sensor types revealed that values from the SoilVUE10 sensors were consistently less than TDR-315 values for all irrigation treatments. Underestimation at the near surface (5 and 10 cm depths) was attributed to loss of soil to electrode contact possibly associated with clay shrinkage during periodic drying following irrigation. Although soil to electrode contact can be problematic at greater depths, the explanation for chronic underestimation of VWC was less obvious except to note that underestimation occurred immediately after installation, which indicated poor electrode-soil contact after installation despite use of manufacturer guidelines and tools. Other possible reasons include challenges for accurate estimation of soil permittivity for a measured permittivity that includes the plastic sensor body. Results from this study suggest vertically distributed arrays of TDR-315 sensors can provide profile water content values adequate for monitoring soil water status for irrigation scheduling in a clay loam soil. The chronic underestimation observed for the SoilVUE10 sensors does not support their use for water resources research and irrigation management and could lead to over irrigation. Additionally, the relatively short 1 m length is less than the rooting depth of many regional crops and thus not capable of determining percolation below the root zone. Keywords: Acclima TDR-315, Campbell Scientific SoilVUE, Irrigation scheduling, Neutron moisture meter, Semi-arid, Soil water sensors, Time domain reflectometry, Volumetric water content.

Calibration of Soil Moisture Sensors (ECH2O-5TE) in Hot and Saline Soils with New Empirical Equation

The use of soil moisture sensors is a practice applied to improve irrigation water management. ECH2O-5TE sensors are increasingly being used to estimate the volumetric water content (VWC). In view of the importance of the efficient use of these devices, six main factors affecting the accuracy of sensor measurements were studied: soil moisture levels, soil salinity, temperature, organic matter, soil texture, and bulk density. The study showed that the electrical conductivity of the soil and the temperature independently affect the measurements, while the influence of other factors interferes with that of salinity. This study found that the sensor measurements of the VWC were closest to the actual VWC at the soil ECe and temperatures of 2.42 dS m−1 and 25 °C, with root-mean-square errors (RMSE) of 0.003 and 0.004 m3 m−3. Otherwise, the measured VWC values of these sensor readouts significantly overestimated the actual VWC, with an increasing soil ECe and/or producing temperatures higher than the stated values, and vice versa. Given the importance of these sensors for obtaining accurate measurements for water management, a simplified empirical equation was derived using the data collected from a wide range of measurements to correct the influences of electrical conductivity and temperature on the measurement accuracy of the sensors, while considering the influence of the soil’s texture. Thus, the following equation was proposed: ϴva = θvsaECe2+bECe+c+dT2+eT+f−1. The results concerning the measurement of different VWC levels via these sensors and the proposed L&O correction equation were compared with the corresponding actual VWC values determined by gravimetric methods. It was found that this empirical equation reduced the differences in the RMSE between the sensor readings for the VWC and the actual VWC from 0.072 and 0.252 to 0.030 and 0.030 m3 m−3 for 1 and 5 dS m−1, respectively, with respect to the EC’s influence at 25 °C and reduced the RMSE from 0.053 and 0.098 to 0.007 and 0.011 at 3 and 50 °C, respectively, regarding the effect of the temperature at EC 2.42 dS m−1 at different levels of the actual VWC values.

Forecasting the moisture dynamics of a landfill capping system comprising different geosynthetics: A NARX neural network approach

Engineered landfill capping systems consist of geosynthetics and soil layers, which often experience inconsistent and extreme weather events throughout their service life. Complex moisture dynamics in the capping layers can be created by these weather events in combination with other field conditions and can be detrimental to the system's integrity. The limited data on the hydraulic performance of landfill capping systems is a major challenge that hinders the development, validation, and calibration of models that can be used for realistic forecasting of these dynamics. Using the field-level data collected at the Bletchley landfill site, UK, this study develops a data-driven forecasting approach employing a non-linear autoregressive neural network with exogenous inputs (NARX). The data includes precipitation and volumetric water content (VWC) of the capping soil overlaying different geosynthetic layers recorded from Nov 2011 to July 2012. The NARX network was trained using the VWC data as inputs and precipitation data as the exogenous input. Also, the accuracy of NARX predictions was compared against that of a state-space statistical model. NARX-predicted VWC values for a period of 21-days ahead are distributed with a mean error of 0.05 and a standard deviation of 0.2. In the majority of prediction windows, NARX approach outperforms the state-space model. For all NARX prediction periods, RMSEr has been less than 10% for the cuspated core geocomposite. Comparatively, RMSEr values increased to approximately 15% and 19% for the non-woven needle-punched geotextile and the non-woven needle-punched geotextile with band drains, respectively.

