Comparison of Putting Greens Constructed with Airfield Systems and USGA Designs
BY KEVIN J. MCINNES, KEISHA ROSE-HARVEY, AND JAMES C. THOMAS
Note: The information in this article has been adapted from the original work published in Crop Science titled “Water Storage in Putting Greens Constructed with United States Golf Association and Airfield Systems Designs” (McInnes and Thomas, 2011, 51:1261-1267) and in Hort Science titled “Water Flow Though Sand-based Root Zones atop Geotextiles” (Rose-Harvey et al., 2012, 47:1543-1547). The research was collaboratively funded by Texas A&M University, Airfield Systems (Oklahoma City, OK), and the United States Golf Association.
Airfield Systems offers an alternative to the standard USGA putting green design. Their design utilizes a highly porous, 1-inch deep plastic grid (AirDrain, Fig. 1) in place of a 4 inch deep gravel layer to allow for uniform moisture content of the root zone and rapid lateral movement of excess water to drains.
While voids in AirDrain are very effective in transmitting water, they are much too large for the sand in the root zone to bridge for self-support so a geotextile is used atop the grid to prevent infilling of the void space.
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Use of geotextiles in in putting green construction has been controversial due to the perceived potential for clogging of the fabric by migrating fine particles and eventual loss of permeability.
We became interested in the hydraulic performance of the Airfield Systems design after Texas A&M University constructed a soccer field with the Airfield System design in 2002, and subsequent anecdotal evidence from field managers suggested that the new field required less frequent watering than the University’s football field that had been constructed following the USGA design.
While the two fields were constructed with different root zone mixtures and the physical environments surrounding the fields are quite different, we suspected that there may be a difference in the amount of water stored in root zones on fields constructed with the two designs.
We knew from the physics of water in sand that the amount of water stored in a root zone decreases with increasing tension at the bottom of the root zone, and we expected because of the geometrical and physical differences in the designs that there would be differences in water tension at the bottom of the root zones.
To test this hypothesis, we constructed laboratory- based test cells from 4-inch diameter PVC pipe containing profiles of the Airfield Systems and USGA designs.
Using tensiometers, we were able to demonstrate that the tension that developed at the bottom of the root zone in the Airfield Systems design was less than that in the USGA design. At that point we thought it worthwhile to investigate this finding on a slightly larger scale and a more realistic setting. To this end, we constructed test greens in 14-inch diameter PVC pipe.
Three sands and three gravels were chosen such that they covered the ranges from coarser to finer sides of the USGA recommendations for particle size distribution. To create root zone mixtures, the coarser two sands had peat moss added to increase water retention. The finer sand was not amended. These three root zone mixtures were used in combination with the three gravels to construct test greens of the USGA design. The gravel layer in all of the test greens was 4 inches deep.
The same three root zone mixtures were used in combination with four geotextiles atop AirDrain to construct test greens of the Airfield Systems design. We used the Lutradur polyester geotextile prescribed by Airfield Systems at the time and chose three additional geotextiles that had the same apparent opening size (0.2 mm), but differed in material and/or manner of construction.
Manometer-tensiometers were used to measure pressure or tension that developed at the root zone-drainage layer interface of both designs (Fig. 2). After the test green columns were packed with 12 inches of the root zone mixtures they were sprigged with MiniVerde bermuda grass supplied by King Ranch Turfgrass-Wharton Farms, (Wharton, TX).
Following a period to grow-in the grass, a series of experiments were conducted that measured the amount of water stored in the root zone profiles and the water tension that developed at the bottom of the root zones of the different treatments after irrigation and drainage.
Vertically oriented time domain reflectometry TDR probes were used to measure the amount of water stored in the root zone profiles (Fig. 3).
The test greens were watered until drainage was observed and then the amount of water stored in the profiles and the water tension at the bottom of the root zones were recorded for 24 hours. As with our preliminary lab study, we found that the water at the bottom of the root zones of test greens constructed with the Airfield design was under less tension than the water in test greens constructed with the USGA design, by about 2.2 inches of water tension (Fig. 4). This lower tension was associated with an increase in water storage of about 0.5 inch in the Airfield System design greens above that in the USGA design greens (Fig. 4).
Because of reduced tension at the bottom of the root zone, these results also implied that the tension at which root zone mixtures should be tested for capillary porosity when intended to be used in an Airfield System design green should be reduced to achieve similar moisture retention to greens built according to the USGA recommendations. In doing so, slightly coarser sand would meet specifications for capillary water retention in the Airfield design.
Conversely, problems of having too high of retention could occur if a sand that pushes the fine side of the current USGA recommendations were used in the Airfield Systems design green. In addition, the results suggested that geotextiles can be used support a root zone atop gravel too coarse for the sand to bridge.
The question of whether or not geotextiles used in a green will clog with fines migrating out of the root zone appears to be behind infrequent use of geotextiles in putting greens. To address this issue, we conducted a year-long laboratory experiment to investigate a range of geotextiles that were suited to supporting sand in the Airfield System design and determine whether or not they limit drainage out of the root zone.
In this experiment, 6-inch diameter PVC columns were used to contain combinations of 12 inches of three sand mixes with 10 geotextiles held atop AirDrain (Fig. 5). Manometer-tensiometers again were used to measure pressure or tension at the sand-geotextile interfaces. Mix 1 had a particle size distribution that ran down the center of the USGA specs. Mix 2 was made by blending Mix 1 with a sandy clay loam (9:1) and Mix 3 was made by blending Mix 1 with a sand having excess fines (1:1). Mix 1 and Mix 2 met USGA recommendations. Mix 3 contained twice the recommended amount of very fine sand.
