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Perspective from the Field on the MPD Infiltrometer
February 2013 (volume 8 - issue 2)
Contributed by Anthony Vecchi, Undergraduate Research Assistant, St. Anthony Falls Laboratory, University of Minnesota (Principal Investigator: John S. Gulliver)
Beginning May 2012 I have worked as an Undergraduate Researcher under Dr. John Gulliver. My experience thus far has dealt with measuring the saturated hydraulic conductivity (Ksat) using a Modified Phillip Dunne (MPD) Infiltrometer. This device is a falling head, single ring infiltrometer known for its simple design and ease of use. I was a primary user of the MPDs this past summer, working with teams of undergraduates, graduate students, and even some practicing engineers to measure Ksat. Through my experience with these tools I have gained an interesting perspective on their usefulness in measuring Ksat.
The device itself, for those unfamiliar, is made of a thick plastic cylinder with an easily removable steel base. A ruler is oriented vertically along the wall of the cylinder for easy head measurements. The device is pounded into the ground by placing a steel driver on the base and hitting the driver with a hammer. Once perpendicular with the ground, the plastic cylinder can be easily fit back into the base. A stopwatch is necessary to take the measurements over time. The MPDs used during the aforementioned project are easily kept in a plastic bin. The relatively light design means that they are easy to transport, which facilitates having to haul them in and out of a truck several times a day! To use the MPDs, one needs to measure initial and final moisture content (at the time I started on the project), bulk density, and time the column of water in the cylinder incrementally as it infiltrates.
Beginning in May 2012, these MPDs were used several times a week for approximately six months to measure the saturated hydraulic conductivity (Ksat) in roadside swales along highways throughout the Twin Cities Metropolitan Area. The simplicity of our technique allowed measurements to be taken at about 20 different locations in a 60 ft by 30 ft section of a swale in roughly a three-hour time period. As one can imagine, during this six-month period I was able to provide input to Dr. Gulliver and Ph.D. student Farzana Ahmed that would help to refine the experimental technique. In May and June we were using small soil samples taken before and immediately after the MPD was removed to determine the moisture before and after the test. This technique was found to be inaccurate in the lab and difficult to manage in the field. The new, and current, technique is to gather approximately ten samples with a 2 inch diameter corer (previously used exclusively for bulk density) from random locations around the swale section being tested. From these samples we could gather information about the bulk density of the soil, initial moisture content, porosity and therefore the estimated final moisture content by assuming the soil was fully saturated.
Figure 1: Measuring saturated hydraulic conductivity with MPD infiltrometers in an urban swale.
The new technique provides a simple and powerful way to determine the Ksat, a value otherwise difficult to determine. This allows us to take many samples in a swale, which is important due to the very high variation in Ksat within the swale. For example, at one MPD the column of water may completely infiltrate (approximately an initial head of 40 cm) while another MPD only five feet away may only have the column of water drop 10 cm in three hours. This variation is common, so the more MPDs that can be used the more accurately we will be able to estimate a mean Ksat. Taking few measurements at a single site can lead to inaccurate results for the swale as a whole, because both particle size and structure of the soil are important for infiltration. Although the soil structure averaged over an entire swale may be more or less uniform, at the scale of a few feet there can be significant variation. The MPD Infiltrometers have been vital to the progress of the project in taking many tests each time we go to a new site.
Figure 2: MPD infiltrometers are used in close proximity to each other due to the very high variability in Ksat.
As an undergraduate student with no previous experience with infiltration measurements, I was amazed at how easy the MPDs were to use. In a matter of days I felt confident enough (driving them into the ground, putting them together, storing them, reading them) to be able to teach someone how to use them. While the technique is not perfect and is affected in some way by the variation within the swale, the use of MPDs has expedited the completion of the project while allowing us to test several swales that were not initially planned for testing. The MPD Infiltrometer is a great example of using science to create something that accomplishes a complex task in a simple way.
In a matter of days I learned what a Modified Phillip Dunne (MPD) Infiltrometer can be used for, why it is used, and how to effectively use it. This simple tool aids in the measurement a soil property that is far from simple. After using MPD’s for six months I have begun to take the physically tough design for granted. I have likely driven these devices into the ground hundreds of times, hitting thousands of rocks, and taken countless measurements with little to no change in functionality of the MPD.
We want to hear from you!!!
Let us know your thoughts, experiences, and questions by posting a comment. To get you thinking, here are a few questions:
- What are your experiences with measuring Ksat?
- Have you used MPD Infiltrometers? What was your experience?