Frequently Asked Questions
- Hydraulic conductivity - HYPROP - GENERAL Is hydraulic conductivity at near saturation unreliable for higher suctions?
It’s not that unsaturated hydraulic conductivity near saturation is unreliable but that some measurement methods have difficulty making that measurement, for example: the flow cell vs. the HYPROP. The HYPROP has issues near saturation because there is not enough difference between its two tensiometers. The flow cell approach does a better job of measuring unsaturated hydraulic conductivity near saturation. So it just depends on the method you use.
- Hydraulic conductivity - HYPROP - GENERAL If field and lab measurements are different, is it recommended to complete the hydraulic conductivity curve obtained with HYPROP in the lab with field measurements in the HYPROP-Fit?
This will depend on your research goal. If your goal is to understand individual types of soil (i.e., what are the intrinsic soil properties of a B horizon) then a better approach is to take lab measurements because you have better control. But if your goal is to understand interactions in the field, a better approach is to make field measurements.
- Hydraulic conductivity - HYPROP - GENERAL - KSAT - MINI DISK INFILTROMETER Can you refer me to a reliable measurement method to estimate and predict unsaturated hydraulic conductivity?
- Hydraulic conductivity - SATURO - HYPROP - GENERAL How could one determine the change in hydraulic conductivity at a soil-spoil interface?
There are two approaches you could take. You could measure the two materials independently and see which one is going to be most limiting. Typically, hydraulic conductivity is governed by the most limiting layer. You could also just make the measurement in the field and see the interaction between the two layers. Again this rate will be governed by the most limiting layer.
- Water potential - Soil moisture release curves - WP4C - HYPROP - Volumetric Water Content - GENERAL - LABROS Does field capacity vary depending on whether the soil has previously been in a dry or wet state? If so, what error margin might that cause if I plan irrigation scheduling according to FC?
This is true. What you are looking at is the effect of hysteresis, which is generally not a big concern. Depending on the soil type and how big the hysteresis effect is, it can actually shift the field capacity point slightly. If you are concerned about this, you may want to use water potential to schedule irrigation, for example with the TEROS 21 or a tensiometer. If you’d like more information about this, contact customer support.
- TEROS 21 - Water potential - Soil moisture release curves - WP4C - HYPROP - Volumetric Water Content - GENERAL - T8 Tensiometer - T4 Tensiometer - T5 Tensiometer - LABROS How can you measure capillary water potential?
Capillary water potential is tied to matric potential. So if you are measuring matric potential with a tensiometer or a TEROS 21, you are essentially measuring the effect of the capillaries or those different pore sizes. You can also use the HYPROP. The WP4C will also work assuming the soil has a negligible osmotic potential.
- TEROS 21 - Water potential - Soil moisture release curves - WP4C - HYPROP - Volumetric Water Content - GENERAL - T8 Tensiometer - T4 Tensiometer - TEROS - T5 Tensiometer Do matric potential sensor readings include osmotic potential?
This depends on what type of instrument you are using to measure the potential. For example, tensiometers, granular matric sensors, and the TEROS 21 ONLY measure matric potential. So these sensors are blind to osmotic potential. Laboratory instruments like the WP4C measure both osmotic and matric potential. But in terms of field sensors, there aren’t any that give both components.
- Water potential - Soil moisture release curves - WP4C - HYPROP - Volumetric Water Content - GENERAL - LABROS We monitor soil moisture using water content. How can we integrate this into a soil moisture release curve?
One of the best ways to do this is to take some samples and measure the soil moisture release curve for that soil, generating a functional relationship. Then you can take that curve and use your water content values to set your irrigation points through your release curve function. Another option is modeling it. If you know some information about the soil type and pedology, there are pedotransfer functions you can use by inputting those variables, and it will predict a soil moisture release curve. This method is not as accurate, but it’s a possible option.
- Water potential - Soil moisture release curves - WP4C - HYPROP - Volumetric Water Content - GENERAL Which depths should I consider for active roots in maize for irrigation management?
You can refer to the literature for rooting depths of maize. As for the sensors, we recommend a combination of TEROS 12 soil moisture sensors and TEROS 21 matric potential sensors to get the whole picture.
- Water potential - Soil moisture release curves - WP4C - HYPROP - Volumetric Water Content - GENERAL - LABROS What modeling programs can you use to model soil moisture release curves?
There are a few different models out there to model soil moisture release curves. ROSETTA is a program from the US Salinity lab that has been around for a long time. Hydrus is another tool that can be used to model soil moisture release curves. One thing to remember is that these models don’t take into account all of the factors that can change a soil moisture release curve. So if you decide to model your soil moisture release curve, remember they aren’t perfect.
