The Pendulum Swings
(Golf Course News International, July 2001)
Within the short space of two months saturated soils and waterlogged surfaces have dried out leaving parched grass cover wherever there is no irrigation.
Every year in Britain during the 14-16 weeks commencing about the middle of May quick drying out can threaten the playing surfaces on golf courses and sportspitches. It is essentially the sudden dramatic increase in evapotranspiration accompanying the warmer weather and reduced rainfall that causes this stress. While automatic irrigation systems offer the solution their sophistication and limitations are the root of many problems facing the course manager.
With most golf greens and tees installed with sprinklers covering the entire area water shortage should not be a problem. In fact, generally most greens receive too much water as no measure is made of the amount of water lost daily in the summer and the ‘safe’ approach is to apply more than enough – which can be detrimental to grass cover.
Gauging the Need
Irrespective of the weather, the moisture content in greens and tees should be monitored with a probe regularly during the summer to gauge the moisture in the upper 100mm. At this depth the moisture regime is critical. It can be quickly assessed and an immediate decision can be made whether to irrigate in the evening or leave it another day. However, probing in one location can be misleading as moisture content varies within a green and some areas are prone to dry out more quickly.
With the undulating nature of green surfaces, water run-off after irrigation restricts the amount of water entering the higher points and increases the amount collected in the lower areas. This is a common problem even on new courses with highly permeable rootzones. Accumulating thatch and surface compaction promote surface run-off irrespective of the rootzone composition. Repeat cycles in the same night irrigating for short periods can improve water penetration, but increasing the irrigation period only makes the wet areas wetter while the high points remain dry. Independently operating sprinklers do not overcome this dilemma and the only real solution once water will not enter the soil is to hand water these areas after spiking or aquatine forking and applying a wetting agent. In fact, the longer areas remain dry, the more they repel water as the tension with which the little remaining water is held to the soil particles increases and there is no continuous water film to allow water movement into the green. The phenomenon of dry patch can also be quick to develop.
Limitations in Irrigation Systems
Properly designed, an irrigation system around greens should incorporate sprinkler heads spaced so as to give even precipitation over the entire area. With almost head to head spacing even coverage is generally assured, but wind can dramatically distort the water distribution. In windy locations sprinkler placement should allow for prevailing winds and close-by quick coupling points for hose connections are vital when hand watering is necessary.
In addition, where the operating arc of sprinklers employed around the green varies significantly, nozzle sizes should be adjusted to ensure even precipitation throughout the green over a set period. Alternatively, independently working sprinklers set to cover a larger arc (almost full circle) must operate for longer periods than sprinklers covering smaller arcs (small part circle) to deliver the same precipitation. As uneven water distribution often occurs it is important to occasionally measure the contents of collecting cups placed throughout the green.
Nozzle performance, the pressure at the sprinkler head and its rotation are key factors that determine the performance of sprinklers. These should be checked regularly as each can have a significant influence on the distribution of water. Different elevations on the golf course too can affect sprinkler performance and it is the task of the irrigation designer to ensure pipe sizing and pump performance is adequate.
Applying the Right Amount of Water
The objective is clearly to apply only the amount of water needed for healthy growth. Surplus water is either lost into the drainage system or is retained in heavier soils at the expense of air which is a limiting factor in many greens.
Without gauging the amount of water in the rootzone, programming can be ineffectual. Daily applications of scarcely two millimetres precipitation (approximately five minutes) after a hot day can eventually result in a serious depletion of water in the area of root growth and drying out at this depth is far more critical than at the surface. On the other hand, applying in the region of 4mm daily will build up excess water in the rootzone and hasten the reduction of air, vital for healthy growth.
No one can argue that grass cover and putting performance is best a few days after good penetrating rains when the surface has had time to partially dry out. Developing a programme that ensures moist (not saturated) conditions at root depth without constantly wetting the thatched surface creates a healthier turfgrass environment and better putting conditions.
Unfortunately, over watering is common in Britain as the actual water need in the upper 100mm is seldom properly gauged. In addition, with staffing limitations hand watering is often not feasible and the only practical solution is to ensure that the greens have more than enough water. This becomes a recipe for disaster with anaerobic conditions developing and grass cover suffering.
Getting the Best from an Irrigation System
No matter how efficient the system, any restriction in infiltration or movement of water within the soil soon reduces the benefits of applying water.
By virtue of its natural growth habit, turfgrass quickly develops a relatively impermeable thatch layer and concentrated wear, often on wettened surface layers, results in increased compaction. Together, these factors can make it difficult to maintain healthy growth with a planned irrigation programme. Regular aerating and thatch control measures alone are important but they become essential considerations during the summer if the best is to be obtained from the irrigation system without over watering incessantly.
Maintaining a moisture balance or account in the rootzone can be a valuable and interesting exercise even though moisture conditions vary in a green and from one location to another. Logically, converting the irrigation period to an actual measured precipitation in millimetres is necessary – especially as rainfall is measured this way and evapotranspiration losses in millimetres can be roughly estimated using a formula incorporating temperature and wind measurements. With this data readily available it is futile to have no knowledge of the irrigation measurement in millimetres when trying to control the moisture regime and limit watering to the minimum.
Finally, what quantity of water should golf courses use over these critical 14-16 weeks every summer? This becomes an important question when we realise that in the not too distant future the use of potable water from a mainline supply will be severely restricted and alternative water sources will have to be sought. I believe that if properly managed an 18 hole golf course with irrigation on greens, approaches and tees should use less than 10000m3 (2 million gallons) in a year – but I feel sure that many exceed this quantity.