The flow rate depends on many factors, such as the objectives of the experiment, the hydraulic and chemical properties of the soil in the column, and the time available for completing the experiments.For saturated columns, the highest flow rate used is often taken to be equal to the saturated hydraulic conductivity of the soil in the column. The lowest flow rate is often chosen equivalent to a flux of about 1 cm/day. Below this flux rate, transport experiments take much time for completion.For unsaturated experiments, a good initial choice for a flux rate would be 10 cm/day.To put the flux rate in perspective, note that the average recharge rate in the more humid areas of the United States averages about 30 cm per year, equivalent to a flux rate of 0.082 cm/day. Similarly, many irrigated areas have irrigation efficiencies of around 50% or less, although a few irrigation areas have irrigation efficiencies greater than 80%. If 120 cm of irrigation water is used in a year, and between 20% and 50% recharges to the groundwater, the mean annual recharge rate is 24 to 60 cm per year, equivalent to .0065 and 0.16 cm/day, respectively. Of course during the year the flux rate may vary greatly outside of these averages. However, the point is that average, real, recharge rates are low. In practice, it would take too much time to conduct experiments at these low flux rates, and thus a compromise low flux rate of 1 cm/day may be appropriate.
To obtain the low flow rates needed for unsaturated flow experiments, the syringe pump is designed to make one revolution at a time (one pump event). During such a pump event, one or more syringes (up to a maximum of ten syringes) will fill with fluid from a supply bottle and then will discharge the fluid onto the column. The motor, which drives the syringes back and forth, then waits for another signal to repeat the filling and discharging of the syringe(s). One such pump event takes about 9 seconds.The volume of fluid discharged per pump event depends on the size of the syringes used as well as on the travel distance of the syringe plunger. For example, a standard 3 ml syringe can be set to deliver approximately 0.75 ml per event, 1.5 ml per event, 2.25 ml per event, or 3 ml per event.A good initial leaching rate for unsaturated columns is 10 cm/day. Assume the inside diameter of the soil column is 7.6 cm and its surface area is 45.6 sq cm. At a leaching rate of 10 cm/day, a total of 456 ml of fluid would move through the column in one day. If the pump is set to deliver 1.5 ml per event, it would have to pump 456/1.5=305 times in a 24 hour day. This amounts to 1440/305=4.72 minutes, or 283 seconds. Thus set the pump at a pump interval of 283 seconds.If the fraction collector had been set such that effluent is collected for a total of 30 minutes in one test tube (i.e. at 30 minute time intervals), in 24 hours a total of 48 test tubes would be filled. The volume of effluent collected in each of these 48 test tubes is then equal to 456/48= 9.5 ml. However, if each test tube can hold only 10 ml, it may be safer to collect samples at 20 minute intervals (or 72 intervals per day), so that one collects only 456/72= 6.3 ml effluent per test tube. Thus when performing unsaturated column leaching studies, it is important to synchronize the pumping rate with the interval setting on the fraction collector, as well as the size of the fraction collector test tubes.For column leaching studies, the researcher often has flexibility in setting the leaching rate, which makes it possible to find a practical combination of pumping rate, column size and effluent collection interval. Once a suitable leaching rate is set, it should remain constant for the duration of the experiment. The syringe pump is designed to do this.Once the pump is set, the actual pumping rate can be determined by pumping fluid into an empty bottle, and then weigh the bottle with fluid over time.To check the flow rate during an experiment, one should weigh the volume of fluid collected from the column. After removing the test tubes from the fraction collector, one should weigh the test tubes with fluid as well as the empty test tubes. One could also determine the average weight of the empty test tubes before starting the experiment, and subtract this average weight from the test tubes with effluent.
