The increasing restrictions upon the use of potable water for irrigating golf courses are a concern in many parts of the world. The most up-to-date irrigation systems with built in weather stations are a positive move towards better water conservation. But, asks Gordon Jaaback, consultant agronomist, are they successful? – And are course managers fully aware of all the factors that determine the planning of an irrigation programme? An old golf iron shaft with a notch ground out in the bottom 75mm for use as a soil probe is an invaluable tool that should be used daily in the critical weeks of an hot summer
Daily light sprinkling
Applying light sprinkling for five to ten minutes daily during the hot summer is a pretty common approach. If part-circle sprinklers around a green of 500 sq. m. deliver about 2.4 l/sec for 5minutes every day – provided the distribution pattern is even – this would apply approximately 2mm precipitation. Does anyone believe that this amount of water can penetrate the thatch normally expected on a golf green? – and what proportion of that 2mm is lost by evaporation and so is unavailable to the turfgrass plant?
Water losses can be categorised in three ways:
- water penetrating the rootzone and exiting through the drainage system
- water lost by runoff, particularly at the time of quick sharp downpours or when irrigation extends beyond the period when infiltration is possible
- water lost by the increased evapotranspiration(ET) resulting from daily watering. This quantity can for exceed the amount in 1 and 2 above
A simple water balance
Monitoring a water balance in the green is a logical consideration if you want to keep pace with daily performance – much the same as monitoring a bank balance of your funds. However, this can become very technical.
The recording of daily rainfall requirement – particularly the intensity – is vital. It is important here to gauge the effective rainfall after allowing for runoff. Actual ET losses are often not correctly assessed by weather stations but even without this equipment rough approximations can be adequate.
The precipitation applied by irrigation can be satisfactorily approximated for the time spent sprinkling. For practical purposes and without getting involved in regular weather measurements the following are some basic assumptions that can give a good idea of the water regime to the depth of the roots.
- available water reservoir to a root depth of 75mm is in the region of 9mm
- the daily evaporation lost in the summer or hot days greater than 20Â°C can amount to between 2.5 and 3.5mm
- with part-circle sprinklers around a green operating for five minutes the precipitation is in the region of 2mm if there is even distribution.
- effective or penetrating rainfall can be estimated:
- up to 4 to 6mm gentle rain is effective and penetrating
- greater than 6mm per day, allow 20% loss by runoff
- quick sharp storm allow 80% runoff
At least with appreciation of these estimates a good idea of the moisture regime to the depth of the roots is possible. More importantly it gives an indication of whether there is a build up of excess water or a chronic shortage is developing.
Note: For those enthusiasts who are interested enough to calculate accurate ET values and record rainfall – particularly intensity – the writer would be happy to prepare a simple water balance table and assist in obtaining accurate calculations with the use of accepted formulae.
Planning water use
This is cannot be an exact science. Generally the greenkeeper is the one person who through experience has the best idea of the different conditions on the course. Parts of a green react differently to others because of undulations and variation in shade and exposure. Nevertheless, a base assessment of a standard green gives a good point to start from.
Knowing the depth of moisture to the depth of the roots is vital in planning water application. Even with a wealth of experience it is essential to probe regularly with a simple auger to gauge the moisture content. The USGA some time ago introduced the novel idea of an old golf iron shaft with a notch ground out in the bottom 75mm for use as a soil probe. This tool should be used daily in the critical 16 to 18 weeks of the hot summer over different parts of the green in order to assess the water regime in the critical top 75mm.
The top portion of the green can even be drying out so long as the rootzone depth is adequately supplied with water. A dry zone below wet thatch must be avoided – that we know is the beginning of dry patch. Simply if the soil flows out of the notch like sand it is far too dry and dry patch is imminent. If wetness is clearly evident on your fingers when pressing the soil in the notch then the soil is far too wet.
The soil in the notch should be just moist and not wet. Technically a water supply half way between field capacity and wilting point is ideal – but the trick is to keep it that way, as this supply can be short lived if daily losses are neglected when making applications.
Duncan Kelso, course manager at Kings Hill Golf Club in east Kent, adopts the principle of applying half the weekly ET losses in sufficient amount to get adequate penetration provided the expected rainfall – effective that is – has fallen. Keeping record of this rainfall is therefore vital in his planning of water applications.
The turfgrass plant shows little visual change in appearance with quite a variation in the water content in the soil. Logically it is not necessary to maintain a higher water regime than required and apart from the increasing cost and diminishing availability of potable water application should be limited to the essential need to maintain a healthy grass cover.
That means not too much but enough just to keep it actively growing. In fact grass cover offers a better putting surface when the plant is less succulent and a degree of hardiness is apparent. The plant is healthier too when the roots are extending down into the rootzone in search of water and not being supplied with a continual surface supply of water only.
With the uneven distribution of water in a green, water needs are not as easily assessed in the water balance. Quickly drying areas should be hand watered with the aid of spiking and the use of wetting agents. Increasing the irrigation period only makes wet areas wetter and the high points continue to shed the water.
Only water penetrating below the thatch depth is really effective. Aeration measures will naturally help to improve water penetration but the sure test must be to probe regularly.
With thatch accumulation, compaction and surplus organic matter in the top 50mm, penetration of water by sprinkler irrigation can only be achieved with repeat cycles if water runoff is to be avoided. The timing of each cycle can only be established with experience. As soon as runoff begins water is lost to the plant.
Sandy greens give the impression they can take in limitless quantities of water. That they can – but they cannot hold it and the surplus is quickly lost to the drainage systems below. They do not hold as much water as built-up greens but because of their higher permeability they have naturally no maximum intake – the more that is applied the more that is lost and the drying upper portion of the rootzone always appears the same.
The secret is to apply only the amount needed to penetrate the thatch to the depth of the roots below where a moisture regime persists indefinitely. It is particularly important on sandy greens to have an idea of the likely water balance after rainfall, irrigation and expected ET losses. Without this awareness a reservoir of about 9mm cannot give comfort when daily losses by ET can be between 2.5 and 3.5mm.
It should be clear that a sound knowledge of the movement of water in and out of the soil is essential and daily recording of rainfall and irrigation applications are vital if a healthy putting surface is to be maintained. During the hot summer months nothing is as important as the regular probing necessary to assess the moisture regime to the depth of the roots. In brief:
- It is very easy to over water during the summer. Yet in a week it is not difficult to gauge the approximate ET losses, the effective rainfall and the amount of irrigation applied. A simple calculation indicates whether there is too much or too little water to the depth of the roots.
- Indications of the water balance only give an idea in the basic condition. High or low points, shady or exposed areas must be judged accordingly from the base assessment.
- Remember there is only a reservoir of about 9mm of available water to the depth of the roots at 75mm – so it can be quickly over or under supplied.
- Water penetration remains the most important requirement. Adequate irrigation by repeat cycles if necessary and assessment of the effective rainfall are the most important considerations.
- Hand watering with the aid of spiking and wetting agents is the only way to overcome localised dry areas. Increasing the irrigation period can make the situation worse with continual shedding of water from the high areas and excessive water accumulation in the low areas.
Gordon Jaaback, consultant agronomist and project manager is based in Kent, England. Tel 01732 350351 firstname.lastname@example.org
Gordon Jaaback www.turfandgrass.com