Soil Water Content Sensors Research Articles

Sensor-to-Sensor Variability of the ECHO EC-5, TE, and 5TE Sensors in Dielectric Liquids

Low-budget sensors used in wireless soil water content sensor networks typically show considerable variation. Because of the large number of sensors in sensor network applications, it is not feasible to account for this variability using a calibration between sensor response and soil water content. An alternative approach is to split the calibration into two parts: (i) determination of sensor response–permittivity relationships using standard liquids with a defined reference permittivity, and (ii) site-specific calibration between permittivity and soil water content using a subset of sensors. In this study, we determined sensor response–permittivity relationships for several ECH2O, EC-5, TE, and 5TE sensors by Decagon Devices (Pullman, WA). The objectives of this study were to determine (i) the sensor-to-sensor variability and precision of these sensor types, and (ii) the increase in accuracy when a sensor-specific calibration is used instead of a single calibration. The results showed that the sensor-to-sensor variability was significantly larger than the measurement noise for each sensor type. When a sensor-specific calibration was used, the RMSE expressed in (equivalent) soil water content ranged from 0.008 cm3 cm−3 for the TE sensor to 0.014 cm3 cm−3 for the EC-5 sensor in a permittivity range between ∼2 and 35. When a single calibration was used, the RMSE was higher and ranged from 0.01 cm3 cm−3 for the 5TE sensor to 0.02 cm3 cm−3 for the TE sensor. An improvement in accuracy of nearly 0.01 cm3 cm−3 can be reached in the high-permittivity range for each sensor type by calibrating each sensor individually.

Field Performance of Nine Soil Water Content Sensors on a Sandy Loam Soil in New Brunswick, Maritime Region, Canada

An in situ field test on nine commonly-used soil water sensors was carried out in a sandy loam soil located in the Potato Research Center, Fredericton, NB (Canada) using the gravimetric method as a reference. The results showed that among the tested sensors, regardless of installation depths and soil water regimes, CS615, Trase, and Troxler performed the best with the factory calibrations, with a relative root mean square error (RRMSE) of 15.78, 16.93, and 17.65%, and a r2 of 0.75, 0.77, and 0.65, respectively. TRIME, Moisture Point (MP917), and Gopher performed slightly worse with the factory calibrations, with a RRMSE of 45.76, 26.57, and 20.41%, and a r2 of 0.65, 0.72, and 0.78, respectively, while the Gypsum, WaterMark, and Netafim showed a frequent need for calibration in the application in this region.

A solar-powered wireless cell for dynamically monitoring soil water content

In various agricultural activities, such as irrigation scheduling, tillage operation and nutrient management, soil water content is regarded as an essential parameter. For the demand of monitoring soil water dynamics at field scale, this study presents a solar-powered wireless cell that contained a water content sensor, two temperature sensors, an optical sensor, a communication module and a data-logger. To evaluate its feasibility in the field, three cells were independently installed in a bare plot, a maize plot and a sugarbeet plot in the summer of 2008. By a heavy rain event (20 mm, 6 h) during the experimental period, different responses of soil water content sensors were dynamically recorded and then analyzed under each canopy condition. In addition, to explore the possibility of reducing the dimensions of the cell under the condition of sufficient energy supply, various sizes of the solar panel were tested. The experimental results showed that the cell is promising to become a node of wireless network in the field.

Water and salt movement in soil driven by crop roots: a controlled column study

AbstractWater deficit and salt accumulation in soil presents serious problems to crop production in semi-arid regions. These problems depend on the active transpiration stream and the selective absorption of ions by crop roots. In this study, a large sized soil column system was used to examine the dynamics of water and ion transport and salt accumulation in soil layers. Special reference was placed on the effects of the active and selective absorption by roots of different crops (i.e., corn plants, sunflower plants and no plants). The column system was equipped with on-line systems for the control of groundwater level. Soil water content sensors enabled time-course evaluations of the volumetric water content and hence upward flux of the groundwater in the soils at different depths. Furthermore, the distribution and accumulation of ions in soil layers, plant organs and xylem sap were analyzed using ion chromatography. In this column experiment, diurnal and longer term changes in water movement and ion accumulation in soil, affected by root absorption characteristics of plants, were evaluated quantitatively. The results demonstrated that the column system was applicable for the quantitative analysis of the effects of root absorption by different crops on water deficit and salinization in soils.