- Water potential - Soil moisture release curves - WP4C - HYPROP - Volumetric Water Content - GENERAL - LABROS Now VWC trends are used to determine field capacity and stress onset. Is this a more accurate method than water potential?
This is one approach to take. The issue with using water content measurements is you have to wait until you observe stress occurring to make this type of set point. We recommend a physical water potential measurement as a better way to determine a stress set point. As for field capacity, you can still use the physical measurements to set your field capacity point. The most important thing to understand is that the traditional -33 kPa point for field capacity is not a good rule of thumb to follow. Read why here.
- Water potential - Soil moisture release curves - WP4C - HYPROP - Volumetric Water Content - GENERAL - LABROS How do you develop a soil moisture release curve in highly variable soils?
If you have a site with highly variable soils you will need to generate a curve for each individual soil type. One approach would be to map out the site and select the soil types of most importance and then create soil moisture release curves for those soils.
- Water potential - Soil moisture release curves - WP4C - HYPROP - Volumetric Water Content - GENERAL - LABROS Do private chemical labs carry out soil water retention curve analysis? Or just university labs?
There aren’t many private labs that offer retention curve services, however, METER does offer soil moisture release curve services. Find it here.
- TEROS 21 - Water potential - Soil Moisture Sensor - WP4C - HYPROP - GENERAL What is matric potential?
Matric potential is the force that would need to be exerted to move a water molecule from the surface of a soil particle. For example, a matric potential of -100 kPa would require a force of -101 kPa to pull that water molecule off of the soil particle. It is one component of the total water potential. Learn more about the different components of water potential here.
- WP4C What are the main differences between WP4, WP4-T, and WP4C?
The WP4, first model, doesn’t have a couple of features of the newer dewpoint water potential models. The second model, WP4-T, has temperature control of the sample. The third model, WP4C, in addition to temperature control of the block, has improved accuracy in the wet range by being able to resolve temperature differences of 0.001 degrees between the sample and the mirror. The WP4-T can only resolve temperature differences of 0.01 degrees between the sample and the mirror. This results in an improvement in accuracy of 0.5 MPa in the WP4C. The range of the WP4C has also been extended to -300 MPa.
- HYPROP - LABROS How do I connect more than one HYPROP sensor to HYPROP View? Connect only one HYPROP sensor at a time to check the Device ID number when you purchase a new sensor. Each HYPROP should have a unique Device ID. Change the Device ID if it is the same number as your other HYPROP sensor. To do this, select “Show Devices”. Right click the sensor, and click Change Device ID.(Note: Older HYPROP software may not be compatible with newer sensor units. If needed, upgrade the HYPROP View software).Use the Help Wizard to see each step with pictures (or view this video).
- WP4C - GENERAL - LABROS How do you convert MPa to pF?
You can convert MPa to cm of suction by dividing MPa by -9.787×10-4. pF is then the log base 10 of cm of suction.
- WP4C - LABROS Can the WP4C be used to measure the water potential of plant leaves?
Yes. Please refer to our detailed application note.
- WP4C - LABROS How can I use the WP4C to make a soil water retention curve?
- HYPROP - LABROS Why does the scale not record the weight during the measurement?
There are two possibilities:
- The scale was not switched on. The scale will be detected by the software but cannot log any data. When the scale is switched on, the weight appears in its display.
- The scale is being used in a wrong mode and therefore cannot log the weight. In Single Balance Mode, there is no automatic measurement of the weight. The weight changes need to be taken one by one and recorded by the software. In Multi Balance Mode, the weight changes will be recorded automatically. Review the section “Measuring” in the user manual for more information.
- The default settings on the scales were changed. Check to ensure that the scale settings match the defined settings shown in the user manual (section “Preparing a Measurement” under “Default Settings”).
- HYPROP - LABROS How do I complete a HYPROP measurement?
You can finish a measurement in three ways:
- You can stop when the top tensiometer shaft has reached the cavitation phase (see illustration 1). Then you will not use the air entry point.
- If you are willing to use the air entry point. Then there are two possibilities:
- The air entry point of the top tensiometer shaft has been reached and the bottom tensiometer shaft is still in the regular measurement range (phase 1) or in boiling delay (phase 2). In this case, the HYPROP can calculate the median value of the top tensiometer and bottom tensiometer curve (see illustration 2).
- When the air entry point of the top tensiometer shaft has been reached and the bottom tensiometer shaft is still in the cavitation phase (phase 3), the median value of the two cannot yet be calculated. If this is the case, wait until the air entry point of the bottom tensiometer shaft has been reached (see illustration 3).
- HYPROP - LABROS What four phases should I see if the tensiometer shafts and sensor unit are properly degassed?
Phase 1: Regular measurement range
In phase 1, the tension value increases without flattening until it reaches the boiling point of water.