Four-Year Monitoring Study of Shallow Landslide Hazards Based on Hydrological Measurements in a Weathered Granite Soil Slope in South Korea

To build a comprehensive understanding of long-term hydro-mechanical processes that lead to shallow landslide hazards, this study explicitly monitored the volumetric water content (VWC) and rainfall amount for a weathered granite soil slope over a four year period. From the 12 operational landslide monitoring stations installed across South Korea, the Songnisan station was selected as the study site. VWC sensors were placed in the subsurface with a grid-like arrangement at depths of 0.5 and 1.0 m. Shallow landslide hazards were evaluated by applying an infinite slope stability model that adopted a previously proposed unified effective stress concept. By analyzing the variations in the monitored VWC values, the derived matric suctions and suction stresses, and the calculated factor of safety values, we were able to obtain numerous valuable insights. In particular, the seasonal effects of drainage and evapotranspiration on the slope moisture conditions and slope stability were addressed. Preliminary test results indicated that continuous rainfall successfully represented the derived matric suction conditions at a depth of 1.0 m in the lower slope, although this was not the case for the upper and middle slopes. The significance of a future study on cumulative field monitoring data from various sites in different geological conditions is highlighted.

Evaluation of installation procedures of volumetric water content and matric potential probes: fundamentals for obtaining field and laboratory accordance

The current paper aims to analyze the influence of installation procedures when it comes to the accordance of the Soil Water Retention Curve and field monitoring data. The method comprises testing three different installation procedures: with driving the rod into the soil; with the application of mud inside the auger hole; and with a hardened steel gauge. Further, is evaluated the influence of the variation of Bulk density on volumetric water content values by using the Proctor and a double ring hydraulic equipment. To analyze the soil-rod coupling, a microtomography imaging routine was performed. The results point out that the probe’s data are connected to the Bulk density of the material, producing higher volumetric water content values with the increase of Bulk density. Comparing results of different installation methods with laboratory results, it is possible to conclude that driving the rod directly into the soil is the best way to install the equipment since the probe underestimates the volumetric water content data by 2,5%, while the mud application by 4%, and the gauge method by 5%.

Variability in spatial–temporal recharge under the observed and projected climate: A site-specific simulation in the black soil region of Russia

The main purpose of this work is to evaluate diffuse groundwater recharge and its temporal dynamics at sites with different landscapes and soil profiles under observed and projected climate variability. Three typical sites with different landscapes and topsoil profiles were chosen for the field study. Field work consisted of pit development and soil sample collection for laboratory study using the centrifuge method. Soil hydraulic parameters were obtained by the RETC code using the observed values of volumetric water content vs. pressure head. HYDRUS-1D code was used to estimate groundwater recharge based on a 70-year (1945–2015) meteorological dataset with daily resolution (including values of precipitation, air temperature, wind speed, and humidity). This dataset was preprocessed using our SURFBAL code to calculate the surface and topsoil water and energy balance. Retrospective historical simulation results showed that there was a considerable variation in the mean annual groundwater recharge values (49–104 mm/year) for the three studied profiles. The temporal change in groundwater recharge occurred relatively synchronously at all sites and was governed by the change in the current annual aridity index. The results of this historical simulation did not reveal any traces of climate change in the groundwater recharge of the studied region in the last 30–40 years. To predict the recharge variations in the second half of the 21st century, the LARSWG forecast weather generator with the climate projections of the 5th General Circulation Models (GCMs) from the Coupled Model Intercomparison Project 5 (CMIP5) family was used. On average in the case of climate development under the maximum greenhouse gas emissions RCP8.5 scenario, there will be a reduction in groundwater recharge in the studied region in 2060–2080, caused by two factors: change of aridity index, which determines evapotranspiration during a warm season and the accumulation of snow and subsequent meltwater infiltration during the snowmelt periods.

