Introduction
Irrigation is the application of water to the turf surface, but may also include subsurface application in some high specification pitch constructions.
Why Irrigate?
Irrigation may be required when rainfall is insufficient to compensate for evapo-transpiration losses, otherwise this would reduce the ability of the grass plant to function properly.
Some reasons for carrying out irrigation include:
- to reduce the effects of drought and hot weather;
- to maintain adequate plant health and prevent the plant dying;
- to allow nutrients to be taken up by the plant from a soil solution;
- to ensure a suitable sward density is maintained;
- to maintain adequate wear tolerance;
- to reduce the potential for Dry Patch development;
- to maintain playability; and
- to maintain and improve the overall standards of the facility.
Watering should take place before the turf shows any sign of stress: An adequate amount needs to be applied on each occasion.
If irrigation is only given to a shallow depth, then this will encourage surface rooting of the grass and also reduce its potential to withstand dry conditions and wear.
Indications of Inadequate Water Availability
A number of signs can be used to indicate that inadequate water is available to the grass plant:
- the grass initially becomes a darker colour, eventually turning a yellowish green to brownish colour;
- a shiny tinge, or sheen, appears on the grass leaves; this can also be the result from constant wind exposure;
- the amount of clippings removed by the mowing operation are reduced: This is probably the best, or at least the most tangible, indicator during the growing season ;
- the drying of high spots or exposed edges, such as around a bowling green or green-side bunker;
- shallow rooted areas will dry out and yellow quicker than deeper rooted areas;
- a very dry soil profile will be evident when a soil core is extracted;
- footprinting is pronounced, with the grass taking a relatively long time to stand back up after being walked on.
The Watering Process
It is beneficial for a certain amount of stress to be placed onto the grass plant, as this encourages it to extend its roots deeper into the rootzone in an exploration for more water.
This deepening of root growth leads to a healthier plant, as well as giving it a greater ability to withstand drought conditions.
It is not wise to apply water on a too frequent basis, for two main reasons:
1. If too little water is given on frequent a basis, this results in only a small depth of the surface being wetted. This encourages:
- shallow rooting,
- the development of thatch,
- a greater likelihood of disease susceptibility,
- the development of Poa annua,
- the encouragement of root break, and
- a greater potential for Dry Patch development.
2. If too much water is applied on a frequent basis, this can give similar results to those above, except that the soil profile is wetted to a much greater depth, however, it is not allowed to dry out sufficiently to allow for the deep penetration of grasses. There is little chance of Dry Patch developing because the soil is constantly wet.
With too much water being applied the end result will be a very soft surface, which has a slow speed of play; euphemistically called a 'pudding'.
Frequent applications will be required where seedlings are newly germinated and becoming established. This will help to prevent desiccation of the seedling plant.
Effective Watering
To carry out effective watering it is sensible to adopt a policy similar to the following:
- Do not wait until the soil is too dry before irrigating as this can make the soil difficult to rewet;
- Apply an adequate amount of water per application, aiming to wet to a depth of some 100mm or more. A minimum application rate of 10 -11 litres per m² (2 gallons per yd²) is generally suggested, although this will depend upon the soil type and its infiltration potential, as well as the size of the irrigation tank available;
- The surface should be allowed to dry out to some extent before further watering takes place, as this will encourage the deeper rooting of the grass and reduce the potential for disease attack. As a guide, during the main part of the growing season, turf will ideally be watered twice per week, but preferably on no more than 3 occasions per week. Selected hand watering may be needed to address high spots and dry patch areas;
- Water in the early or late part of the day to reduce evapo-transpiration losses with the added benefit that there will be less disruption to play at these times. In particular, the production of slower playing surfaces will be evident until the water has managed to infiltrate down to lower levels;
- Surface waterlogging should not occur whilst irrigation is taking place because an uneven distribution of water will occur;
Ensure that the turf has been aerated prior to irrigation, especially if an undesirable thatch layer is present. Shallow aeration, such as sarel type spiking, can help the water to penetrate that much quicker into the soil profile by providing a means of speedily passing through the top 25mm or so.
Water applied to a turf surface tends to bring the surface layer to field capacity before advancing further into the soil profile as more water is applied. Thatch has a very high capacity to retain water and therefore it is essential to provide 'bypass' channels, in the form of aeration holes, to ensure the irrigation water adequately reaches the soil profile beneath the sponge-like absorbing thatch layer.