This is best determined with tensiometers in the column. Ideally one should have nearly the same matric potential near the top and bottom of the column. The top matric potential is mostly a function of the column flow rate. Once the flow rate is established, determine the matric potential in the upper tensiometer and then read the matric potential in the lower tensiometer. If the lower matric potential is less negative (soil is wetter) than the upper matric potential, increase the vacuum in the chamber. If the lower matric potential is more negative than the upper matric potential, decrease the vacuum in the chamber. Continue adjusting the vacuum in the vacuum chamber until the upper and lower matric potentials in the column are approximately the same.
References:Gaber, H.M., S.D. Comfort, W.P. Inskeep, and H.A. El-Attar. 1992. A test of the Local Equilibrium Assumption for Adsorption and Transport of Picloram. Soil Sci. Soc. Am. J. 56:1392-1400.Risler, P. D., Wraith, J. M., and Gaber, H. M. 1996. Solute Transport under Transient Flow Conditions Estimated Using Time Domain Reflectometry. Soil Sci. Soc. Am. J. 60:1297-1305.Van Genuchten, M. Th., Wierenga, P.J. 1986. Solute Dispersion Coefficients and Retardation Factors. Klute, A. (ed.) Methods of Soil Analysis, Part I, 2nd ed. Agron. Monogr. 9, ASA and SSSA, Madison, WI p. 1025-1054.
The flow cells are designed for pressures up to 1 bar. However, the porous membrane has a bubbling pressure less than 1 bar, so the flow cell will start leaking if subjected to air pressures of 1 bar or more.
Yes, they can be used if they are relatively thin (less than 0.2 mm thick).
If many flow cells are required, SMS can make special flow cells to fit the non-standard cores.An alternate approach would be to place the non-standard cores inside larger flow cells, which are long and wide enough to accommodate the cores. This works well for determining water retention data.
Tubing connectors may be ordered with the flow cells. They can be screwed into the outlets, which are standard 1/8 inch NPT, or 1/4 inch NPT (National Pipe Thread) threaded outlets. Quick release connectors are recommended for hydraulic property determinations. They are especially useful when determining soil water retention (soil water release curves) curves. These connectors fit directly into the 1/8 or 1/4 inch NPT outlets of the flow cells.
Yes, tensiometers can readily be used. When ordering the flow cells, specify the number of access ports and tensiometers required, as well as the desired placement of the access ports in the flow cell wall (i.e. 7 cm from the top of the flow cell, 15 cm from the top of the flow cell, etc.). SMS column tensiometers may be ordered with a three-way valve to make it easy to fill the tensiometers with water, to readily remove all air from the tensiometers, and to attach the pressure transducers.
Yes, extra holes, either threaded or straight, in the top and bottom can be ordered.
Separate the disc from the infiltrometer tower.Close the tubing clamp on the bubble tower and close the bottom outlet of the water tower with a stopper.Inflate the infiltrometer to about 60 to 100 cm water pressure (60 mbar to 100 mbar). Hold the complete unit under water and check for leaks.Next, check the infiltrometer disc for leaks. (Note: Before installing the nylon mesh screen material, make sure the disc is free from small particles. These may cause leaks. Install a new mesh screen membrane if necessary).Connect 1/4" tygon tube (60 cm long) to the outlet in the center of the disc.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.Close the open end of the tube with a tubing clamp or with a small stopper.Remove the disc with attached tube from dishpan.Now turn the disc, so the screen is facing up.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.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.
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 itscalibration.
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 thlaboratory.
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.
References:Ankeny, M.D., T.C. Kaspar, and R. Horton. 1988. Design for an automated tension infiltrometer. Soil Sci. Soc. Am.J52:893-896.Ankeny, M.D., M. Ahmed, T.C. Kaspar, and R. Horton. 1991. Simple field method for determining unsaturated hydraulic conductivity. Soil Sci. Soc. Am. J55:467-470Casey, F.X.M. and N.E. Derby. 2002. Improved design for an automated tension infiltrometer. Soil Sci. Soc. J66:64-67.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 Researc29:2779-2786.Logsdon,S.D. and D.B. Jaynes. 1993. Methodology for determining hydraulic conductivity with tension infiltrometers. Soil Sci. Soc. Am. J57:1426-1431.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 Scienc44:11-24.Perroux, K.M. and I. White. 1988. Designs for disc permeameters. Soil Sci. Soc. Am. J52:1205-1215.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. RiversideCA.Reynolds, W.D. and D.E. Elrick. 1991. Determination of hydraulic conductivity using a tension infiltrometer. Soil Sci. Soc. Am. J55:633-639.Wooding, R.A. Steady infiltration from a shallow circular pond. 1968. Water Resour. Res. 4: 1259-1273.