시설재배 토양의 수분 조절을 위한 자동 수분제어시스템 개발

This study was conducted to develop an automatic soil water content control system for greenhouse, which consisted of drip irrigation nozzles, soil water content sensors, an on/off valve, a servo-motor assembly and a control program. The control logic adopted in the system was Ziegler-Nichols algorithm and rising time, time constant and over/undershoot ratio as control variables in the system was selected and determined by various control experiments to maintain small delay time and low overshoot. Based on the experimental results, it was concluded that the control system developed in the study could replace the unreliable conventional greenhouse soil water management.

Evaluation of a low-cost soil water content sensor for wireless network applications

Summary Wireless sensor networks are a promising new in situ measurement technology for monitoring soil water content changes with a high spatial and temporal resolution for large areas. However, to realise sensor networks at the small basin scale (e.g. 500 sensors for an area of 25 ha), the costs for a single sensor have to be minimised. Furthermore, the sensor technique should be robust and operate with a low energy consumption to achieve a long operation time of the network. This paper evaluates a low-cost soil water content sensor (ECH2O probe model EC-5, Decagon Devices Inc., Pullman, WA) using laboratory as well as field experiments. The field experiment features a comparison of water content measurements of a forest soil at 5 cm depth using TDR and EC-5 sensors. The laboratory experiment is based on a standardized sensor characterisation methodology, which uses liquid standards with a known dielectric permittivity. The results of the laboratory experiment showed that the EC-5 sensor has good output voltage sensitivity below a permittivity of 40, but is less sensitive when permittivity is higher. The experiments also revealed a distinct dependence of the sensor reading on the applied supply voltage. Therefore, a function was obtained that allows the permittivity to be determined from the sensor reading and the supply voltage. Due to the higher frequency of the EC-5 sensor, conductivity effects were less pronounced compared to the older EC-20 sensor (also Decagon Devices Inc.). However, the EC-5 sensor reading was significantly influenced by temperature changes. The field experiment showed distinct differences between TDR and EC-5 measurements that could be explained to a large degree with the correction functions derived from the laboratory measurements. Remaining errors are possibly due to soil variability and discrepancies between measurement volume and installation depth. Overall, we conclude that the EC-5 sensor is suitable for wireless network applications. However, the results of this paper also suggest that temperature and electric conductivity effects on the sensor reading have to be compensated using appropriate correction functions.

Determination of lower limits for irrigation management using in situ assessments of apparent crop water uptake made with volumetric soil water content sensors

In situ approaches for determining lower limit (threshold, refill) values for irrigation management using continuously monitored data from volumetric soil water content (SWC) sensors were evaluated. Four indices were derived from SWC data: (i) apparent daily crop water uptake (ADCWU), the reduction in SWC during daylight periods; (ii) daily soil water loss (DSWL), the reduction in SWC during 24 h periods; (iii) previous overnight redistribution (POR), overnight drainage; and (iv) ADCWU normalised for reference evapotranspiration (ADCWU:ET o). Indices were calculated for 0–20 and 20–40 cm soil depths in four drying cycles applied to melon and to autumn and spring tomato crops. In each drying cycle, there were well-watered and un-watered irrigation treatments. In drying soil, values of ADCWU and DSWL followed a decay curve with three phases: (1) of rapid decline when drainage occurred; (2) of a much slower decline, after drainage ceased, with smaller values influenced by climatic factors; and (3) of constantly very low values not influenced by climate. The actual commencement of crop water stress was indicated by the first statistically significant difference in midday Ψ leaf between un-watered and well-watered plants. A protocol was developed to identify commencement of crop water stress using ADCWU or DSWL in un-watered plants. POR was used to identify cessation of internal drainage. Then, the first reduction in ADCWU or DSWL while climatic variables (solar radiation, VPD) indicated a constant or increasing atmospheric evaporative demand was considered an “indication” of crop water stress, the second such reduction as “confirmation”. In the four cycles studied, “indications” of crop water stress using both ADCWU and DSWL for 0–20 cm soil were 1 (twice), 2 and 6 d after commencement of water stress. When there was not appreciable climatic variation during phase 2, ADCWU and DSWL detected crop water stress within 1–2 d of Ψ leaf. Generally, climate-mediated fluctuations in ADCWU and DSWL during phase 2 delayed identification of the commencement of water stress. Normalising ADCWU for ET o reduced effects of climatic fluctuation. Using a 25% relative reduction in ADCWU:ET o as a threshold, this parameter was more sensitive than ADCWU or DSWL using the proposed protocols for detecting commencement of water stress. “Breaking points” in the rate of reduction of SWC, and variations in stem diameter were also compared for detecting commencement of crop water stress. This study suggested that reductions in SWC in drying soil can be used to identify lower limits for irrigation management using SWC sensors, and that stable climatic conditions are required to optimise these approaches.