The laboratory calibration of a soil moisture capacitance probe in sandy soils

Determining and mitigating landslide risk is a technical-scientific objective, particularly for the protection and proper territorial management and planning. The slope stability depends on the pore pressure distribution, which is influenced by the saturation front propagation through the unsaturated zone, whose monitoring is useful to understand any possible instabilities. Such monitoring may be undertaken by sensors based on the measurement of the relative dielectric permittivity. Reliable relationships between the measurement and the soil moisture are necessary. The main objective of this study is to assess a laboratory calibration protocol for a specific capacitance sensor (Drill & Drop, Sentek Sensor Technologies). Two monogranular sands have been selected for the calibration purpose. The laboratory tests were performed under three relative density values (D<sub>R</sub> equal to 40%, 60% and 80%) for seven volumetric water content values (θ<sub>v</sub> ranging from 0.00% to 36.26%). Based on the experimental measurements, the soil-specific calibration curves were determined at an assigned relative density value; in particular, a simple power law is adopted to describe the probe’s reading as a function of the volumetric water content. The results point out that the relative density values slightly affect the tests, thus, the soil-specific calibration curves are derived based on a simple regression analysis fitting the whole set of the laboratory tests validated for each sand. The calculated coefficient of determination (R<sup>2</sup> = 0.96÷0.99) and root mean square error (RMSE = 1.4%÷2.8%) values confirm the goodness of fit. In order to propose more general fitting curves, suitable for both the investigated sands, multiple linear regressions are performed by considering θ<sub>v</sub> and the mean grain size, D<sub>50</sub> as independent variables; again, the R<sup>2</sup> and RMSE values equal to 0.97 and 2.41%, respectively, confirm the suitability of the calibration curve. Finally, the laboratory calibration curves are compared with the manufacturer-supplied curves, thus, enhancing the need for the soil-specific calibration.

Effect of root density of wheat and okra on hydraulic properties of an unsaturated compacted loam

Root‐induced changes of soil hydraulic properties, that is soil water retention curve (SWRC) and unsaturated hydraulic conductivity (K(ψ)), depend on plant species and root density. This study aimed to investigate the effects of wheat roots and okra roots on both SWRC andK(ψ) of soil within an extended range of matric potential (−300 ~ 0 kPa) and to investigate the effects of root density of wheat (i.e. grass species) and okra (i.e. shrub species) on SWRC andK(ψ) of a loamy soil. The SWRC andK(ψ) of soil planted with wheat and okra were measured by the simplified evaporation method. Soil matric potential was measured directly by high‐capacity tensiometers. Different root densities were obtained by establishing different seeding densities in the soil. Meanwhile, root characteristics in planted soil were obtained using image analysis. The results showed that wheat‐ and okra‐planted soil had significantly larger volumetric water content (VWC) within the range of matric potential from −90 to 0 kPa than that of unplanted soil. Within the range of matric potential from around −300 to −90 kPa, the effects of wheat and okra roots on SWRC were not significant. The effects of wheat and okra roots onK(ψ) were not significant within the range of matric potential from around −300 to −10 kPa. Wheat‐planted soil with a root length density (RLD) of 10.04 cm cm−3had larger values of VWC (ψ = 0, −33, −50 kPa) than those of wheat‐planted soil with RLD of 2.69 cm cm−3. The difference in hydraulic properties between okra‐planted soil and the two RLDs, 0.89 and 2.96 cm cm−3, was not significant. Okra roots were more effective in increasing saturated water content (θs) and field capacity (θfc) than wheat roots. This might be because okra roots are relatively coarser than wheat roots.HighlightsEffect of root density of wheat and okra on SWRC andK(ψ) was investigated.TheK(ψ) of soil planted with wheat and okra was measured at matric potentials less than −100 kPa.Okra roots were more effective in increasingθsandθfcthan wheat roots.Effect of wheat and okra roots onK(ψ) was not significant at matric potentials from −300 to −10 kPa.

Analyzing High‐Frequency Soil Respiration Using a Probabilistic Model in a Semiarid, Mediterranean Climate

AbstractHigh‐frequency (subhourly) changes in soil respiration (Rs), a critical component of the carbon cycle, are not well documented in semiarid ecosystems. We investigate the response of Rs to subhourly soil temperature (Tsoil) and soil volumetric water content (VWC) variation in a semiarid ecosystem across a year of highly variable weather. We propose a probabilistic model to estimate the likelihood of Rs exceeding its annual mean (1.2 μmol CO2/m2s), conditioned on different Tsoil and VWC values. The results show that Rs follows a Gaussian‐like pattern as Tsoil increases, where it follows an upward trend until ~18 °C, and then begins to decline. Rs remains constant at Tsoil beyond ~27 °C. Using our novel conditional probability model, we show both the increasing and decreasing response of Rs to rising Tsoil through two ranges (14–17 and 20–23 °C) where Rs observations show opposing responses. We demonstrate that an increase in Tsoil from 14 to 17 °C causes a rise in Rs by a factor of 1.4 relative to the mean. The proposed model also describes how Rs reduces as Tsoil continues to increase above ~18 °C. Considering VWC and Tsoil into the proposed model, we show that when Tsoil is low (e.g., 17 °C), a rise in VWC yields a decrease of Rs by a factor of 3.6. However, when Tsoil is high (e.g., 23 °C), Rs increases with increasing VWC by a factor of 2.5. Overall, the probabilistic model enables us to detect and characterize changes in Rs distribution in response to different environmental variables and thresholds.