The apparent opening sizes of the geotextiles used ranged from 0.15 to 0.43 mm. After the sands were added to the columns they were regularly irrigated. Periodically, the rate that 1-inch of irrigation water drained from a column was measured. For the first six months, any particles that washed out of the sand through the geotextiles were accumulated and analyzed for total dry weight and particle size distribution.
At the end of the year-long study, saturated hydraulic conductivity of the sand-geotextile combinations were measured. Statistical analyses showed that drainage rate, saturated hydraulic conductivity, and mass of eluviated particles were not affected by the properties of the geotextiles, but rather by the properties of the sands.
The size distribution of eluviated particles also was dependent on the properties of the sand, but not the properties of the geotextiles (Fig. 6). Drainage rates from the columns containing the sand without added fines increased over the year, presumably because pore channels in the sand were widened when fines washed out of the profile. Drainage rates of the columns containing the two sands with additional fines decreased over the year, but the decrease was not statistically related to the properties of the geotextiles. It appeared that the sands themselves were clogging.
To test this, saturated hydraulic conductivities were measured as layers of sand were removed from columns. Since saturated hydraulic conductivity would not depend on the depth of sand in a hydraulically uniform column of sand, any observed changes would be due to difference in the conductivity of the layers removed compared to those remaining. We found that when surface layers were removed the saturated hydraulic conductivity increased, indicating that the surface layers had lower conductivities.
This was not too surprising as the majority of inter-particle pores of sand meeting USGA recommendation, as determined from the change in capillary water retention with change in capillary diameter (estimated from water tension), are smaller than the apparent opening sizes of the geotextiles we tested.
In conclusion, the results of our studies gave no reason to prevent more widespread use of Airfield Systems’ design as an alternative method for putting green construction. The data also support the use of properly sized geotextiles to support sand based root zones in putting greens. Geotextiles with apparent opening size of 0.2 mm worked well in our test greens and a woven geotextile with an apparent opening size twice as large (0.43 mm) performed satisfactorily in our laboratory experiments to test for potential clogging.
Figure 1. Airfield Systems’ highly pours, 1-inch deep AirDrain.
Figure 2. Test greens constructed in 14-inch PVC pipe with either gravel or geotextile atop AirDrain as the drainage materials. Both types of test greens contained a pair of porous cups connected to plastic tubing that formed manometer-tensiometers to allow measurement of water pressure or tension at the root zone-drainage layer interface.
Figure 3. Test green with vertically installed, 1-ft long TDR probe used to measure average water content within the root zone profile.
Figure 4. Range in the mean amount of water stored in 12-inch root zone profiles in Airfield Systems (Geotextiles atop AirDrain) and USGA (Gravels) design test greens. Means were of the three root zone mixture treatments and variation shown is from drainage-type treatments (i.e., type of geotextile or gravel). Stored water in the profile was measured by TDR and water tension was measured with manometer-tensiometers.
Figure 5. Columns used to measure potential clogging of geotextiles by fines migrating out of the root zone.
Figure 6. Size distribution of particles washed out of the three sand mixes through the geotextiles. Solid lines represent means and dashed lines represent one standard deviation each side of the mean.
Michael,
Dr. McInnes submitted an excellent article on the Airfield System for putting greens last month. Jim Moore and I both plan to print the article; however, we had it reviewed by some of our agronomists. Since it is a very technical article, we probably need to add a few sidebars with less technical language.
So, we are finally making some progress.
Thanks,
Mike
Michael P. Kenna, Ph.D.
Director, Green Section Research
United States Golf Association
The following comments on the Airfield systems article are offered below:
- I very much like the research conducted and it appears the evidence supporting the use of the Airfield systems is growing. I think the article will be challenging to read for superintendents, based on the technical nature of the article. I might suggest including some practical information on the Airfield system compared to the gravel system, such as the following:
- A diagram of each system showing the soil profile and individual layers used to build the system.
- Highlight the difference in the moisture retention capabilities of the Airfield system vs. the gravel. Mention the perched water table concept and explain the Airfield system will promote a more distinct perched water table and therefore may increase plant available water.
- A simple, practical paragraph on the geotextile fabric would be beneficial. I have heard from drainage engineers the geotextile fabric has been very successful and this study seems to confirm the fabric has a place in this system as well. Most in the industry have a different opinion. This is a good opportunity to change those views.
- Their design utilizes a highly porous, 1-inch deep plastic grid (AirDrain, Fig. 1) in place of a 4 inch deep gravel layer to allow for uniform moisture content of the root zone and rapid lateral movement of excess water to drains.
- Is the highlighted statement based on data? This statement indicates the Airfield system will provide more uniform moisture in the sand root zone, but is this actually true? The data in this article does not indicate the moisture is more uniform in the Airfield system.
- Using tension meters, we were able to demonstrate that the tension that developed at the bottom of the root zone in the Airfield Systems design was less than that in the USGA design.
- Following this sentence, this might be a good opportunity to explain to the layman reader that the root zone immediately above the Airfield system contains more water than the same root zone when constructed over gravel. Furthermore, this would be a good opportunity to mention the “perched water table” concept. The Airfield system will create MORE of a perched water table than the gravel because the gravel does have some tension where the Airfield system does not. I think the readers will find that very interesting.
- Because of reduced tension at the bottom of the root zone, these results also implied that the tension at which root zone mixtures should be tested for capillary porosity when intended to be used in an Airfield System design green should be reduced to achieve similar moisture retention to greens built according to the USGA recommendations. In doing so, slightly coarser sand would meet specifications for capillary water retention in the Airfield design.
If you have any questions please contact AirField Systems at 405-359-3775.