The summer of 1997 has been a rather wet one and this has obviated the need to use irrigation systems on a regular basis. However, the recent dry spell, albeit a short one, has seen sprinklers in full flow and this is always the signal for vibrant debate among club members on irrigation in general.

The debate is wide-ranging and includes questions such as whether there is a need for irrigation systems at all. Didn't golf courses survive for decades without such artificial aids? The effect on water tables and also the effect on botanical composition of irrigated areas is frequently debated.

Suffice it to say that irrigation systems are essential if golf courses are to be maintained in acceptable condition during the summer months, ensuring that water tables are not influenced to any great degree, and that proper use of irrigation will most definitely not alter botanical composition. It must also be conceded that it is possible to overwater golf courses, greens in particular, but it is also possible to unduly stress turfgrass by underwatering.

The greenkeeper is responsible for ensuring that neither scenario manifests itself at his golf course. To be successful, it is necessary to fully understand the requirement for water and also soil-water relationships. This is a complex matter and space does not allow all matters to be dealt with in one article, so I'll expand on the matter next month.

Plants require water for survival and the amount required varies with species. Water is required for photosynthesis (the process by which plants manufacture food), is used as a conveyor of nutrients throughout plant tissue, confers elasticity to the plant, and also helps to regulate temperature. In short, plants which are totally deprived of water will die.

Water is supplied naturally by rainfall and artificially by irrigation. It is lost from the root zone through drainage and by a process known as evapotranspiration. The latter is a term used to describe the combined losses brought about by direct evaporation from the soil and by transpiration through the leaves of plants. Both processes are driven by solar heat. In winter, water accumulates in the soil as rainfall always exceeds losses; in summer, losses can exceed rainfall and moisture deficits can occur.

In Ireland, the long term average rainfall varies from 750mm on the East coast to 1600mm on the West coast. Winter rainfall is of little interest as regards irrigation - the April to September figures are the important ones! These range from 350mm to 700mm for the period in question, quite a large range. So, irrigation requirements vary with location. Potential evapotranspiration for the soil is a much more constant figure with the range confined to 400mm along the coast. It is useful to have these figures. However, it should also be remembered that the losses referred to are averages and there can be considerable variation on a daily basis.

Comparing gains with losses, it is evident that over the growing season the deficit is not huge and the figures even suggest that western areas would have no requirement for irrigation. However, it must be borne in mind that these are six month figures and do not take account of variations in rainfall patterns. It is quite normal in this country to experience three weeks of hot dry weather followed by a similar period of very wet weather.

Approximately 50 per cent of soil comprises mineral and organic matter, the balance is air space (pore space). A measurement of the amount of pore space is called porosity. This figure varies with soil type.

Pore size ranges from quite large spaces, macro pores, to microscopic sized spaces. Again, soil type influences the range. These pores line up into channels and through these water moves to the water table. But not all water drains away.

Following extremely heavy rainfall, all pore spaces are filled with water and the soil is said to be at saturation point. If this situation were to remain for long periods, plant growth would be restricted as plants require air in the rootzone. When rainfall ceases, water moves out of the large pores rapidly and eventually all macro pores are empty of water. The water in the root zone is then confined to the smaller pore spaces and the soil can hold this water against gravity. When this point is reached, the soil is said to be at "Field Capacity".

This is an important term for greenkeepers. It matters little how much artificial drainage is in place, the water held at this level can only be removed by plants or evaporation. In the absence of further precipitation, soil moisture levels fall and plants begin to struggle for water. "Wilting Point" is reached when plants wilt during the day in an effort to conserve water. Grasses turn brown and, again, this is a survival tactic. Green colour is caused by Chlorophyll and without this pigment photosynthesis ceases and this results in a saving of water. At "Wilting Point," there is water for survival, but eventually "Permanent Wilting Point" is reached and, when this happens, plants die. The soil will still contain some water, but it will be unavailable to plants.

Therefore, at Field Capacity, there is plenty of water available for plant growth. At Permanent Wilting Point, there is none. Soils must be retained at a level somewhere between Field Capacity and Permanent Wilting Point, and, for quality turfgrass, between the two.

It has been predicted that summers are to become hotter and drier in the future. As this happens, the greenkeeper will become more and more reliant on his irrigation system. I'll deal with how to ensure that the proper system is installed and how to use it to the best advantage of the golf course in next month's article.