Phase 2: Boiling delay phase
In phase 2, the tension value increases to the boiling delay area (above ambient air pressure). This is nice to have, but in general not necessary for the evaluation.
Phase 3: Cavitation phase
In phase 3, water vapor is generated in the tensiometer shaft, and then the tension value drops abruptly down to the boiling point. After this, the tension value decreases only slightly.
Phase 4: Air entry phase
In phase 4, the tension value drops abruptly to zero as air penetrates the ceramic. The air entry point is a material characteristic of the ceramic and amounts to about 8800 hPa (880 kPa). This point can also be used for the evaluation.
- HYPROP - LABROS Why does my HYPROP read 4000 hPa (400 kPa)?
The pressure sensor may be damaged, so the sensor unit should be checked. Contact METER Group support or your local dealer. The sensor unit will likely need to be sent in for repairs.
- HYPROP - LABROS What should I do if the software does not find any sensor units in the Single Balance Mode?
Disconnect the sensor units one by one and have the software show the device tree. Check the device tree to see if one or more sensor units have the same address. Review the section “Preparing a Measurement” in the user manual for information about changing the sensor unit address.
- HYPROP - LABROS What can cause the bottom tensiometer to “overtake” the top tensiometer?
The tensiometer shafts may have been switched around. There is no need to interrupt the measurement since you can correct this in HYPROP-FIT.
- HYPROP - LABROS What do I need to check if data stops recording in the middle of a measurement?
Check the cable connection to the USB port. Change the energy management of your computer to Continuous Operation (which is typical when using a laptop). If using a USB hub, make sure it is properly powered.
- HYPROP - LABROS What if the tension value exceeds atmospheric pressure (e.g., 1000 hPa or 100 kPa)?
This is not a problem, and it is a result of proper degassing and causing a boiling delay. This means you can extend beyond the “normal” measuring range of the HYPROP.
- WP4C - LABROS What measurement mode should I use to read my samples?
It depends on the expected water potential range of your sample. Very dry samples (< -40 MPa) can be run in fast mode with no loss of accuracy. Precise mode should be used for optimum accuracy of samples up to ~ -0.50 MPa. Continuous mode is recommended for wetter samples that require extreme temperature equilibrium for maximum precision.
Please note that the time to completion is not determined in continuous mode; the user must determine when the reading levels off and the sample has reached equilibrium.
- KSAT - LABROS Why does the fitted falling-head curve not match my KSAT data?
There can be a variety of reasons for this:
- If your sample is not mounted properly, it might be not tightly sealed at its base. If this is the case, the water pressure will not approximate the value of zero hPa at the end but will tend to go to a negative value.
- Solution: Remount the sample properly.
- NOTE: In KSAT early releases, a bottom plate was used that sometimes failed to provide a tightly sealed connection to the sample, particularly if steel cylinders were scratched or dirty. The plate was replaced in summer 2015 by a new plate with a soft rubber seal. Only this updated plate should be used to ensure a tight connection between sample and dome.
- In some soils, particularly of loamy texture, almost all water passes through a very small part of the soil sample (i.e., through macropores). Water flow in these macropores becomes turbulent if the pressure gradient becomes too large. If this is the case, the water flow is no longer proportional to the pressure gradient. Consequently, the change of the hydraulic head with time is not exponential, and Darcy’s law is not valid. If this is the case, the exponential function will not fit the data: the fitted function will be less curved than the experimental results. Also, you will notice in such cases that the smaller pressure heads give a larger calculated conductivity.
- Solution: Under very small gradients, flow still might be laminar. So, repeat the measurement with a small gradient (i.e., an initial pressure head < 5 cm).
- Soils are fragile porous systems, and their permeability might change during the measurement process. There are different reasons for this:
- a. If flow takes place primarily through macropores, these might erode during the measurement process (i.e., conductivity increases). This will lead to a result similar to #2, however, the effect (increasing conductivity) will be lasting.
- b. Due to preferential flow, macropores can become sealed by sediment particles. In this case, conductivity will decrease during the measurement process. This will be indicated by an apparent misfit of the exponential function, but in this case, the fitted exponential curve will be more curved than the data.
- The offset of your pressure transducer might not be equal to zero. You may have a temperature drift if all components of the measurement (i.e., KSAT, used liquid, and soil samples) were not equilibrated at the same temperature.
- Solution: Equilibrate all components to the same temperature, and perform the offset recalibration before the measurement.
- If your sample is not mounted properly, it might be not tightly sealed at its base. If this is the case, the water pressure will not approximate the value of zero hPa at the end but will tend to go to a negative value.
- KSAT - LABROS It’s best to measure saturated hydraulic conductivity in the field, as this covers the entire pore system of a soil. How can you measure Ks (Kf) with only a soil core?
