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Frequently Asked Questions

Frequently Asked Questions

KSAT

  • Hydraulic conductivity - HYPROP - GENERAL - KSAT - MINI DISK INFILTROMETER Can you refer me to a reliable measurement method to estimate and predict unsaturated hydraulic conductivity?
  • KSAT - LABROS Why does the fitted falling-head curve not match my KSAT data?

    There can be a variety of reasons for this:

    1. 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.
    2. 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).
    3. 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.
    4. 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.
  • 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.