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Turfgrass irrigation – principles and practices

If naturally occurring rainfall is insufficient, irrigation is necessary. To decide how much to apply, we need to know the moisture content of the soil and how much water the turfgrass is using. Regular monitoring will support irrigation scheduling, indicating when and how much water to apply to maintain growth.

Water use

Evapotranspiration (ET) is the quantity of water transpired from the plant and evaporated from the soil surface. Reference evapotranspiration (ET_O) is derived from climate data and the use of mathematical models. It is necessary to adjust ET_O values to estimate turf ET (ET_T). We do this by multiplying ET_O by the crop coefficient, Kc:

ET_O × Kc = ET_T

The Kc that is frequently used for sports turf is 0.7.

For example, if the long-term climate data showed that ET_O for a particular day of the year was 4.5 mm, the actual evapotranspiration from the turf, ET_T, would be 4.5 × 0.7 = 3.15 mm.

This shows us that if 4.5 mm of water had been applied to the surface to make up for that day’s deficit, then the turf would have been overwatered, at a cost to the environment – use of a resource which is not needed, and potentially damaging soil structure – and a financial cost.

In most parts of Australia, evapotranspiration exceeds rainfall for most of the year. There will therefore be a water debt in the soil. Figure 1 shows this debt before the crop coefficient is applied.

Figure 1. Average monthly rainfall (green) and evapotranspiration (line) at ‘Tocal’, Paterson, NSW. This graph indicates that effective rain falls between February and June. The area above the green and below the line above it is the water deficit.

Soil water availability

All soils can hold a certain amount of water, dependent on texture, which will vary constantly with weather and crop use. A typical sandy soil may have a field capacity of 29% and a permanent wilting point of 19%. The amount of water readily available to the plant is therefore 10%.

If the plant can extract water down to 200 mm in the soil, then the amount of soil moisture available to the plant is 10% × 200 mm = 20 mm.

If we assume in this example a constant ET_T of 4 mm a day, there is therefore sufficient water in the soil to keep the turfgrass growing healthily for 5 days.

However, the transpiration rate of a plant reduces as the plant begins to respond to a lack of water. So the turf would last longer than 5 days, but its condition and ability to survive further stress, such a playing injury, are reduced. Research cited by Hull (1996) found that as the soil dried, the ET_T rate of turfgrass remained fairly constant for the first 8 days, declined rapidly for a further 8 days, levelled off briefly, and finally fell to zero, resulting in injury and death.

Response to water deficit

A turfgrass’s response to water deficit will vary according to the severity and duration of the deficit. Healthy turfgrass has a water content of between 75% and 90%, depending on species. A small reduction in this content, as little as 10% (Danneberger 1993), can kill the grass.

A turfgrass’s response to a water deficit is not the same as its drought resistance. Drought resistance is the plant’s ability to tolerate drought through its drought avoidance mechanisms, such as a deep root system, stomatal closure and waxy cuticles.

Wilt is often the first visible sign that a turfgrass is approaching a deficit. Wilt occurs when the transpiration rate exceeds the rate of water uptake through the roots. This can happen even if there is sufficient water in the soil if the loss is faster than the roots’ ability to draw up more water. Physical signs of wilt are a rolling or folding of the leaves, and drooping as turgidity is lost. The leaves may also turn a bluish-green.

Response to drought

During drought, turfgrasses slow down or stop growing. There is a perception that during these periods the roots will go looking for water. However, roots will not grow into dry soil.

As drought continues, cover thins out, and remaining plants show low vigour and poor ability to recover. Once the sward has been weakened in this way it is less able to cope with other stresses such as disease, wear and even some maintenance practices.

Overwatering

Excessive water can be as damaging as too little water: over-irrigation may physically damage the surface through runoff and washouts; a wet surface is more prone to damage from players’ spikes and studs; ball marks disrupt the playing surface; and even maintenance equipment may cause damage.

In addition, frequent watering encourages shallow rooting and therefore reduces drought-tolerance.

Irrigation strategies

Water use efficiency can be maximised by determining the point at which the grass needs to be irrigated before it is unduly stressed.

The simplest method of irrigation scheduling is to apply the same amount of water that is lost daily. But this technique has drawbacks. The upper part of the root zone is likely to remain wet, in turn promoting shallow rooting. Turfgrass benefits from drying-down periods between irrigation cycles, as this promotes deeper rooting.

Irrigation can be delayed until after ET rates begin to decline without causing damage to the turf. This allows for schedules which apply more water but with greater intervals between applications, allowing time for water to move into and through the root zone, promoting deeper rooting.

One potential side-effect of longer irrigation run times is runoff. To avoid this, measure how long it takes for water to run off from a particular area. Use this time as the maximum run time for the irrigation cycle. If further irrigation is needed, irrigations need to be shorter to allow the water time to soak into the root zone, but this means that irrigation needs to be repeated more often to prevent the soil from drying too quickly.

Finally, the quantity of water applied should not be enough to return the root zone to field capacity, as this could lead to problems with compaction, surface damage and shallow rooting.

References

Danneberger, KT. 1993. Turfgrass Ecology and Management. Franzak & Foster, Cleveland, OH, USA.

Hull, R. (1996). Managing turf for minimum water use. TurfGrass Trends 5(10): 1–9.

Acknowledgment

This article has been adapted for Australian conditions from Pitchcare magazine, UK, with the kind permission of the author, Steve Prinn.

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