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Enabling better global research outcomes in soil, plant & environmental monitoring.

Tension Infiltrometer

The SMS tension infiltrometer is designed to add water to soil at a range of tensions, which can be set by the operator of the instrument. By performing infiltration experiments at multiple tensions, one obtains data on the unsaturated hydraulic conductivity at the various tensions.

The range of tensions that can be set is (for practical reasons) limited to tensions between 0 and –30 cm H2O. By setting the tension at or close to zero, one could obtain an infiltration rate that approximates the saturated hydraulic conductivity.

Features:

  • On-site determination of hydraulic properties
  • Disc can be separated from the water tower or mounted under the water tower
  • Requires low volume of water
  • Adjustable tension settings
  • Flow rates are determined from changes in the water level inside the water tower
  • Flow rates can also be determined from changes in the water level recorded with a datalogger, connected to a pressure transducer and attached to the water tower
  • Polycarbonate and acrylic materials
  • Available with nylon or stainless steel mesh screen membranes
Specifications

20 cm Model

8 cm Model

Diameter Disc

20 cm

8 cm

Inside Diameter Water Reservoir

5.1 cm

2.54 cm

Inside Diameter Bubbling Tower

2.54 cm

2.54 cm

Length Water Reservoir

81 cm

81 cm

Bubbling Pressure Membrane

30 cm H2O

30 cm H2O

Carrying Case Dimensions(optional)

28cm x 36cm x 107cm

28cm x 36cm x 107cm

Pressure Transducer

1 psi (67 cm H2O)

1 psi (67 cm H2O)

Tension infiltrometers are designed to measure the unsaturated hydraulic properties of soils. Water is allowed to infiltrate soil at a rate, which is slower than when water is ponded on the soil surface. This is accomplished by maintaining a small negative pressure on the water as it moves out of the infiltrometer disc into the soil.

In contrast, the saturated hydraulic conductivity of surface soils is often determined with single or double ring infiltrometers. With single or double ring infiltrometers, water at atmospheric pressure is allowed to infiltrate soil, initially fast, and at a slower rate once the open spaces in the soil fill up with water. When the rate of infiltration has stabilised, the infiltration rate is measured and used to compute the saturated hydraulic conductivity. However, because with ring infiltrometers water is ponded on the soil surface, a good portion of the water might infiltrate through cracks or wormholes, and a reduced amount will infiltrate through the soil matrix. By maintaining a small negative pressure (or tension) on the soil as water is infiltrating, water will not enter the large cracks or wormholes as much, but will infiltrate through the soil matrix. When not all soil pores, cracks and wormholes are filled with water, the soil is considered unsaturated.

Field Set-up

Before placing the infiltrometer on the site where a measurement is to be made, the site is levelled and cleaned of debris. A ring (either 8 or 20 cm in diameter- to be ordered separately from SMS) is placed on the levelled surface.  The area within the ring is filled with fine sifted sand from the testing site or with silica sand (a few mm in thickness). The sand is levelled carefully and the ring removed. A perfect flat surface, 8 or 20 cm in diameter, is formed for placement of the infiltrometer. The sand layer should result in good contact between the base of the infiltrometer and the soil below.

For the 20-cm infiltrometer only, make sure the area where the water tower is going to be placed should be at the same elevation as the top of the sand layer for the disc. One can accomplish this by adding or removing soil from this area. It may be convenient to use a carpenters level to make sure these two surfaces are at the same level. Prepare the spot for the water tower using the carpenters level to ensure the correct height of the water tower, before removing the soil ring.

By raising or lowering the tube in the bubble tower, the tension that will be maintained at the bottom of the base plate can be set. The maximum tension is generally less than 30 cm. Many researchers start with the highest tension (often 20 cm). One should note that at the highest tension, the hydraulic conductivity is the lowest, and thus it may take some time for the instrument to start “bubbling”. If it takes too long for bubbles to appear one may want to reduce the initial tension. Some investigators moisten the soil surface with a very fine spray.