Application of Sensors to Assess Soil Conditions in a Korean Paddy Field

Precision agriculture, also called as site-specific crop (or field) management, is a recent trend in crop production that uses field information collected at different within-field locations to optimize amount, timing, and location of agricultural inputs according to the site-specific requirements. Recent development of soil property sensors has facilitated sensor-based data collection for SSCM in many countries around the world. In this study, commercial soil strength, electrical conductivity, and water content and temperature sensors were applied to a Korean rice (Oriza Sativa L) field and spatial and non-spatial statistical techniques were used to assess soil conditions and the variability, and investigate optimum sampling intensity. Results of the study would be useful for establishment of data collection schemes and better application of soil property sensors to Korean paddy fields for successful precision agriculture.

Dielectric Loss and Calibration of the Hydra Probe Soil Water Sensor

Widespread interest in soil water content (θ, m3 m−3) information for both management and research has led to the development of a variety of soil water content sensors. In most cases, critical issues related to sensor calibration and accuracy have received little independent study. We investigated the performance of the Hydra Probe soil water sensor with the following objectives: (i) quantify the inter‐sensor variability, (ii) evaluate the applicability of data from two commonly used calibration methods, and (iii) develop and test two multi‐soil calibration equations, one general, “default” calibration equation and a second calibration that incorporates the effects of soil properties. The largest deviation in the real component of the relative dielectric permittivity εr′ determined with the Hydra Probe using 30 sensors in ethanol corresponded to a water content deviation of about 0.012 m3m−3, indicating that a single calibration could be generally applied. In layered (wet and dry) media, εr′ determined with the Hydra Probe was different from that in uniform media with the same water content. In uniform media, θ was a linear function of √εr′ We used this functional relationship to describe individual soil calibrations and the multi‐soil calibrations. Individual soil calibrations varied independently of clay content but were correlated with dielectric loss. When applied to the 19‐soil test data set, the general calibration outperformed manufacturer‐supplied calibrations. The average θ difference, evaluated between and , was 0.019 m3m−3 for the general equation and 0.013 m3m−3 for the loss‐corrected equation.

Laboratory and field evaluation of five soil water sensors

Accurate in situ determination of soil water content is important in many fields of agricultural, environmental, hydrological, and engineering sciences. As numerous soil water content sensors are available on the market today, the knowledge of their performance will aid users in the selection of appropriate sensors. The objectives of this study were to evaluate five soil water sensors in the laboratory and to determine if laboratory calibration is appropriate for the field. In this study, the performances of five sensors, including the Profile Probe™ (PP), ThetaProbe™ , Watermark™, Aqua-Tel™, and Aquaterr™ were compared in the laboratory. The PP and ThetaProbe™ were more accurate than the other soil water sensors, reproducing soil water content using factory recommended parameters. However, when PP was installed on a loamy sand in the field, the same soil that was used for the laboratory evaluation, it overestimated field soil water, especially at depth. Another laboratory experiment showed that soil water content readings from the PP were strongly influenced by soil bulk density. The higher the soil bulk density, the greater was the overestimation of soil water content. Two regression parameters, a0 and a1, which are used to convert the apparent dielectric constant to volumetric water content, were found to increase linearly with the soil bulk density in the range of 1.2 to 1.6 Mg m-3. Finally, the PP was calibrated in the field and a good calibration function was obtained with an r2 of 0.87 and RMSE of 2.7%. The values of a0 and a1 obtained in the field were different from factory recommended parameters (a0 = 2.4 versus 1.6 while a1 = 12.5 versus 8.4) and were independent of soil depth, bulk density, and texture. As such, individual field calibration will be necessary to obtain precise and accurate measurement of soil water content with this instrument. Key words: Soil water content, Profile Probe, calibration, soil water content sensor

Micro-variability of hydrological processes at the maize row scale: implications for soil water content measurements and evapotranspiration estimates