Effects of Light, Soil Moisture, and Nutrition on Greenhouse Propagation of Twinflower

Twinflower (Linnaea borealis) is an understory subshrub native to northern regions of North America, Europe, and Asia. Some growers report that this native plant is difficult to propagate. Although twinflower prefers partial shade and grows in areas with naturally variable moisture, there has been no greenhouse propagation work testing the impact of light or soil moisture conditions on root development of this plant or whether fertilizer impacts root development or root:shoot ratios during propagation. The goal of the first experiment was to propagate twinflower under a variety of daily light integrals (DLI)—27.6, 14.4, or 5.8 mol·m−2·d−1—and soil volumetric water content values (θ = volume of water ÷ volume of soil) 0.30, 0.35, 0.40, and 0.45 L·L−1, both parameters aimed at reproducing a range of natural conditions. The largest roots were grown at DLIs of 5.8 and 14.4 mol·m−2·d−1 and θ values of 0.30 and 0.35 L·L−1. In the second experiment, twinflower plants were grown in substrates with 0, 2.1, or 5.0 g·L−1 of incorporated controlled-release fertilizer (14N–6.1P–11.6K). Root and shoot dry weight increased at both treatment rates. The relative percentages of nitrogen, phosphorus, and potassium, and the total concentrations of manganese in parts per million, increased in foliage, as well. In both experiments, the source of cuttings impacted results. In the first experiment, cuttings taken from the source that was in the most light were least likely to survive (26% survival rate) compared with cuttings taken from stock plants growing in partial shade (65% or 82% survival rates, by site). In the second experiment, cuttings taken from source plants that were most intensively managed for removal of weeds and competing plants had the highest survival rate and the greatest shoot and root dry weight. We recommend propagating twinflower with moderate rates of fertility (i.e., 2.1 g·L−1 of incorporated controlled-release fertilizer) under some shade (5.8–14.4 DLI) and a moderate θ (0.30–0.35 L·L−1).

Fiber Optic Thermo-Hygrometers for Soil Moisture Monitoring.

This work deals with the fabrication, prototyping, and experimental validation of a fiber optic thermo-hygrometer-based soil moisture sensor, useful for rainfall-induced landslide prevention applications. In particular, we recently proposed a new generation of fiber Bragg grating (FBGs)-based soil moisture sensors for irrigation purposes. This device was realized by integrating, inside a customized aluminum protection package, a FBG thermo-hygrometer with a polymer micro-porous membrane. Here, we first verify the limitations, in terms of the volumetric water content (VWC) measuring range, of this first version of the soil moisture sensor for its exploitation in landslide prevention applications. Successively, we present the development, prototyping, and experimental validation of a novel, optimized version of a soil VWC sensor, still based on a FBG thermo-hygrometer, but able to reliably monitor, continuously and in real-time, VWC values up to 37% when buried in the soil.

Volumetric Water Content Measurements as Obtained from Remote Sensing and In Situ Instrumentation

A single-polarization, vertical wave propagation transmitted-vertical wave propagation received (VV), terrestrial imaging radar and time domain reflectometry (TDR) equipment were used to monitor the fluctuations of water content within two thin (15.24 centimeters) compacted clay test sections (each 30.48-m long, by 15.34-m wide). Radar imagery was obtained in 2013 on March 22, June 26, July 7 and July 10 using a Gamma Remote Sensing Portable Radar Interferometer version II (GAMMA Remote Sensing and Consulting AG, Bern, Switzerland). In situ observations were acquired hourly using TDR probes. The background, methodology, and comparison of the results of the remote sensing and in situ measurements are presented. Specifically, two remote sensing data reduction algorithms were considered. The water content results obtained from these algorithms were compared with the water content values derived from correlations with the dielectric permittivity values measured by the TDR probes. Key findings include: 1) there were differences between the values obtained from the data reduction methods, 2) there were differences between the values obtained from the remote sensing methods and the values obtained from the in situ method, and 3) the identification of additional avenues of research. The remotely sensed measurements were, on average, higher than the in situ measurements (the average volumetric water content values were 0.2 and 0.5 depending on the method, as obtained via the remotely sensed methods; the average volumetric water content value in the compacted native soil test sections was 0.22, as obtained via the in situ method).

