Infiltrometer Tests
by: Sebastien Fortin, E.I.T., M.Sc.
Tension Infiltrometer
Tension (disk) infiltrometers (TI) have been widely used for in situ determination of the field-saturated hydraulic conductivity (Kfs) of soils under near-saturated conditions in the vadose zone (Meiers, 2002). Near-saturated conditions refer to measurements made over the negative pressure range, -20cm to 0.0cm, where water contents are nearly as high as those for saturation (Reynolds and Elrick, 1991). Tension infiltrometers determine the steady-state infiltration rate into the soil through a porous plate on which a constant negative water pressure (tension) is applied.
As for the Guelph permeameter, the tension infiltrometer is also capable of giving estimates of several useful parameters for characterizing soil structure. Soil Moisture Equipment Corp. manufactures the Guelph Tension Infiltrometer Adapter (Model 2825KI), which is designed to attach directly to the GP reservoir (Figure 6).
Tension infiltrometers are especially useful for quantifying the effects of macropores and preferential flow paths on infiltration in the field. A new method has recently been established for determining the Kfs from the TI method (Reynolds and Elrick, 1991, Ankeny et al. 1991). In this method, sequences of steady infiltration measurements are taken at several tensions on a single infiltration surface are used for calculating the Kfs.
Figure 6. Components of the tension infiltrometer (left) and the GP reservoir connected to the TI (adapted from Meiers, 2002).
Principles
White et al. (1992) reviewed the use of tension infiltrometers and presented alternative methods of measurement and analysis of the resulting data. Figure 7 presents a schematic description of the tension infiltrometer and its components. All Mariotte-type instruments operate on the same physical principles. The major components of a TI are (i) the bubbling tower, which contains the air-entry tubes that control tension at the soil surface, (ii) the water reservoir, in which the water level falls as water flows into the soil, (iii) the porous base plate, which establishes hydraulic continuity with soil, and (iv) data logger and pressure transducers (optional). Tension (negative pressure) in the air pocket at the top of the water reservoir is linearly related to the height of water in the column (Ankeny, 1992), e.g. a 1mm drop in water level corresponds to a 1mm decrease in tension in the air pocket. Thus, cumulative infiltration can be monitored by recording tension changes over time.
Figure 7. Example of a single reservoir tension infiltrometer with conical roof in base (after Evet et al., 1999).
Measurements conducted with the TI require that the negative water pressure in the soil be transferred to the TI through the porous membrane, which requires a good hydraulic contact. A flat surface, free of debris or coarse particles, is favorable for making "good" contact with the TI membrane. In order to achieve these conditions, the soil surface is prepared by removing any occasional fragments (>2 mm) from the site or by locating a 20 cm diameter area free of coarse fragments (Meiers, 2002).
A support ring slightly larger than the TI foot assembly is placed on the ground. The bottom edge of the support ring is inserted about 0.5cm into the ground; this acts to support the contact material so a uniform thickness can be established (Reynolds, 1993). Once the foot assembly is positioned in the testing area and in good contact with the soil, the sides of the foot assembly must be sealed to the ground using a saturated soil paste made onsite with saturated material. When determining Kfs from a tension infiltrometer, the radius of the support ring is used for calculating the area of the infiltrating surface. The applied tension must be corrected when using a contact material. The following correction factor is given by Meiers (2002):
The use of a contact material is required in situations where the surface is irregular or not level. Contact materials should only be used where necessary since they can cause a difference between the pressure head set on the tension infiltrometer membrane and the pressure head at the soil surface (Azevedo et al., 1998). Contact materials shall have an air-entry value greater than the greatest suction being applied and a hydraulic conductivity greater than the material being tested (i.e. contact material is hydraulically "transparent"). Contact material, usually a silicate sand, should have a grain size distribution in the range of 53mm to 105mm (Meiers, 2002).
Poor contact results in poor data. The sand should have a conductivity greater than that of the soil being measured to avoid impeding flow. If too fine a sand is used, conductivities may be underestimated because of the impedance of the contact layer. Coarse sand, however, may not wet fully, which could also lead to underestimation of infiltration rates. If contact material conductivity is greater than soil conductivity, the maximum error in water potential at the contact-soil interface is the thickness of the contact layer itself. Therefore, the thickness of the contact layer should be kept to a practical minimum (Arkeny, 1992).
Summary of Field Procedures
The typical approach to infiltration measurements using the tension infiltrometer is briefly summarized here (adapted from Ankeny, 1992):
1. Soil surface preparation: The surface crust or top 10 or 20m is carefully removed unless the crust itself is being tested.
2. Ring insertion: A sharpened ring is pushed a short distance (~1cm) into the soil to define the area of the infiltration surface and prevent lateral surface flow of ponded water, and cheesecloth is placed in the ring.
3. Filling with contact material: If necessary, contact sand is added to fill the inside of the inserted ring and leveled.
4. TI installation: A Tension infiltrometer is centered over the sand-filled ring, and the legs of the device are pushed into the soil until contact is made with the sand. This step can be adapted when using the TI-kit to adapt to the Guelph permeameter.
5. Measurements: Measurements are typically made from low to high tension.
Note: The air contained in the porous base plate must be removed by submerging the apparatus in water for a period of 24 hours.
A complete step-wise description of the field set up and operating procedures is provided by Ankeny (1992).
The time required to reach steady-state in unconfined infiltration measurements depends on initial soil water content and on hydraulic properties of a given soil. In general, drier soil and lower hydraulic conductivity result in the need for a longer infiltration period in order to reach steady-state infiltration. The change in rate over time should therefore be monitored to confirm that steady rates are reached. Not reaching steady-state results in an overestimate of hydraulic conductivity (Kfs). Typically, Arkeny et al. (1990) suggest collecting data for 1000 seconds at each tension measuring from low to high tension under most condition. This methodology is normally adequate except for very dry and/or high bulk density porous media. In very porous or sandy soil, steady-state rates are reached much faster and times can be shorter. As a practical field guide, if a third of the 25,4 mm-diameter water reservoir has emptied, most likely more than enough water has been added to the soil wetting bulb for the infiltration rate to approach steady-state.
Analysis of Field Data
Wooding's equation for steady-state unconfined (three dimensional) infiltration rates is used in calculation hydraulic conductivities. The Kfs can be calculated by the following equation (Meiers, 2002):
Note: When using equation 12 (i.e. steady-state flow from two applied negative heads), the recommended procedure indicates that the lowest negative head (or lowest tension, e.g. -5cm) is applied first, followed by the greater negative head (e.g. -10cm) (Arkeny, 1992).
Consult the reference list on Tension Infiltrometers.
Forward to Single-Ring Infiltrometer.
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