The four experimental greens in Wooster, Ohio, received simulated rainfall to allow comparisons of root-zone construction methods and determine water-flow trends below ground.

Water content Water requirements

Although greens with no gravel drainage layer require greater amounts of rainfall to initiate steady outflows from drainworks, they don't necessarily require more water than USGA greens to fully "charge" the root zone. Research on water behavior in root zones offers insights into the water quantities required to leach salts from putting greens root zones. For the greens with no gravel drainage layer, maximum moisture content reached about 37 percent in this study, whereas USGA-style greens reached only about 31 percent.

Root zones composed primarily of sand-sized particles are the preferred growing media for putting greens. Sandy root zones provide superior playing conditions by generally meeting the agronomic demands of turf. The overarching factor dictating the use of high-sand root zones is that compaction does not reduce their permeability (1).

Profile design can improve the performance of sand-based root zones. A putting green soil profile consists of the putting surface, the sandy root zone, a subsurface drainage system and perhaps the underlying native subsoil. There are two popular types of putting green profiles: those that employ an underlying gravel layer, and those that do not.

Profiles with a gravel layer are often generically referred to as USGA profiles because they generally reflect the recommendations of the USGA (6). Profiles that do not employ such a layer are often referred to as "California profiles" because they generally reflect the recommendations of the University of California's "pure sand putting green" (2).

In experimental greens having otherwise identical root zones, the presence of the gravel layer, as in a USGA profile, yields greater maximum drainage rates and laterally more uniform soil moistures than does a profile without the gravel layer (5). The paths of water flow exiting the root zone are relatively short and principally vertical in green designs containing the gravel layer. Without this gravel layer, water must move laterally through the root zone for a substantial distance before reaching a drain line and exiting the system. Referring to this gravel layer as a gravel drainage blanket is clearly appropriate considering its major role in root-zone drainage.

During the course of our water drainage and redistribution study (5), we collected data on drainage outflow and soil moisture accumulation immediately after irrigation commenced. This research allows us to examine the influence of putting green profile design, root-zone composition, green slope and rainfall rate on irrigation requirements for charging the root zone with water and leaching excess salts.

Material and methods
This study employed four greens profile designs, including:

	A profile with no gravel drainage layer containing nine parts of sand and one part sphagnum
	A profile with no gravel drainage layer containing six parts sand, two parts compost and two parts topsoil
	A USGA-style profile (i.e., employing a gravel layer) containing nine parts sand and one part sphagnum
	A USGA-style profile containing six parts sand, two parts compost and two parts topsoil

The sand-sphagnum blend had a lab permeability of 20.8 inches per hour and was designated the "high-permeability" mix, whereas the sand-compost-topsoil blend had a lab permeability of 12.6 inches and was called the "low-permeability" mix. When tested by an accredited laboratory, both mixes met the particle size and performance criteria for a USGA root zone.

The high-permeability mix, although not a pure sand, met recently proposed performance criteria for a California root zone (4). The four greens construction treatments were each replicated three times for a total of 12 experimental greens. The turf was a 15-month-old Penncross creeping bentgrass (Agrostis palustris) stand maintained at ³/16 inch.

The rectangular experimental greens were 4 feet by 24 feet, with drain lines 15 feet apart at 2 and 17 feet from one end. The drain lines 2 feet from the end fed tipping-bucket rain gauges to monitor drainage outflow. Additionally, each root zone had soil moisture probes at three depths (3, 6 and 9 inches) and five locations (2, 7, 12, 17 and 22 feet), for a total of 15 probes per green, allowing continuous monitoring of soil moistures.

The set-up allowed monitoring of water drainage and soil moisture accumulation within the root zone as influenced by profile style, root-zone composition, green slope and rainfall rate. The overall study was conducted as a series of 18 individual experimental runs. During an experimental run, one green from each replication was sloped by zero, 2 or 4 percent.

Each green received simulated rainfall of at least 1.9 inches per hour for three hours to ensure a constant drainage rate. Rainfall measurements were collected during this period. Drainage outflow was measured every five minutes, and soil moisture was measured every 20 minutes for the duration of the rain event.

The tipping-bucket data were converted to graphs of cumulative outflow vs. time after rainfall commenced for each treatment combination in the study. From these graphs, we determined the times when drainage outflow started and when a constant drainage rate occurred. These times were converted to rainfall depths by multiplying by the measured rainfall rate. These rainfall depths and corresponding soil moistures were examined for treatment effects.

Results and discussion
For greens without any slope, outflow occurred after about 1.2 inches of simulated rain had fallen, regardless of profile design or root-zone composition. Greens with no gravel drainage layer sloped at 4 percent, however, required slightly less rain to initiate outflows, and sloped USGA-style greens required slightly more.

This general similarity between greens disappeared when we estimated the rain depth needed to achieve constant drainage: The USGA-style greens required more than 2 inches less than greens with no gravel drainage layer. And as slope increased from 0 to 4 percent, rain depths needed for constant drainage rate increased as well.

These results may be interpreted by examining mean soil moistures in the various systems at selected times during the rainfall period. With the exception of the greens with no gravel drainage layer containing the low-permeability root zone, all greens had similar moistures of about 21 percent by volume before the rainfall simulation began.