The dual chamber models can be installed at depths of up to several hundred meters (500 feet or more). The single chamber models should be installed no deeper than six meters (about 20 feet).
Many researchers use sifted fine sand removed from the depth of lysimeter installation as contact material. If such sand is not available, silica flour may be used as contact material. It is best to add water to the contact material first, resulting in a slurry that can be poured down a plastic pipe temporarily placed in the access hole.
Model SW-074 requires a vacuum source (i.e. a vacuum pump) to draw pore water from the surrounding medium into the lysimeter, and from there up into a collection bottle, located on the soil surface.Model SW-071 can also be operated with just a vacuum source, if it is placed at a relatively shallow depth (i.e. less than 3 to 4 meter), and if a collection bottle is located on the soil surface, as for model SW-074.However, SW-071 models are usually operated such that the pore water is collected in the lysimeter itself. In that case a pressure pump (hand pump or electric pump) is needed to apply pressure to the vacuum/pressure tube of the lysimeter. This forces the water from the lysimeter up into the collection bottle. A good bicycle can be used for this purpose.The dual chamber models SW-070 and SW-070A need both vacuum (to draw the pore water into the lysimeter and into the upper chamber of the lysimeter), and pressure. Pressure is needed to force the pore water sample up through the fluid return tube to the collection bottle at the soil surface. For a lysimeter placed at a depth of 30 meters (98 feet) one needs an air pump that can deliver a pressure of at least 30 meters, equivalent to 3 bars or about 45 psi. Note that most bicycle pumps can deliver at least 100 psi or 6.6 bars.
If all tubing to the lysimeters is airtight and if all connections are airtight, one pump can maintain an approximate negative pressure of 300 mbar in many lysimeters. However, even small leaks can severely reduce the efficiency of the system. Thus great care is required in installing lysimeters in the field. Leak testing is done by applying positive pressure to the tubing and connectors, and by holding all connections under water. Appearance of air bubbles in the water is proof of leaks.
This pump can reduce the pressure in a 1 liter bottle to -100 mbar in 11 seconds. It takes approximately 30 seconds to reduce the air pressure in a 1 liter bottle to -200 mbar.
One should always place an overflow bottle in the fluid return tube right before the vacuum pump. This ensures that no water from the lysimeters can enter the pump. Water will damage the pump.
For permanent installations line powered pumps (110V or 220V) are preferable. The portable, battery powered pump is particularly suitable for intermittent pore water collection, and/or where suction lysimeters are widely dispersed in the field.
In principle, one can run up to four columns simultaneously with this fraction collector. To do this, one has to modify the test tube racks so that they advance one rack at a time (there are 17 six-position racks), rather than one position at a time. Furthermore, one has to modify the cover plate of the vacuum chamber, so that the four columns can drip simultaneously into four different test tubes. Although this is possible for small columns, it is not very practical for larger columns. The reason is that with this arrangement there are only 17 tubes for each column. If each of these tubes holds 10 ml effluent, then only 170 ml can be collected from each column before one needs to open the vacuum chamber to replace the filled tubes with empty ones. This is less than one pore volume for many columns filled with soil (note that the volume of one collected pore volume depends on the size of the column and its water content). Thus one has to open the vacuum chamber and replace the full test tubes frequently.
Soil Measurement Systems412 Olive Ave #125Huntington Beach CA 92648, USA