tensiometer sensor system campaign operation for measuring soil water retention and hydraulic conductivity

Protocol for Soil Hydraulic Property Measurement via Evaporation

Soil water retention and hydraulic conductivity within natural and human-altered environments help us understand and observe changes in soil health and functionality. Quantifying hydraulic properties through the soil water retention curve (SWRC) and soil water conductivity curve offers insight into key drivers of soil physical behavior and water movement characterization1. The relationship between volumetric water content (&#952;) and matric head (h) is represented within an SWRC, and the ranges within the curve describe the saturation point, field capacity, and permanent wilting point2. Soil management practices, amendments, agroecosystem types, and environmental conditions can all have an impact on soil hydraulics3,4. These factors can in turn influence solute transport5 and plant available water6, soil respiration and microbial activity7, as well as wetting and drying cycles8. As an important piece in quantifying healthy and functioning soil, proper analysis of the SWRC is imperative for obtaining an informed understanding of soil hydraulic properties.
A variety of measurement techniques currently exist for developing a reliable SWRC, with the hanging-water column and pressure plate methods being common traditional approaches for determining the pore size distribution of soil2. Traditional methods can be time-consuming, usually taking weeks or months to analyze a small set of samples9. Moreover, once analysis is complete, these methods result in only a few data points that inform the SWRC9. Additionally, the accuracy of producing representative data using traditional methods such as pressure plates can become a concern at lower matric potentials, in particular, with fine-textured soils10,11. More modern techniques, which involve the simple evaporation experiment approach using tensiometers and the chilled-mirror dew point method, tend to deliver more reproducible data across a wide range of soil textures2. Initially developed by Wind in 1968, the simple evaporation experiment involved measuring water mass changes and tension changes through tensiometers in the soil sample over time12. As evaporation occurs, soil sample mass measurements are taken at specific time intervals to create an SWRC. Later refined by Schindler (1980), the method involved only two tensiometers placed at different pressure heads within the soil sample. The modified method was then tested and validated as capable of being used in scientific analysis13,14. A key benefit of the simple evaporation experiment is the potential to easily produce data across a large portion of the soil moisture curve (0 to -300 kPa), with more data points than with traditional methods. 
These modern methods involve automated instruments that take numerous data points throughout the sample analysis period and produce data using a software interface. The hydraulic property instrument is a contemporary instrument that creates water retention curves and conductivity curves from sample data15. By employing a simple evaporation experiment using the hydraulic property instrument, the relationship between water content and water potential in the soil can be evaluated1. In this experiment, water present within the tensiometer shaft exists in an equilibrium with water in the soil solution. As evaporation of soil water occurs and the soil sample dries, cavitation takes place in the tensiometer, and the experiment ends. There is a limitation of the hydraulic property instrument in the dry range of the SWRC, as the instrument is only capable of operating within matric potentials of 0 to -100 kPa. This can be remedied with the inclusion of data generated with a chilled-mirror dew point experiment using a soil water potential instrument16, which can extend the data range to -300,000 kPa or the permanent wilting point. All these data are brought together in the modeling software post processing to cohesively inform the SWRC from null tensions to higher tensions even beyond wilting point. The SWRC and hydraulic conductivity curves are then generated based on matric potential data points taken throughout the measurement period, allowing a complete curve projected from saturation to permanent wilting point to be generated. 
The method described here presents a succinct operating procedure for soil analysis with a hydraulic property instrument. This method has been conducted in a number of scientific settings, including quantification of soil health in a broad range of agroecosystems3,17,18,19, and efforts have been made to understand best practices beyond the instrument user manual20. Here, a standardized protocol is outlined for all steps of the procedure, including field sampling, sample preparation, software function, and data processing. Following this method will ensure a successful campaign that results in reliable data. Critical steps for ensuring quality data, common challenges, and best practices are presented to ensure proper implementation.

Author Spotlight: Advancing Agricultural Land Ecosystem Research with a Hydraulic Property Analyzer to Assess Soil Health

Measurements of Soil Water Potential and Conductivity based on a Simple Evaporation Experiment using a Hydraulic Property Analyzer

At the Sustainable Land Ecosystems Lab, our research focuses on finding sustainable solutions to problems affecting land ecosystems, including croplands and grasslands. Solving this complex problem involves researching soil health, a key part of all agricultural land ecosystems. Obtaining a soil water retention curve that extends into the Dry Range is a challenge.Currently, it is necessary to combine the data from this experiment with additional data on another instrument. This can be less than ideal for some research objectives. The main advantage here is the amount of data the instrument can produce with relatively low user time and input.Compared to other techniques that attempt to create a soil water retention curve with few data points, this instrument provides more precise data.

Tensiometer Sensor System Campaign Operation for Measuring Soil Water Retention and Hydraulic Conductivity

To begin, place the saturated soil samples in the tensiometer sensor unit. Once each sensor unit is set up, input the sample identification present on the metal core. Then click Browse to save the file location.After clicking Start, take the initial weight reading after two tensiometer readings have been completed. First, unplug the connection cord from the sensor unit. Place the unit on the weight scale when a dialogue box appears on the software.Remove the sensor unit once the software indicates the weight reading has been taken and plug it back into the connection cord. Once the sample has reached the air entry point, take a final weight measurement for each sensor unit and click Stop. The measurement campaign revealed distinct tension curves for two soil samples, showcasing optimal and suboptimal results.Soil water retention curves and hydraulic conductivity data were generated from the experiment with optimal data demonstrating clear air entry and cavitation points. The application of the van Genuchten-Mualem model to the output data provided fitted curves, which were used to derive soil properties.

