Abstracts

Bogena, H.R., Huisman, J.A., et al. 2007, 'Evaluation of a Low Cost Soil Water Content Sensor for Wireless Network Applications', Journal of Hydrology, vol. 344, pp. 32-42.

Wireless sensor networks are a promising newin situmeasurement 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 aswell as field experiments. The field experiment features a comparison ofwater 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 functionwas obtained that allows the permittivity to be determined fromthe 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.

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Borhan, M.S., Parsons, L.R., et al. 2004, Evaluation of a Low Cost Capacitance ECH2O Soil Moisture Sensor for Citrus in a Sandy Soil, 25th International Irrigation Show, Tampa, Florida. 14-16 November 2004 pp. 447-458.

Most citrus in central Florida is grown on sandy soils that have very low water holding capacities. A small change is soil volumetric water content can greatly affect available water. The purpose of this study was to determine if a moderately low cost sensor (ECH₂O probe) can perform well in this sandy soil. Three water stress treatments (irrigated, non-irrigated and non-irrigated with rain exclusion) were imposed on Valencia orange trees in the fall and witner (2003-2004) to determine the effects of stress on sugar accumulation in the fruit. Five ECH₂O probes were installed in each treatment plot at depths ranging from 10 to 90 cm. Sensors were calibrated in the laboratory. Real time probe responses due to irrigation, rainfall and water uptake by the plants were collected and analyzed. These probes were able to detect small changes in soil water content at the lower end of the soil water regime and performed well in this soil.

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Campbell, C.S., Application Note: Response of ECH₂O Soil Moisture Sensor to Temperature Variation Decagon Devices pp 1-6

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Campbell, C.S., Application Note: Response of the ECH2O Soil Moisture Probe to Variation in Soil Water Content, Soil Type, and Solution Electrical Conductivity Decagon Devices pp 1-5

Researchers familiar with commercial water content probes will often ask three questions when approached with a newly developed dielectric sensor: what is the accuracy of the instrument, how does it react to differing soil textures and electrical conductivity, and how much does it cost? In fact, the first two questions are closely related, as often the properties of a soil can determine the accuracy of volumetric water content reading from a dielectric probe. Poor results from probes that measure dielectric in soils with high electrical conductivity and salinity are well documented. The third question has considerable importance as well because the cost of water content sensors can limit the number of sites where water content is monitored.

A new inexpensive dielectric sensor (trade name: ECH₂O) developed by Decagon Devices, Inc. uses specialized circuitry to measure the dielectric of media surrounding a thin, fiberglass-enclosed probe. The objective of the experiment was to determine the calibration of several dielectric probes with respect to soil water content and examine the effects of soil texture and salinity on the stability of that calibration.

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Christensen, N.B. 2005, Irrigation Management using Soil Moisture Monitors, Western Nutrient Management Conference, Salt Lake City, UT, USA. pp. 46-53.

Different methods of irrigation scheduling have been developed. These methods have evolved from a checkbook method of estimating soil water balance and irrigation timing to real time monitoring of soil moisture with the use of radio telemetry to archive the data. The checkbook method estimates soil water balance by summing rainfall and irrigation and subtracting crop water use. The method can be cumbersome and time consuming for practical use by retail agronomist. Real time methods of soil moisture determination and data archiving have made soil moisture monitoring easier. The C-probe and ECH₂O probes have been evaluated and used. The ECH₂O probes were the easiest to install and have a good balance of cost and reliability. ECH₂O were used in irrigation management of silage corn and potatoes in southern Idaho. Good irrigation management has prevented deep peculation and subsequent movement of nitrate out of the root profile.

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Czarnomski, N.M., Moore, G.W., et al. 2005, 'Precision and Accuracy of Three Alternative Instruments for Measuring Soil Water Content in Two Forest Soils of the Pacific Northwest', Canadian Journal of Forest Research, vol. 35, pp. 1867-1876.

We compared the accuracy and precision of three devices for measuring soil water content in both natural and repacked soils and evaluated their temperature sensitivity. Calibrations were developed for a capacitance instrument (ECH₂O), a time domain reflectometry cable tester (CT), and a water content reflectometer (WCR) in soils collected from the Wind River and H.J. Andrews Experimental Forests. We compared these calibrations with equations suggested by manufacturers or commonly used in the literature and found the standard equations predicted soil moisture 0%-11.5% lower (p < 0.0001) than new calibrations. Each new calibration equation adequately predicted soil moisture from the output for each instrument regardless of location or soil type. Prediction intervals varied, with errors of 4.5%, 3.5% and 7.1% for the ECH₂O, CT and ECR respectively. Only the ECH₂O was significantly influenced by temperature for the range sampled: as temperature increased by 1°C, the soil moisture estimate decreased by 0.1%. Overall, the ECH₂O performed nearly as well as the CT and thanks to its lower cost, small differences in performance might be offset by deployment of a greater number of probes in field sampling. Despite its higher cost, the WCR did not perform as well as the other two systems.