As the steady rainfall continued, soil moisture increased in the root zones until it was sufficient to initiate drainage from the greens. This moisture content was about 30 percent by volume in all greens except in the no-slope, low-permeability profile with no gravel drainage layer (which required slightly more soil moisture to produce steady outflows) and the no-slope, high-permeability USGA profile (which required slightly less moisture).

The greens were not at their maximum wetness at this point, however, but continued to build water content as the simulated rainfall continued. When the greens achieved a constant drainage rate, the rainfall inputs were balanced by drainage, and the greens were at their maximum moisture content. For the greens with no gravel drainage layer, this state occurred when mean water contents were about 37 percent, regardless of root-zone composition or slope. For the USGA-style greens, maximum moisture content was about 31 percent, with slightly higher values in low-permeability root zones and with greater slope.

The greater water contents for the greens with no gravel drainage layer were principally the result of greater soil moisture accumulation between drain lines, which delayed constant drainage as more water accumulated. Finally, the effect of slope is simply explained by the greater distance water must travel to reach a drain line in the 4-percent vs. the zero-percent slope configuration.

The rainfall required to fully "charge" a putting green root zone would seemingly equal the amount required to achieve maximum water content and constant drainage. In our study, this ranged from about 6 inches for a green with no gravel drainage layer with a high-permeability root zone at 4-percent slope to 2 inches for the unsloped USGA-style greens. Of course, if pre-rainfall moistures had been drier, greater rain depth would be required to charge the root zone, and greater beginning moisture would have meant less rainfall was needed.

We can't assume, however, that a green with no gravel drainage layer is "charged" only when steady outflows occur. Such greens continued to drain over a 48-hour period, with a steady reduction of soil moisture occurring midway between the drain lines. This root-zone moisture was thus "lost" to the turf and is in excess of what's required to replace moisture that is evapotranspired. Consequently, the rain depths needed to charge a root zone with no gravel drainage layer might be similar to the requirements of a USGA-style green.

Many superintendents judge the irrigation depths needed to charge a root zone by fitting an inspection port on the main drain exiting the green and observing outflow during irrigation. When outflow just reaches maximum, the root zone is judged fully charged. This study suggests that this would be a useful tool for a USGA green. On the other hand, this approach may overestimate the needs of a green with no gravel layer, leading to excess application of water that ultimately drains from the green and therefore becomes unavailable to the turf.

The rain depth required to achieve constant drainage also represents the amount of rain necessary to initiate leaching of the root zone. It is only when this depth of water is applied that the entire root zone, regardless of location relative to a drain, is contributing flow to an outlet. The depth of rainfall required to completely replace root-zone moisture when the root zone is at its maximum water content (see table) was determined by calculating the total water held in the root zone between the drain lines and dividing by the corresponding area of the experimental green.

Compared to the USGA-style greens, the greens with no gravel layer required about 0.85 inches more rain to completely leach their root zones -- or to equal a leaching fraction of 100 percent (3). Further, root-zone composition and slope had little influence on this leaching depth in the greens with no gravel layer, whereas increasing slope and use of the low-permeability root zone served to increase leaching depths for the USGA-style greens. The difference between profiles is again because of higher water contents of the greens with no gravel layer when the root zones have achieved a constant drainage rate.

Putting the information together implies that, to achieve 100 percent leaching fraction for a green with some existing moisture, one needs to add the rain depth required for constant drainage to the depth required to replace root-zone moisture. Thus, in this study's conditions, a USGA-style green with a high-permeability root zone and 4-percent slope needed 3.4 + 3.8 = 7.2 inches of rain to leach the root zone completely. Of course, leaching requirements are often a smaller percentage of the leaching fraction than what was estimated here and can best be determined by soil testing. Consequently, the values we estimated to "replace root-zone water" (see table) would typically be multiplied by this smaller percentage (10 to 25 percent) to determine the actual leaching requirements.

Clearly, the results of this study depend on existing moisture, root-zone permeability and slope and cannot be adopted at face value when deciding water depths to charge or flush a root zone. They do, however, provide some general guidelines for understanding greens behaviors that might aid the superintendent in water management.

Literature cited

1. Adams, W.A., V.I. Stewart and D.J. Thornton. 1971. The assessment of sands suitable for use in sportsfields. Journal of the Sports Turf Research Institute 47:77-86.
2. Davis, W.B., J.L. Paul and D. Bowman. 1990. The sand putting green: construction and management. Publication No. 21448. University of California Division of Agriculture and Natural Resources.
3. Harivandi, M.A., J.D. Butler and L. Wu. 1992. Salinity and turfgrass culture. p. 207-229. In D.V. Waddington, R.N. Carrow and R.C. Shearman (eds.) Turfgrass Agronomy Monograph 32. ASA-CSSA-SSSA, Madison, Wis.
4. Hummel, Norman W., Jr. 1998. Which root zone recipe makes the best green? Golf Course Management 66(12):49-51.
5. Prettyman, G.W., and E.L. McCoy. 1999. Subsuface drainage of modern putting greens. USGA Green Section Record 37(4):12-15.
6. USGA Green Section Staff. 1993. The 1993 revision, USGA recommendations for a method of putting green construction. USGA Green Section Record 32(2):1-3.

Guy Prettyman recently received his master's degree in soil science from Ohio State University. Ed McCoy, Ph.D., is an associate professor of soil science at Ohio State University. The authors wish to acknowledge the GCSAA and USGA for their support of this research project.