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36 Views. To begin, place the saturated soil samples in the tensiometer sensor unit. Once each sensor unit is set up, input the sample identification present on the metal core. Then click Browse to save the file location.After clicking Start, take the initial weight reading after two tensiometer readings have been completed. First, unplug the connection cord from the sensor unit. Place the unit on the weight scale when a dialogue box appears on the software.Remove the sensor unit once the...

Video: Tensiometer Sensor System Campaign Operation for Measuring Soil Water Retention and Hydraulic Conductivity

The measurement of soil hydraulic properties is critical in understanding the physical components of soil health as well as integrated knowledge of soil systems under various management practices. Collecting reliable data is imperative for informing decisions that affect agriculture and the environment. The simple evaporation experiment described here uses instrumentation in a laboratory setting to analyze soil samples collected in the field. The soil water tension of the sample is measured by the instrument, and tension data is modeled by software to return soil hydraulic properties. This method can be utilized to measure soil water retention and hydraulic conductivity and give insight into differences in treatments or environmental dynamics over time. Initial establishment requires a user, but data acquisition is automated with the instrument. Soil hydraulic properties are not easily measured with traditional experiments, and this protocol offers a simple and optimal alternative. Interpretation of results and options for extending the data range are discussed.

This article features a simple evaporation experiment using a hydraulic property instrument for a soil sample. Through efficient means, measurements can be taken over a series of days to generate high-quality data.

The measurement of soil hydraulic properties is critical in understanding the physical components of soil health as well as integrated knowledge of soil ...

Preparation of Soil Sample and Preparation of Tensiometer for Analyzing Soil Health and Hydraulic Properties with Evaporation Experiments

To begin, remove the plastic cover from the flat edge side of the metal sampling core, and place a paper coffee filter on top, followed by a saturation plate. Saturate the soil samples for at least 24 hours inside the sample cores in a large plastic container with degassed deionized water. Invert the core and saturation plate into the container, and fill it with degassed deionized water, leaving a one centimeter space from the top of the soil sample.Using a 20 milliliter syringe and fine-tip needle, fill both tensiometer shaft ports with degassed deionized water. Shine a light into the port and check for debris to confirm if the pressure transducer is clean. Place the acrylic top on the sensor unit and fasten the metal clips.Insert the syringe containing degassed deionized water into the opening of the acrylic top and fill just below the top of the acrylic head. To attach the acrylic top to the degassing unit, connect the T tube on the degassing unit to the top of the acrylic head. Place stage mounted cups into both positions on the degassing unit, and fill three quarters of each cup with degassed deionized water.Screw the tensiometers into the threaded acrylic holders, and adjust the black O ring to the top of the acrylic holder. Place the tensiometers into the degassed deionized water in the stage mounted cups. Turn on the vacuum pump until 0.4 bar is reached.Then, turn off the vacuum pump and allow the system to equalize. Turn on the vacuum pump again until 0.8 bar is reached. After 24 hours, remove the tubing from the acrylic top and take out all tenters from the acrylic holders.Place the short and tall tensiometers into separate beakers of degassed deionized water.

Establishment of Tensiometer in the Sensor Unit for Evaporation-Based Soil Hydraulic Property Analysis

To begin, saturate the soil samples with degassed deionized water and prepare the tensiometers for installation. For sensor unit setup, plug the sensor unit assembly into the system connection cords. Release the metal clips, and remove the acrylic top.Click the data measurement software to open the program, and click the Show devices icon. Click the Refilling wizard icon to open the user interface and navigate to the appropriate sensor unit from the dropdown menu. Select a tensiometer from the beaker.Ensure there are no visible air bubbles in the shaft, and a convex meniscus of water forms at the top. Add more degassed deionized water with a syringe if there is no convex meniscus. Then, move the black O-ring on the tensiometer to the center of the threads.Invert the tensiometer into the standing water on the sensor unit, while keeping the convex meniscus intact. Install the short tensiometer in the port indicated with a short line, and the tall tensiometer in the port with a long line. Carefully screw the tensiometer into the port while observing the pressure readings on the Current readings tab.Next, place a silicon bulb filled with degassed deionized water on the top of the tensiometer to prevent the tip from desiccating during installation.

Placement of Soil Sample in the Tensiometer-Sensor Units for Assessing Soil Hydraulic Conductivity and Water Retention Using Evaporation

To begin saturate the soil samples with degassed, deionized water. After setting up the tensiometers and the sensor, place the saturated sample and the corresponding saturation plate on a working surface. Place the auger guide on top of the sample ring.Insert the tensiometer shaft auger into the hole of the auger guide, and turn it completely to remove soil. Then remove the auger guide and ensure that the soil sample has not collapsed in the hole. Remove the silicon bulbs from each tensiometer and place the silicon disc on top of the sensor unit.Place an absorbent material such as a towel under the connected sensor unit. Invert the sample core and place it on top of the sensor unit, fitting the sample onto the tensiometers. Next, remove the coffee filter and saturation plate.Secure the soil core with metal clasps on the side of the sensor unit.