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Decagon Inc., Application Note: ECH₂O Dielectric Probes vs Time Domain Reflectometers (TDR) Decagon Inc. pp 3 pp.

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Kizito, F., Campbell, C.G., et al. 2008, 'Frequency, Electrical Conductivity and Temperature Analysis of a Low-cost Capacitance Soil Moisture Sensor', Journal of Hydrology, vol. 352, pp. 367-378.

This study evaluated the family of ECH₂O sensors (EC-5 and ECH₂O-TE) for measurement of soil moisture content (θ), bulk electrical conductivity (EC_b) and temperature for a range of soils, across a range of measurement frequencies between 5 and 150 MHz. Measurement frequency is one of the primary factors affecting the sensitivity of capacitance sensor measurements to soil variables such as soil texture, electrical conductivity, and temperature. Measurements in both soil and solution demonstrated that the ECH₂O EC and TE measurements were accurate. Using a measurement frequency of 70 MHz, a single calibration curve was determined for a range of mineral soils, independent of soil salinity, suggesting there might be no need for a soil specific calibration. When combining all data for each soil type, the R² values remained high (R² = 0.98) with little probe to probe variability. After laboratory calibration, the error for θ was about 2%, independent of soil EC_b, up to a soil solution EC of about 12 dS/m. Our results showed that a single calibration curve could be used for all tested mineral soils, independent of soil salinity. The bulk soil EC_b – water content data were excellently described by a polynomial expression. Measurements of temperature sensitivity to soil water content and EC_b were sufficiently small. For example, for a temperature change of 10°C, measurements of θ and EC_b were affected by about 0.02 cm³ cm^-3 and 0.02 dS/m, respectively. Limited sensor calibration requirements are important, when large networks of soil moisture sensors are being deployed. It is concluded that an accurate, cost-effective soil moisture sensor is available that operates at a measurement frequency of 70 MHz, with a low sensitivity to confounding soil environmental factors.

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Laurent, J.P., Olivier, F., et al. 2005, Monitoring Moisture Content in Municipal Solid Waste: Results of a Preliminary Test under Laboratory Conditions, International Workshop on Hydro-Physico-Mechanics of Landfills, Grenoble Univerisity. 21-22 March

This paper presents the results of the tentative testing of various moisture content monitoring probes in Municipal Solid Waste (MSW). The experimental test was conducted on the basis of a 0.07m³ sample of shredded waste subject to compression (up to 130 kPa) and a cycle of water saturation - drainage under laboratory conditions. First results indicate that TDR and capacitance probes are sensitive to volumetric variations of moisture content. Further works will be needed to calibrate the response of the latter probes from various types of waste and test conditions.

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Luedeling, E., Nagieb, M., et al. 2005, 'Drainage, Salt Leaching and Physico-chemical Properties of Irrigated Man-made Terrace Soils in a Mountain Oasis of Northern Oman', Geoderma, vol. 125, pp. 273-285.

Little is known about the sustainability of irrigated oasis agriculture in northern Oman. The objective of this study therefore was to examine which factors allowed agricultural productivity to be apparently maintained during the two millenia of a mountain oasis' existence. Soil moisture and physico-chemical properties were measured in a typical flood-irrigated field sown to alfalfa (Medicago sativa L.). Particle size, organic (C_org) and inorganic carbon content, pH and electrical conductivity (EC) of the soil progile were analyzed at 0.15, 0.45 and 1.00 m. Saturated hydraulic conductivity and the soil's apparent bulk density and water potential were determined from undisturbed samples at 0.05, 0.25 and 0.60 m. During irrigation cycles of 6-9 days, volumetric water contents ranged from 30% to 13%. A tracer experiment with potassium bromide revealed that 52-56% of the irrigation water was stored in the upper 0.4 m of the soil. The rest of the water moved further down the profile, thus providing the necessary drainage to avoid the build-up of toxic salt concentrations. Due to differences in pore size, plant-available water in the topsoil amounted to 18.7% compared to 13% and 13.5% at 0.25- and 0.60-m depth, respectively. The aggregate structure in the upper 1.0 m of the profile is likely preserved by concentrations of calcium carbonate (CaCO₃) from 379 to 434 mg kg-1 and Corg from 157 to 368 mg kg^-1 soil. The data indicate that the sustainability of this irrigated landuse system is due to high water quality with low sodium but high CaCO₃ concentration, the elaborate terrace structure and water management which allows adequate drainage.

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Mattson, E.D., Baker, K.E., et al. 2006, 'A Flexible Water Content Probe for Unsaturated Soil Column Experiments', Vadose Zone Journal, vol. 5, pp. 805-808.

A commercially available soil moisture probe was modified by replacing rigid electrode waveguides with flexible electrode traces that can be attached to the interior of soil column walls. This new design minimizes soil packing difficulties and potential bias in water and solute flow pathways commonly associated with rigid probe installations in column experiments. Tests demonstrated that the modified flexible electrode design maintains a voltage response that is proportional to the electrode trace length and is reduced by approximately one-half when only one side of the electrode trace is in contact with the soil media. Laboratory experiments confirmed that the longer electrode traces attached to the interior of a column wall have a comparable sensitivity of that of a commercially available rigid probe inserted horizontally across the diameter of a soil column. The replacement of the rigid electrode with a flexible electrode offers an improved method of measuring the water content in solute transport experiments while minimizing sensor intrusion into laboratory soil columns.

