Lately I have fielded questions about irrigation strategies and drainage characteristics in putting greens built with sand over gravel and drainage and sand over native soil and drainage. This update provides an easy-to-read summary of the differences in these two construction models. Additionally, the perched water table concept and leaching versus deep watering is also discussed.
The perched water table: A sand over gravel system like the USGA green provides a reservoir of water for the turf to use and allows the profile to still drain well. Contrary to some belief, water continually flows from the sand through the gravel due to gravity and adhesion forces from the gravel – i.e., water flow does not entirely stop at this interface. In a sand over native soil profile, the water may not flow into the native soil depending on the native soil characteristics. If the subsoil has very poor water permeability, water will accumulate in the rootzone in between the drain lines and drainage therefore is less uniform than when using a sand over gravel design.
A coarse sand will have a shallower perched water table than a finer-textured sand. However, the coarser and rounder the gravel (less adhesive forces), the deeper the perched water table. If the depth of the sand is not greater than the height of the perched water table, the profile remains chronically wet. This situation is commonly observed in bunkers when using the durable liners with only 4-5 inches of sand. In low-lying areas such as near the front of greens, deepening the rootzone to 14-16 inches has proven useful to raise the sand depth well above the height of the perched water table. The deeper sand depth in low areas can be beneficial in a sand over gravel or sand over native soil construction method because it will decrease the tendency for these low areas to hold excess moisture.
Flushing salts versus deep watering: The “ideal” way to flush salts is to replace the water in the rootzone with irrigation water. When additional water is applied, the added weight of the water will help break the tension at the sand and gravel interface and expedite flow through the gravel. If the goal is to wet a 12-inch rootzone, a considerable amount of water must be added. Some superintendents will use portable, low-precipitation-rate sprinklers to optimize leaching events and avoid adding water to green surrounds and bunkers. You can calculate the amount of water needed to leach fairly easily. If you know the total porosity from a soil physical test, let’s say it is 45%, multiply 0.45 by the depth of the rootzone in inches. A fairly new green may have a 12-inch rootzone and at 45% porosity would need about 5.4 inches of water to leach the entire profile. For older courses, the rootzone is often 16-18 inches, but the porosity could be lower at maybe 40%. An 18-inch rootzone with 40% porosity would need about 7 inches of water to entirely leach the profile. If these calculations make your head hurt like mine does, then use a 4- to 5-inch guideline for a good leaching event and ideally you will apply this water slowly, over at least a 10-hour period (1/2 inch per hour).
Deep watering does not require the same amount of water as a leaching event. In a deep-water event, you are trying to fill the rootzone with water, not replace all the water in the rootzone. A 12-inch rootzone dried to 20% volumetric water content will still contain 2.4 inches of water (12 X 0.20). If the porosity of the rootzone is 40%, the green can hold about 5 inches of water, so in this case you would need to add about 2.6 inches of water to fill the rootzone. Again, it is not necessary to make these calculations – a good guideline is to apply 1 to 2.5 inches of water during a deep irrigation event. These events wet the profile and help to reestablish greater moisture consistency across the entire green.
There are many different putting green construction methods that can be managed to deliver good performance. While there are pros and cons of each method, I strongly encourage using science (soil physical testing and the advice of a trained laboratory professional or USGA agronomists) when selecting construction methods and advice on irrigation management. Please reach out if you have any questions on the water relations in different putting green construction mixtures and designs.
West Region Agronomists:
Brian Whitlark, senior consulting agronomist – bwhitlark@usga.org
Cory Isom, agronomist – cisom@usga.org
Information on the USGA’s Course Consulting Service
Contact the Green Section Staff