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 ring infiltrometers, water at atmospheric pressure is allowed to infiltrate soil. Once the rate of infiltration has stabilized, this steady infiltration rate is then measured and used to compute the saturated hydraulic conductivity. However, because water is ponded on the soil surface with ring infiltrometers, a large portion of the water might infiltrate through cracks or wormholes, and thus result in very large saturated hydraulic conductivity values, which are not representative of 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, but will infiltrate through the soil matrix. As a result, the measurements obtained with a tension infiltrometer are more representative of the soil as a whole.
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 should obtain an infiltration rate close to the saturated hydraulic conductivity of the soil. Thus, the tension infiltrometer can also be used to estimate the saturated hydraulic conductivity.
There are a number of methods that can be used to calculate the hydraulic properties from the tension infiltration data. One method is based on the assumption of a log-linear relationship between tension and hydraulic conductivity, as first described by Gardner (1958). This is a valid assumption for the optimum tension range of the infiltrometer. This method can be found in the literature, and is described in detail in the manual that comes with the SMS infiltrometer.
Other methods to calculate the hydraulic properties from the infiltration data use inverse parameter estimation methodology to calculate the van Genuchten parameters (Simunek et al. 1994). A computer program by Dr. Simunek to calculate hydraulic properties from tension infiltrometer data is available from SMS.
Before placing the infiltrometer on the site where a measurement is to be made, the site is leveled and cleaned of debris. A ring (either 8 or 20 cm in diameter- to be ordered separately from SMS) is placed on the leveled 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 leveled 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.
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.
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 infiltration rate measured with the infiltrometer. A continuous record with frequent readings is important if one is interested in the early, transient infiltration behavior.
Water level data can be recorded with a Campbell Scientific (www.campbellsci.com) datalogger or similar device. Dataloggers that provide a constant exitation 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.
Frequently asked questions:
Question 1: How can I test my infiltrometer for leaks?
1. Separate the disc from the infiltrometer tower.
2. Close the tubing clamp on the bubble tower and close the bottom outlet of the water tower with a stopper.
3. Inflate the infiltrometer to about 60 to 100 cm water pressure (60 mbar to 100 mbar).
4. Hold the complete unit under water and check for leaks.
5. Next, check the infiltrometer disc for leaks (Note: Before installing the mesh screen material, make sure the disc is free from small particles. These may cause leaks. Install a new mesh screen membrane if necessary).
6. Connect 1/4" tygon tube (60 cm long) to the outlet in the center of the disc.
7. Immerse the disc and tube in a dishpan full of water. The tube should be completely full of water. Make sure there is no air under the membrane or in the tube.
8. Close the open end of the tube with a tubing clamp or with a small stopper.
9. Remove the disc with attached tube from dishpan.
10. Now turn the disc, so the screen is facing up.
11. Position the tube so the end of the tube is at the same level as the top of the screen. Open the tube and slowly lower the end of the tube. Watch if air bubbles appear below the screen. Air bubbles should start appearing when the open end of the tube is 25-30 cm below the level of the screen. This is the bubbling pressure of the nylon membrane.
12. If air bubbles appear when the tubing outlet is less than 20 to 25 cm below the screen level, then there is a leak in the screen. Replace the screen, making sure that no loose particles are lodged between the screen and the screen support or between the o-ring and the screen.
Question 2: Should I calibrate my tension infiltrometer?
You can, and probably should calibrate it. However, all SMS tension infiltrometers are made using the same diameter tubing, and thus the calibration for all tension infiltrometers should be approximately the same. To start with, you can set the tube in the bubble tower such that its outlet is 4.0 cm below the desired tension. For example, if a tension of 5 cm is desired at the level of the membrane, move the tube in the bubble tower up or down till its outlet is at 9.0 cm below the water level in the bubble tower. After you have had some experience with the tension infiltrometer, you may want to check its calibration.
Question 3: What is the difference between the 8-cm model and the 20-cm model?
The 8-cm model is smaller and uses less water. It can also be used in a smaller space. However, the disc surface is considerably smaller than the disc surface of the 20-cm model, causing greater variance in the measurements. The 8-cm model is good for measurements between crop rows, and is also very good for teaching purposes. This model can further be used to control the tension on top of soil columns in the laboratory.