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McMichael, B. and Lascano, R.J. 2003, 'Laboratory Evaluation of a Commercial Dielectric Soil Water Sensor', Vadose Zone Journal, vol. 3, pp. 650-654.

Development of management strategies for efficient water utilization of crop production requires measurements of changes in soil water content on a dynamic basis. Many of the methods currently used for measuring these changes are destructive, slow, or relatively expensive for large-scale investigations. A commercially available, low-cost, nondestructive soil moisture sensor for measuring changes in soil volumetric water content (VWC) on the basis of changes in the dielectric constant of the soil water was evaluated under laboratory conditions for two soil series (Amarillo fine sandy loam [fine-loamy, mixed, superactive, thermic Aridic Paleustalfs] and Pullman clay loam [fine, mixed, thermic Torretic Paleustolls]) and a potting material across a wide range of water contents. Probes were placed in containers filled with deionized water and soil. Containers with Amarillo fine sandy loam were placed in a programmable temperature chamber and subjected to a series of changes in both temperature and VWC. Containers with Pullman soil and potting material were only subjected to changes in VWC at a constant temperature. Probe output at a constant temperature between air dry and a VWC of 0.25 m³ m^-3 was linear for the Pullman soil and potting material and nonlinear for the Amarillo soil. When the Amarillo soil temperature varied between 15.9 and 39.1°C^-1 at a constant VWC, probe output changed the equivalent of 0.10 m³ m^-3. The temperature sensitivity was 0.5 mV °C^-1 for air-dry and about 5 mV °C^-1 for wet Amarillo soil. We conclude that probe output is soil specific and, given the nonlinear response to increasing water content on some soils and sensitivity to temperature, will require soil-specific calibration equations.

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Sakaki, T., Limsuwat, A., et al. 2008, 'Empirical Two-point α-mixing Model for Calibrating the ECH₂O EC-5 Soil Moisture Sensor in Sands', Water Resources Research, vol. 44

Recently improved ECH₂O soil moisture sensors have received significant attention in many field and laboratory applications. Focusing on the EC-5 sensor, a simple and robust calibration method is proposed. The sensor-to-sensor variability in the readings (analog-to-digital converter (ADC) counts) among 30 EC-5 sensors was relatively small but not negligible. A large number of ADC counts were taken under various volumetric water contents (θ) using four test sands. The proposed two-point α-mixing model, as well as linear and quadratic models, was fitted to the ADC - θ data. Unlike for conventional TDR measurements, the effect of sensor characteristics is lumped into the empirical parameter a in the two-point α-mixing model. The value of a was fitted to be 2.5, yielding a nearly identical calibration curve to the quadratic model. Errors in θ associated with the sensor-to-sensor variability for the two-point α-mixing model were ±0.005 cm³ cm^-3 for dry sand and ±0.028 cm³ cm^-3 for saturated sand. In the validation experiments, the highest accuracy in water content estimation was achieved when sensor-specific ADC_dry and ADC_sat were used in the two-point a-mixing model. Assuming that α = 2.5 is valid for most mineral soils, the two-point α-mixing model only requires the measurement of two extreme ADC counts in dry and saturated soils. Sensor-specific ADC_dry and ADC_sat counts are readily measured in most cases. Therefore, the two-point a-mixing model (with α = 2.5) can be considered as a quick, easy, and robust method for calibrating the ECH₂O EC-5 sensor. Although further investigation is needed, the two-point α-mixing model may also be applied to calibrating other sensors.

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Smith, P. and Christies, J. 2006, Practical Irrigation Scheduling of Pastures under Centre Pivot Irrigators, Irrigation Australia 2006 Conference, Brisbane.

A survey of 27 dairy farms utilising centre pivot irrigation in the Hunter Dairy Development Group Area found that water use efficiency ranged from 3.4 kg DM/mm/ha, to 40.1 kg DM/mm/ha, average 14.5 kg DM/mm/ha (standard deviation = 9.0). Irrigation scheduling for dairy pastures commonly relies heavily on subjective visual assessments of plant and soil conditions, climatic conditions and farmer experience and maybe contributing to this wide range of water use efficiency. Centre pivots commonly apply only small quantities at each event, penetrating to fairly shallow depths. This suits most grass based pastures where the root zones are relatively shallow. But it presents difficulties using current soil moisture monitoring technology as this has limited accuracy at shallow depths.

To investigate how well this equipment reflects water applied for pastures, and therefore how useful it may be for objective irrigation scheduling by managers, a field trial, commenced in November 2004, is being conducted on a dairy farm at Upper Manilla in northern NSW. This paper reports findings from commencement to February 2006, which covers one winter and almost two summers.

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