Data Collection

The SMS infiltrometers are designed to collect data manually or automatically. Data are collected manually by recording the water level in the supply tower over time. One simply reads the water level in the supply tower at fixed time intervals (i.e. one minute: more frequent early on and less frequent during the steady state phase), and records the information together with the time passed since the start of the experiment.

For multiple sites, it is advantageous to use a datalogger to record the data. The water level in the supply tower is then recorded by installing a 1 psi (66 mbar) differential pressure transducer connected to a suitable datalogger. The pressure difference between the air in the top of the water tower and the water near the bottom of the water tower is recorded. With such a pressure transducer system, the effects of air bubble induced noise are reduced. As the water level in the water tower decreases (as it infiltrates into the soil), the negative pressure in the water tower becomes less negative. Thus, the pressure transducer output is linearly related to the water level in the water tower. Output recorded from the pressure transducer therefore provides a continuous record of the volume of water entering the soil as a function of time. A continuous record with frequent readings is important if one is interested in the early, transient infiltration behaviour.

Water level data can be recorded with a datalogger or similar device. Dataloggers that provide a constant excitation voltage (between 2.5 and 12 volts DC) to the transducer and record the millivolt output from the SMS pressure transducers with an accuracy of at least 0.1 millivolts can be used.

Ankeny, M.D., Ahmed, M., Kaspar, T.C. and Horton, R. 1991, ‘Simple Field Method for Determining Unsaturated Hydraulic Conductivity‘, Soil Science Society of America Journal, vol. 55, pp. 467-470.

Ankeny, M.D., Kaspar, T.C. and Horton, R. 1988, ‘Design for an Automated Tension Infiltrometer‘, Soil Science Society of America Journal, vol. 52, pp. 893-896.

Bagarello, V. and Iovino, M. 2003, ‘Field Testing Parameter Sensitivity of the Two-term Infiltration Equation using Differentiated Linearization‘, Vadose Zone Journal, vol. 2, pp. 358-367.

Bodhinayake, W., Si, B.C. and Noborio, K. 2004, ‘Determination of Hydraulic Properties in Sloping Landscapes from Tension and Double-Ring Infiltrometers‘, Vadose Zone Journal, vol. 3, pp. 964-970.

Casey, F.X.M. and Derby, N.E. 2002, ‘Improved Design for an Automated Tension Infiltrometer‘, Soil Science Society of America Journal, vol. 66, pp. 64-67.

Fuentes, J.P., Flury, M. and Bezdicek, D.F. 2004, ‘Hydraulic Properties in a Silt Loam Soil under Natural Prairie, Conventional Till, and No-Till‘, Soil Science Society of America Journal, vol. 68, pp. 1679–1688.

Logsdon, S.D. and Jaynes, D.B. 1993, ‘Methodology for Determining Hydraulic Conductivity with Tension Infiltrometers‘, Soil Science Society of America Journal, vol. 57, pp. 1426-1431.

Perroux, K.M. and White, I. 1988, ‘Designs for Disc Permeameters‘, Soil Science Society of America Journal, vol. 52, pp. 1205-1215.

Rachman, A., Anderson, S.H., Gantzer, C.J. and Thompson, A.L. 2004, ‘Influence of Stiff-Stemmed Grass Hedge Systems on Infiltration‘, Soil Science Society of America Journal, vol. 68, pp. 2000-2006.

Ramos, T.B., Gonçalves, M.C., Martins, J.C., van Genuchten, M.Th. and Pires, F.P. 2006, ‘Estimation of Soil Hydraulic Properties from Numerical Inversion of Tension Disk Infiltrometer Data‘, Vadose Zone Journal, vol. 5, pp. 684-696.

Reynolds, W.D. and Elrick, D.E. 1991, ‘Determination of Hydraulic Conductivity using a Tension Infiltrometer‘, Soil Science Society of America Journal, vol. 55, pp. 633-639.

Wang, D., Yates, S.R. and Ernst, F.F. 1998, ‘Determining Soil Hydraulic Properties using Tension Infiltrometers, Time Domain Reflectometry, and Tensiometers‘, Soil Science Society of America Journal, vol. 62, pp. 318-325.