Understanding principles of irrigation
By Steve Prinn
If naturally occurring rainfall is insufficient to meet the needs of the turfgrass, irrigation needs to take place. In order to take a decision on whether or not irrigation is required the amount of moisture in the soil needs to be known. If this is monitored then irrigation scheduling can take place, deciding when and how much water to apply to maintain turfgrass growth. Before this is can be applied, the amount of water that is present in the soil and how much water is being used by the turf grass needs to be known.
The Turf Grass and Water Use:
Evapotranspiration, (ET), is a measure of the water use of the plant. It is the quantity of the water transpired from the plant and evaporated from the soil surface. Reference evapotranspiration (ETO), is derived from climate data and the use of mathematical models. It is necessary to adjust ETO values, to estimate turf ET (ETT). This process is undertaken by multiplying ETO by an adjustment factor or crop coefficient (Kc).
ETO x Kc = ETT
The Kc that is frequently quoted for use in the U.K. is 0.7.
If therefore the climate data showed the ETO rate for a day in July was 4.5mm, actual evapotranspiration from the turf ETT, would be:
ETo x Kc = ETT
4.5 x 0.7 = 3.15mm
It can be seen that if 4.5mm of water had been applied to the surface to make up for that days deficit then the turf would have been over watered, at a cost to the environment - use of a resource which is not needed - and potentially leading to an increase in damage to the soil structure, there is also a financial cost.
Although in the U.K. average rainfall is fairly constant, in the summer months the evapotranspiration rates, (the water used by the turfgrass), exceeds the rainfall. There will therefore be a water debt in the soil, (figure I). Once the crop coefficient is applied however, the debt while still present is not so great.
Figure I. A comparison of average monthly rainfall/Evapotranspiration rates and Adjusted ET rates. Figures are based on the York area.
All soils will hold a varying amount of water, according to the texture. A typical sandy soil may have a field capacity (FC) of 29% and a Permanent wilting point (PWP) of 19%. The amount of water readily available to the plant (RAW) is therefore 10%.
If the depth of the soil that is able to have water extracted from it by the plant is 200mm deep, the amount of moisture in the soil available can be calculated, in the 200mm of soil there is 10% of available moisture:
10 x 200 = 20mm
For this example assuming a constant ETT of 4mm a day, there is therefore sufficient water in the soil to keep the turfgrass growing healthily for 5 days.
It must be noted that the ET rate of a given plant will vary as the plant begins to respond to a lack of water, ET rates will slow down as a direct response to water deficits. Resistance to transpiration is controlled by the boundary layer and stomatal closing. The closing stomata will reduce transpiration. The turf would last longer than 5 days but its condition and ability to survive further stress, such a playing injury is doubtful. Research from Aronson (1986), cited by Hull (1996), found that as the soil dries, the ETT rates for turfgrass remained fairly constant for the first 8 days after which they decline rapidly for a further 8 days before levelling off prior to their final decline, which results in serious injury and death.
ETT represents the ET from a turfgrass which is well supplied with moisture. The turf grasses response to water deficits will vary according to the severity and duration of the deficit. Turf grass has a water content of between 75-90%, depending on species.
A small reduction in this amount can cause the death of the turfgrass. Danneberger (1993) puts this figure as low as a 10% drop in the water content.
The way in which turf grasses respond to a water deficit should not be considered a measure of its drought resistance. Drought resistance should be considered to be the grasses ability tolerate drought through its drought avoidance mechanisms, such as, deeper root systems, stomata closure mechanisms and waxy cuticles.
Wilt is often seen as the first sign that the turf grass is approaching a deficit. Wilt will occur when the transpiration rate exceeds the rate of water uptake through the roots. It is worth noting that this may still happen when there is sufficient water in the soil. Physical signs of wilt are a rolling or folding of the leaves. The leaves of the grass may also droop as turgidity is lost. Slight discolouration may also be seen as the plant appears a bluish-green colour.
Drought is a result of prolonged water stress, and it is during these times that the turf grasses response is to slow and stop growth, there is a perception that during these periods the roots of the grass will "go looking for water" however, roots will not grow into dry soil. Should conditions continue, thinning out of the surface will occur with remaining plants exhibiting low vigour and having poor recuperative ability.
Once the sward has been weakened in this way it is less able to cope with other stresses such as disease, wear or even some maintenance practices.
Excessive water can be as damaging as too little water, over irrigation may physically damage the surface through run off and washouts, wet sports turf surfaces increases the potential for damage from players spike and stud damage, ball marks disrupt the playing surface and even maintenance equipment may cause damage, Water lying on the surface may evaporate in high summer temperatures causing scorching, a brown/tan discolouration of the grass is a typical indicator this has happened.
Shallow rooting of the turfgrass also occurs if excessive water is applied regularly.
It is due to these factors, that irrigation quantities applied must be correct. These quantities can not be calculated unless the moisture status of the soil is known.
Water use efficiency can be maximised by determining the point at which the grass needs to be irrigated, without causing undue stress on the plant.
The simplest method of irrigation scheduling would be to apply the same amount of water that was lost daily. This technique has drawbacks. The upper part of the root zone is likely to remain wet which in turn, would promote shallow rooting leading to the all the problems associated with this. Turfgrass benefits from drying down periods between irrigation cycles, as this promotes deeper rooting.
For irrigation scheduling purposes 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 periods of time between cycles. This should allow time for water to move into and through the rootzone promoting deeper rooting.
One side effect of longer irrigation run times is that run-off may become an issue. Measurements should be made of the time taken for run-off to occur for a particular turfgrass area, this time is then taken as the maximum run time for the irrigation cycle. If further irrigation is needed, irrigation cycles need to be smaller and more frequent to allow the water time to soak into the rootzone, however these cycles need to be repeated over a short period of time before the soil dries completely.
The final aspect of the irrigation cycle is that the quantity of water applied should not be enough to return the rootzone back to FC, as this could lead to problems with compaction, surface damage and shallow rooting.
Danneberger, K.T. (1993) 'Turfgrass Ecology and Management', Clevelend: Francak & Foster
Hull, R. (1996). Managing Turf for Minimum Water Use', TurfGrass Trends, 5/10, 1- 9.