Potential (Evapo-) Transpiration (PT)
This is the theoretical maximum amount of water vapour that is passed to the atmosphere by the combined processes of evaporation and transpiration from the turf, with there being plenty of available water in the soil (i.e. water is not limiting, which is reflected in the Penman formula and is the basis of the practical implementation of potential transpiration calculations).
Potential moisture loss by evapo-transpiration varies depending upon geographical location and the prevailing climate.
Whilst climate data is gradually being updated over time, some typical potential transpiration losses, in this example for the south coast of England, are given in the Table below (The data has been adapted from Smith, L.P., 'Potential Transpiration', MAFF Technical Bulletin 16, HMSO, pp36-37):
| Month | PT losses, in mm, per month | Average PT losses, in mm, per day |
| April | 61 | 2 |
| May | 84 | 2.7 |
| June | 98 | 3.3 |
| July | 98 | 3.2 |
| August | 83 | 2.7 |
| September | 53 | 1.8 |
Actual evapo-transpiration losses on fine turf have been found to be less than the estimated, or calculated figures.
As an example, one project found that evapo-transpiration rates were on average only 65% of the Meteorological Office's calculated evapo-transpiration rates for turf. (For further information see, Lodge & Baker (1992), 'Soil moisture content and evapotranspiration rates of three types of golf green construction in response to different rates of irrigation', Journal of the Sports Turf Research Institute, Vol.68, pp104-113).
The actual amount of irrigation applied to greens can also differ from the original design estimate for the irrigation system. One research project found that during May and June, just 43% of the calculated irrigation was needed. The amount applied was still said to be sufficient to maintain sward colour and vigour. (For further information see, Sainsbury, (1993), 'Accurate Irrigation', Greenkeeper International, August, pp37-39).
As water in the soil becomes less available, plants adapt to this by the rolling or closing of their leaves, closing of stomata and a consequent reduction in photosynthesis. As a further result, less water is also used. Calculated figures for potential transpiration are based on there being a non-limiting supply of water and do not take into account the plant's ability to reduce moisture loss. This inevitably results in overestimates of the actual evapo-transpiration rate and irrigation requirements.
Soil Water Terms
Field Capacity
This is the amount of water held by the soil after drainage by the force of gravity. This process is usually achieved between 1 and 3 days after rainfall in a reasonably well drained soil.
Permanent Wilting Point
The water content of a soil is held so tightly by the soil particles that it is unavailable to the plant. The plant will wilt and not recover without additional water.
Available Water Capacity
Water that is freely available to the plant and is the difference between permanent wilting point and field capacity.
Water Availability
If the water content of the soil exceeds its field capacity, air is displaced from the remaining pore space and aeration becomes a limiting factor. This excess water eventually drains away.
The pore space within a soil generally ranges between 25 and 50%. (A compacted sandy loam type of soil will typically have a porosity of 35-40%).
If the water content falls much below field capacity, the water begins to be held firmly by the soil particles making it less available to the plant, eventually reaching a stage, i.e. permanent wilting point, where the plant can no longer extract any more water.
As a guide the available water capacity of soils used for fine turf areas can typically range from up to 10% for those of a high sand content with medium to coarse sized particles, to 20% for those with medium to fine particles.
What this means is that within a rootzone (say a medium-fine one) of 100mm, 20% (which is the equivalent of 20mm) of this content is water which can be used by a plant.
Soils have a range of available water capacities, depending upon their texture, i.e. particle size distribution, as well as the soil structure. A well structured soil will generally hold more available water than a poorly structured one.
The inclusion of organic matter in a soil will also increase the amount of available water.
As an example, a soil might have the following parameters:
- Field Capacity = 28%;
- Permanent Wilting Point =10%; and
- Available Water Capacity = 18%.
If this is used as a worked Example, then
In a rootzone of 100mm depth, there will be 18mm of water available to the plant.
If actual evapo-transpiration is say 2.5mm per day, there will theoretically be 18 ÷ 2.5 days (or 7.2 days) supply of water.
In practice, as the supply becomes less and less, the grass plant modifies itself to take account of the increasing effort required to extract moisture that is retained on the soil particles, resulting in the plant being able to sustain itself for longer than the above simple calculations.
Player usage decreases the plants ability to maintain itself in a satisfactory condition and the plant will therefore require irrigation before all available water is used up if a satisfactory playing surface is to be maintained.