Question 4: Can I replace the membrane in the field?
Yes this can be done quite easily. Remove the old membrane after loosening the holding ring. Wet the new membrane (this makes it much easier to install), place the new membrane over the disc, replace the holding ring, tighten the screw, and you are done.
Ankeny, M.D., T.C. Kaspar, and R. Horton. 1988. Design for an automated tension infiltrometer. Soil Sci. Soc. Am.J. 52:893-896.
Ankeny, M.D., M. Ahmed, T.C. Kaspar, and R. Horton. 1991. Simple field method for determining unsaturated hydraulic conductivity. Soil Sci. Soc. Am. J. 55:467-470.
Casey, F.X.M. and N.E. Derby. 2002. Improved design for an automated tension infiltrometer. Soil Sci. Soc. J. 66:64-67.
Gardner, W.R. 1958. Some steady state solutions of unsaturated moisture flow equations with application to evaporation from a water table. Soil Sci. 85:228-232.
Hussen, A.A., and A.W. Warrick. 1993. Algebraic models for disc tension permeameters.
Water Resources Research 29:2779-2786.
Logsdon,S.D. and D.B. Jaynes. 1993. Methodology for determining hydraulic conductivity with tension infiltrometers. Soil Sci. Soc. Am. J. 57:1426-1431.
Messing, I. and N.J. Jarvis. 1993. Temporal variation in the hydraulic conductivity of a tilled clay soil as measured by tension infiltrometers. Journal of Soil Science 44:11-24.
Perroux, K.M. and I. White. 1988. Designs for disc permeameters. Soil Sci. Soc. Am. J. 52:1205-1215.
Simunek, J., T. Vogel and M.Th. van Genuchten.1994. The SWMS-2D code for simulating water flow and solute transport in two dimensional variably saturated media. Version 1.2. Res. Report 132. U.S. Salinity Laboratory, USDA-ARS. Riverside, CA.
Reynolds, W.D. and D.E. Elrick. 1991. Determination of hydraulic conductivity using a tension infiltrometer. Soil Sci. Soc. Am. J. 55:633-639.
Wooding, R.A. Steady infiltration from a shallow circular pond. 1968. Water Resour. Res. 4: 1259-1273.
The Wooding infiltrometer is an automated ring infiltrometer, designed to determine the infiltration rate and/or the saturated hydraulic conductivity of field soils, as well as of soil in laboratory columns.
With the Wooding infiltrometer, water at atmospheric pressure is allowed to infiltrate into soil. Water is held inside a 15 cm diameter soil ring, pushed less than 0.5 cm deep into the soil. The infiltrometer is placed on top of the ring, and used to maintain a head of water of 1 cm inside the soil ring. As water infiltrates the soil, the water level in the infiltrometer tower decreases. This decrease in water level with time is recorded, either with a pressure transducer connected to a data logger or manually. From these data the depth of water entering the soil in a given time is calculated. This information is then used to compute the saturated hydraulic conductivity of the soil. The infiltrometer can be used on larger soil columns and in the field.
For soil columns having an inside diameter of 15 cm, water flow is one-dimensional. However, because the cross sectional area of the water tower (45.4 cm2) is smaller than the cross sectional area of the soil column (176.7 cm2) a 1 cm drop in the water level in the water tower causes a 0.257 cm depth of infiltration. Thus, to get the final infiltration rate, one has to multiply the drop in water level in the water tower (observed during the later part of the measurements in a given time period) by 0.257 to get the final infiltration rate.
In the field, water flow out of the soil ring is three dimensional, and thus additional information is needed to calculate the hydraulic conductivity. We use the solution Wooding developed for 3-D infiltration from a circular ponded area, and further assume that initially cumulative infiltration is proportional to the square root of time. Initial and final water contents of the soil in the ring also need to be measured. The infiltration data and water content data are then entered into Wooding’s equation to get the saturated hydraulic conductivity of the soil.
The method itself and the data analyses are straightforward. However, in actual practice the method greatly benefits from continuous data collection with a datalogger. Suitable data loggers are available from Soil Measurement Systems or Campbell Scientific, Inc.
Wooding, R.A. Steady infiltration from a shallow circular pond. 1968. Water Resour. Res. 4: 1259-1273
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