In this paper, we analyse the micro-variability of hydrological processes (i.e. rainfall redistribution, root water uptake and evapotranspiration) at the maize row scale during a long drying period. Then, we study the impact of the variability of these processes on the spatio-temporal dynamics of the soil water content and on the sampling strategies to estimate soil water content and evapotranspiration at the maize row scale. Results of our study firstly show that the rainfall reaching the ground under the maize canopy is very variable with coefficients of variation ranging between 77.87 and 188.7%. Some locations receive up to 4.47 times more water than the incident rainfall. The estimated root water uptake (RWU) is also extremely variable, laterally within the maize rows with CV values ranging between 13.9 and 137%. Well-defined cyclical patterns of RWU due to the row layout are observed at some depths. Throughout the long drying period, maximum RWU progressively moves downwards and towards the middle of the row. Evapotranspiration (ET) is less variable across the rows with CV values ranging between 11.95 and 29.07%. Cyclical patterns are also observed except at the end of the considered period. The spatio-temporal dynamics of soil water content are clearly influenced by the maize row layout, with the progressive appearance of a fingered pattern within the rows due to the spatially variable RWU. The maximum difference of soil water content measured within a maize row is important and ranges between 0.046 and 0.079 cm 3 cm −3. In absolute terms, coefficients of variation for soil water content range between 1.05 and 12.1%. For estimates of soil water content and ET at the maize row scale, we show that the location of soil water content sensors is critical. Estimates of mean row scale soil water content and ET by the use of ‘central’ sampling locations in the middle of each row leads to maximum error of 0.022 cm 3 cm −3 and 9.09 mm, respectively. More research is needed to generalize the results obtained in this study for other climatic conditions and for other soil water content measurement techniques.

Measurement of Soil Water Content with a 50‐MHz Soil Dielectric Sensor

The Hydra Probe1 is a relatively inexpensive and widely used soil water content (θ, m3m−3) sensor. It measures both the real (εr) and imaginary (εi) components of the complex soil dielectric constant at 50 MHz. Our objectives were to: (i) determine the accuracy and precision of Hydra Probe dielectric measurements, (ii) establish an electrical conductivity limit for Hydra Probe measurements, (iii) document effects of soil type and temperature, and (iv) relate these results to much more thoroughly studied relationships established for time domain reflectometry (TDR). We evaluated Hydra Probe εrmeasurement precision and accuracy in air, ethanol, butanol, and water. Electrical conductivity effects were established in a series of aqueous KCl solutions. Effects of soil type on calibration were evaluated with four soils. Temperature sensitivity was tested in air, oven‐dried, and nearly saturated soil. Each test was performed with three sensors. We found that, in fluids, the sensors were accurate (εrwithin 0.5), precise (coefficient of variation [CV] < 1%), and that inter‐sensor variability was generally low except in KCl solutions with electrical conductivities >0.142 S m−1(0.01M). There was a strong correlation between θ and εrfor all soils tested but the θ–εrrelationship varied with soil. Deviations of measured θ–εrfrom the Topp equation increased in magnitude with εi, which may be the key to more general calibrations. Temperature effects on εrwere negligible in oven dry soils and different for each soil when nearly saturated. The largest temperature effect relative to 25°C was ±0.03 m3m−3In general, it appears that differences between Hydra Probe and TDR measurements are related to differences in soil dielectric properties at the measurement frequencies of the two instruments.

Measurement of Soil Water Content with a 50-MHz Soil Dielectric Sensor

The Hydra Probe1 is a relatively inexpensive and widely used soil water content (θ, m3 m−3) sensor. It measures both the real (εr) and imaginary (εi) components of the complex soil dielectric constant at 50 MHz. Our objectives were to: (i) determine the accuracy and precision of Hydra Probe dielectric measurements, (ii) establish an electrical conductivity limit for Hydra Probe measurements, (iii) document effects of soil type and temperature, and (iv) relate these results to much more thoroughly studied relationships established for time domain reflectometry (TDR). We evaluated Hydra Probe εr measurement precision and accuracy in air, ethanol, butanol, and water. Electrical conductivity effects were established in a series of aqueous KCl solutions. Effects of soil type on calibration were evaluated with four soils. Temperature sensitivity was tested in air, oven-dried, and nearly saturated soil. Each test was performed with three sensors. We found that, in fluids, the sensors were accurate (εr within 0.5), precise (coefficient of variation [CV] < 1%), and that inter-sensor variability was generally low except in KCl solutions with electrical conductivities >0.142 S m−1 (0.01 M). There was a strong correlation between θ and εr for all soils tested but the θ–εr relationship varied with soil. Deviations of measured θ–εr from the Topp equation increased in magnitude with εi, which may be the key to more general calibrations. Temperature effects on εr were negligible in oven dry soils and different for each soil when nearly saturated. The largest temperature effect relative to 25°C was ±0.03 m3 m−3 In general, it appears that differences between Hydra Probe and TDR measurements are related to differences in soil dielectric properties at the measurement frequencies of the two instruments.