Grasses vary in their ability to adapt to drying conditions: Fescues are very good at effectively becoming dormant and awaiting the coming rains or irrigation, whilst Poa annua will not survive for long in the vegetative state without ample supplies of water and can produce vast quantities of seed heads to ensure survival at times of negligible water availability.
Soil Moisture Deficit
The use of a soil balance sheet can be of assistance in determining how much water to apply. However, before looking at an example of a soil balance sheet, we must first clarify the meaning of Soil Moisture Deficit (SMD):
This is the amount of water that is required to bring a soil back to field capacity once some of the available water has been used up.
Irrigation typically takes place when a predetermined SMD has been reached. This SMD may typically be 30-70% or so of the available water figure.
Example:- If field capacity is 22mm, and the available water capacity is 14mm (this means that there is 8mm of water which is unavailable to the plant!), then you may decide to irrigate when the SMD has reached within the range 4mm-10mm (i.e. between 30% and 70% of 14mm) : If you wait until there is a SMD of 30%, there will be 4mm of available water left in the soil, in this example.
The deficit is not brought back to field capacity in practice, but maybe 80-90% of it as this allows for any unforeseen rainfall that may occur. Thus, 90% of this example field capacity is 20mm, (i.e. 90/100 x 22mm).
Experimentation with various figures is required for sports turf areas to give the best level of help for each individual situation.
Example of a Soil Balance Sheet
The example in the Table below is based on the following data:
- An effective root depth of 100mm;
- An available water capacity of 14% (or 14mm for the rootzone depth of 100mm);
- Field Capacity of 22% and Permanent Wilting Point of 8%; and
- a Soil Moisture Deficit of 10mm.
| Date | Rainfall: mm/day |
P-T: mm/day |
Estimated E-T: (PT x say 0.75, to try and reflect the actual situation on the turf) mm/day |
E-T less rainfall: mm [Add these figures to SMD figures; and subtract from Water available figures] |
Irrigation: mm [Subtract these figures from SMD figures and add to Water available figures] |
SMD: mm [Irrigation activated when SMD reaches 10mm] |
Water available to plant: mm [Irrigation activated when available water reaches 4mm] |
Excess drainage or run-off: mm |
| May | Balance carried forward from April | 6 | 8 | n/a | ||||
| 1 | 0 | 2.7 | 2 | 2 | 0 | 8 | 6 | 0 |
| 2 | 0 | 2.7 | 2 | 2 | 0 | 10*a | 4 | 0 |
| 3 | 2 | 2.7 | 2 | 0 | 8 | 2 | 12 | 0 |
| 4 | 0 | 2.7 | 2 | 2 | 0 | 4 | 10 | 0 |
| 5 | 0 | 2.7 | 2 | 2 | 0 | 6 | 8 | 0 |
| 6 | 5 | 2.7 | 2 | -3 | 0 | 3 | 11 | 0 |
| 7 | 0 | 2.7 | 2 | 2 | 0 | 5 | 9 | 0 |
| 8 | 12 | 2.7 | 2 | -10 | 0 | 0 | 14 | 5*b |
| 9 | 0 | 2.7 | 2 | 2 | 0 | 2 | 12 | 0 |
| 10 | 2 | 2.7 | 2 | 0 | 0 | 2 | 12 | 0 |
| The sum of these figures must add up to the available water capacity figure, which in this case is 14mm | ||||||||
*a) When the relevant SMD is reached (i.e. 10mm) then irrigation is applied to bring the figure up to the chosen percentage level of field capacity, in this case it is 90%. Field Capacity is 22mm, 90% of this is 20mm; with PWP taking up 8mm, this leaves a maximum of 12mm of available water to be achieved when the relevant SMD has been reached. Thus, with 4mm of available water already left in the soil another 8mm has to be applied to reach this figure. (4mm within the soil + 8mm as irrigation + 8mm as PWP = 20mm in total = 90% of the field capacity).
*b) SMD cannot be any less than 0, whilst water available to the plant cannot exceed 14, as any higher figure is greater than the field capacity figure and water will consequently drain away freely, or run-off the surface.
This soil balance sheet can be made more sophisticated by having more accurate potential transpiration figures applied to the situation as well as having various estimated evapo-transpiration figures based on the amount of water available to the plant as with less available water the plant will have a reduced actual E-T rate.
Conclusion
The process of irrigating turf needs a methodical approach to be taken if water is to be applied in an effective manner.