Study of the volumetric water content based on density, suction and initial water content

Abstract The practical application of determination of the soil water retention curves (SWRC) is in seepage modelling in unsaturated soil. The models based on the physics behind the seepage mechanism has been developed for predicting the SWRC. However, those models rarely consider the combined effects of initial volumetric water content and soil density. One of the best routes to study these effects is to formulate the SWRC models/functional relations with volumetric water content as an output and the soil density, initial volumetric water content and soil suction as input parameters. In light of this, the present work introduces the advanced soft computing methods such as genetic programming (GP), artificial neural network and support vector regression (SVR) to formulate the volumetric water content models based on the suction, density and initial volumetric water content. The performance of the three models is compared based on the standard measures and goodness-of-fit tests. The findings from the statistical validation reveals that the GP model performs the best in generalizing the volumetric water content values based on the suction, density and initial water content. Further, the 2-D and 3-D plots, evaluating the main and the interaction effects of the three inputs on the volumetric water content are generated based on the parametric procedure of the best model. The study reveals that the volumetric water content values behave non-linearly with respect to soil suction because it first decreases till a certain point of soil suction and then increases suddenly.

Nonstationarity of the electrical resistivity and soil moisture relationship in a heterogeneous soil system: a case study

Abstract. Understanding the role of vegetation in the interface between the atmosphere and groundwater is the most decisive key in analyzing the processes involved in water transfer. The main effect of vegetation is its root water uptake, which significantly modifies the processes involved in water transfer in the vadose zone. This paper focuses on mapping temporal and spatial changes in soil moisture using electrical resistivity tomography (ERT). The main objective is to assess how electrical resistivity (ER) is useful for mapping water distribution along a heterogeneous toposequence crossed by a hedgerow. Ten ERT were performed over the studied period for a 28 m long toposequence and compared to matric potential and groundwater level measurements. Soil volumetric water content (VWC) was predicted with two methods: (i) from ER using the Waxman and Smits model (ii) and from matric potential using an experimental retention curve fitted by a Van Genuchten model. Probability density functions (PDFs) of our set of data show that the largest change in mean ER and matric potential was observed in the topsoil layer. We then analyzed the consistency between ER and point measurements in this layer by extracting the arrays at the junction of ER grids and point measurements. PDFs of ER maps at each monitoring time (from T01 to T10) were also calculated to select the most contrasting distributions, corresponding to the wettest (T06) and driest states (T10). Results of ER were consistent with matric-potential measurements, with two different behaviors for locations inside and outside the root zone. A consistent correlation between VWC values from the Waxman and Smits model and those obtained from the retention curve was observed outside the root zone. The heterogeneous soil system inside the root zone shows a different pattern in this relationship. A shift in the relationship between ER and soil moisture for the locations outside and inside the root zone highlights the nonstationarity between wet and dry periods inside the root zone. The equivocal behavior of this relationship shows the limitation of using ER to predict soil moisture in a heterogeneous soil system. Such systems were actually related to the high hedgerow root density and also to a particular topographical context (ditch and bank) that is encountered in Brittany and throughout northwestern Europe.

Calibration of Water Content Reflectometer Sensors with a Large Soil Sample

In situ measurement of soil water content is of fundamental importance in vadose zone processes. While time domain reflectometry (TDR) is a universally accepted technique, having been developed since the 1980s, water content reflectometer (WCR) is still a relatively new technique and does not hold a similar background. The main goal of this study was to establish an accurate calibration curve for WCR sensors to be installed in an artificial hillslope designed to study the triggering of shallow landslides. Therefore, high accuracy calibration at the high end of the water content range and for large soil volumes was necessary. An experimental device was specifically designed to provide a calibration procedure for a soil control volume commensurate with the application scale of the probes in the hillslope. A large box container (60 by 50 by 60 cm) was used, inside which three WCR probes and three tensiometers were arranged. A drip emitter produced assigned flow rates, while a load cell at the base measured the time evolution of the infiltrated water mass. Water content was evaluated at approximately steady-state flow conditions for infiltration and drainage experiments, performed with varying degrees of compaction to detect the effect of the porosity on the calibration curve. The experimental results suggest a calibration relationship linearly depending not only on the WCR output signal but also on the porosity. In addition, the calibration curve provided by the manufacturer significantly underestimates the soil water content, with a range of predicted volumetric water content values between 3 and 35%, compared with actual values varying between 19 and 61%.