Many research institutions still measure Ks (Kf) with samples, but field data is always better. If using a soil core, it is necessary to have five replicates to be sure open paths do not falsify the result. Compare the results. If one or two have much higher Ks results, don’t average those in, but average only those readings with lower values. The high conductivity data may result from open paths (pores), which were cut on the top and bottom of the soil core but which are more or less passive in the field.
- KSAT - LABROS Which fluid should I use for my experiments?
Do not use distilled water. In sandy soils, the ionic composition of water is not a big concern, but in fine-textured soils, the width of the diffuse double layer will be greatly affected by the ionic strength and ionic composition of the water. Furthermore, the use of distilled water and its monovalent anions can disperse the sample, greatly reducing its saturated conductivity. In general, we recommend using water with a similar ionic composition to the soil under investigation. This is not always easy to determine. In practice, standard tap water is used in most cases, and it is even better if you can specify the ionic strength. For some investigations, particularly with soil that can undergo dispersion, we recommend an electrolyte solution with bivalent cations (e.g., a 0.01 molar solution with calcium as cation). In addition, always use water that is the same temperature as the lab environment where you perform the measurements.
- KSAT - LABROS How does KSAT calculate the temperature correction to obtain the saturated conductivity at the specified reference temperature?
KSAT uses the temperature dependency of the viscosity of water to recalculate the reference conductivity (at your specified reference temperature) from the measured value (at the measured operation temperature). Details are specified on page 11 in the KSAT operation manual (available as a pdf from the Help menu in the KSAT software).
- KSAT - LABROS Does saturated mean that all soil pores are filled with water?
No! But this is also not the case in the field.
- KSAT - LABROS I cannot measure conductivities because all water passes through the sample before the automatic measurement even starts.
The upper limit of the range of measurable conductivities with KSAT is about 10000cm/d. In this case, the initial water level passes through the sample in about 5 seconds, which is close to the temporal resolution of the KSAT data acquisition. You might try using the Restart Measurement button to manually initiate the data recording immediately after opening the valve. This can slightly accelerate the recording of the first data point and help to push the upper measurement limit slightly higher.
- KSAT - LABROS When is my measurement finished?
Your measurement is finished automatically if either a minimum total pressure head (parameter H_end_abs) or a minimum relative pressure head (parameter H_end_rel) is reached, which is related to the initial pressure head. The default setting is that water percolates until the level goes down to 25 % of the initial value. You can change this setting in the parameter menu. The default values are very conservative. Often, measurements can be stopped much earlier. You can do this anytime by pressing Stop Measurement. As a rule of thumb, measurement can be stopped:
a) if the calculated conductivity becomes a stable value. This means that a sufficient number of measured data have been recorded (> 10) and that the signal shows a clear trend, and
b) if r² is high enough (r² > 0.999).
For samples with low permeability, a decrease by 1 cm pressure head is normally sufficient to stop the measurement. For example, a sample with a conductivity of 2 cm/d will take about 8 hours to reach 0.25 of its initial pressure head. In practice, you can start with 20 cm initial head and stop when reaching 19.5 cm (either manually, or by setting H_end_rel = 0.975), which occurs after approximately 15 minutes.
- KSAT - LABROS Can I visualize my data externally?
Yes. All your data and all parameters are written into an ASCII file in the csv format. You can use these data in order to re-visualize the measurement and the fitted curve with your own visualization software.
- HYPROP - LABROS What can cause the tension value to reach 500 – 700 hPa (50 – 70 kPa) and then drop?
This can happen if water in the tensiometer shaft was not degassed well enough. Check the degassing process to ensure you are reaching full vacuum (above 85 kPa depending on your elevation). This can also happen if the red O-ring that seals the tensiometer shaft is not sealing properly. Check for damage, and if necessary, replace the O-ring (review “Cleaning and Maintenance” in the user manual).
- HYPROP - LABROS What steps should I take to remove bubbles from the tensiometer shaft?
Repeat the refilling step for tensiometers, and use degassed water. If bubbles remain in the shaft, check for leakage around the shaft, and fix the leak if one is found. You may need to replace the red O-ring of the tensiometer shaft.
- WP4C - LABROS What causes long read times in my WP4C?
Contamination of the sample chamber is the primary cause of long read times. The WP4C relies on equilibration of water vapor in the chamber with the sample. A dirty sample chamber can have samples that adsorb or desorb water vapor. This can lead to longer read times but is usually rectified by a good cleaning.
Unstable temperatures can also be a problem. Take care to provide a stable temperature environment for your WP4C and to keep your samples close to the temperature at which you intend to read them.
- HYPROP - LABROS How can I combine my WP4C data with data from my HYPROP®?