New Field Method to Determine Streamflow Timing Using Electrical Resistance Sensors

Electrical resistance (ER) sensors were constructed to monitor streambed saturation to infer ephemeral streamflow timing. The sensors were evaluated in an ephemeral stream through comparison with temperature‐based methods, a stream gauge, and soil water content sensors. The ER sensors were more accurate at estimating streamflow timing and the resultant data required less interpretation than data from temperature‐based methods. Accuracy was equivalent to timing methods using stream gauge and soil water content measurements. The ER sensors are advantageous for use in ephemeral stream channels because they are inexpensive, deployable above or below the sediment surface, insensitive to depth, and do not require connecting wires to an external datalogger or power source. On the basis of these results, we conclude that ER sensors may be used to monitor changes in soil water content within the vadose zone. Additionally, the sensors can be used to infer the presence of surface water in diversion canals, storm‐water sewers, and in the form of overland runoff.

New Field Method to Determine Streamflow Timing Using Electrical Resistance Sensors

Electrical resistance (ER) sensors were constructed to monitor streambed saturation to infer ephemeral streamflow timing. The sensors were evaluated in an ephemeral stream through comparison with temperature-based methods, a stream gauge, and soil water content sensors. The ER sensors were more accurate at estimating streamflow timing and the resultant data required less interpretation than data from temperature-based methods. Accuracy was equivalent to timing methods using stream gauge and soil water content measurements. The ER sensors are advantageous for use in ephemeral stream channels because they are inexpensive, deployable above or below the sediment surface, insensitive to depth, and do not require connecting wires to an external datalogger or power source. On the basis of these results, we conclude that ER sensors may be used to monitor changes in soil water content within the vadose zone. Additionally, the sensors can be used to infer the presence of surface water in diversion canals, storm-water sewers, and in the form of overland runoff.

Soil water content determination using a network analyzer and coaxial probe

A small‐volume dielectric constant and soil water content sensor would be desirable in many laboratory experiments. Phase shift of the reflection coefficient in soil and various solutions was measured with a coaxial probe (CP) and a network analyzer operating at a frequency of 795 MHz. The CP had a measurement depth &lt;1 cm. Five soils, varying widely in texture, bulk density, and organic matter content, were tested and comparison was made with other dielectric methods. Synthesized time domain reflectometry (TDR), resonant waveguide, CP, and conventional TDR measurements were in agreement for the sand. A simple mixing model for known dielectrics accurately predicted measurements of the apparent dielectric (Ka) for mixed solutions. A linear function , fit for the water content of all soil data, had a single measurement root mean square error The uncertainty improved when individual linear soil calibrations (singe measurement to 0.99) were used and further improved when repeated measurements were averaged The CP method for measuring Ka is fast, simple, linear, easily repeated, and reasonably accurate, indicating that this instrumentation is useful for studying dielectric behavior of soil and various solutions and for rapid determination of soil water content in a small sample.

Field Calibration of a Capacitance Water Content Probe in Fine Sand Soils

Fine sand soils important to Florida agriculture have volumetric soil water content values (θv) of &lt;0.10 cm3 cm−3 after drainage due to gravity has ceased. Small changes in θv in the range of 0.02 to 0.08 cm3 cm−3 can greatly affect plant available water and, therefore, good calibration of soil water content sensors is necessary. The EnviroSCAN (Sentek Pty. Ltd., South Australia) is a multiple sensor capacitance probe capable of continuous measurement of soil water content by volume (θv). Many fine sand soils in Florida have plant available θv values of ≤0.08 cm3 cm−3 The manufacturer's calibration curve has very few data points &lt;0.10 cm3 cm−3 θv and no data in the 0.02 to 0.04 cm3 cm−3 θv range. Because of the lack of data in this range, a calibration curve from 0.02 to 0.08 cm3 cm−3 θv was developed for Candler fine sand (hyperthermic, uncoated Typic Quartzipsamments), Apopka fine sand (loamy, siliceous, hyperthermic Grossarenic Paleudults), and Immokalee fine sand (sandy, siliceous, hyperthermic Arenic Alaquods) in two locations in Florida. Since calibration curves for the three soils did not differ significantly, data from the three soils were combined. An exponential calibration curve was developed This equation provides substantially different estimates of water content in the 0.02 to 0.08 range than values obtained from the manufacturer's calibration. This improved calibration extends the useful range of the EnviroSCAN to include an important group of soils with very low water holding capacity.