Where has all the water gone?
(Turf Business, May/June 2005)
More often than not water falling on grass is lost or ineffective. If it does not penetrate the grass cover there is a good chance it will be lost by evaporation, be held up in the thatch or lost by run-off. Monitoring effective precipitation becomes critical in the middle of summer when evaporation rates are greatest and rainfalls are generally of short duration. Seldom do managers seek to find the depth of moisture available to the depth of the roots. Decisions are judged on the heat of the day and visible conditions on the grass surface. Most of the water used in irrigation is main line water costing over £1 per cubic meter so it is logical that water should be applied sparingly – in fact sufficient only to maintain the quality of grass cover desired.
The normal irrigation procedure operated on golf greens in the heat of the summer is to sprinkle daily for five to ten minutes. Early summer ‘dry patch’ emerges as a problem on many greens, the water bills soar and overwatering can become commonplace.
How much of the rainfall or irrigation is effective? How much are we losing daily?
Precipitation and run-off
Water reaches the grass either by irrigation or rainfall. Irrigating with four to five part circle sprinklers around a golf green generally applies about 2mm in 5 minutes which is equivalent to nearly 25mm per hour. Travelling sprinklers apply at 4 to 10mm per hour and stationary pop-ups on a football pitch will irrigate at 15 to 20mm per hour. Infiltration is governed primarily by the intensity of thatch, compaction in the upper root zone and the gradient. Though the root zone is highly permeable – generally laboratory tested prior to placement at around 300mm per hour – infiltration on an established green will generally not be much more than 10mm per hour. Contrary to common belief sandy rootzones can be slower than loam soils to take in water when dry. The high water tensions that develop between sand particles make initial entry slow. This extreme condition is seen on golf greens when they almost become hydrophobic (repel water).
Not all rainfall is beneficial. A light shower offers little other than cooling if hot sunshine follows. A heavy downpour often results in most of the water being lost in run-off. Average rainfall intensities in Britain are in the region of 5mm per hour and it may startling to some that over 95% of rainfall is less than 10mm per day with over 50% of that amount being less than 2mm per day. The measure of effective rainfall (that which penetrates) therefore must be seen as vital in assessing the water input.
The phenomenon of water run-off is interesting.
Irrespective of the intensity of precipitation some water falling on the grass is withheld initially in the grass plant and thatch, some collects in the minor soil depressions and some begins to infiltrate into the soil. Frequent surface aeration and tining naturally help in inducing infiltration. As much as 4mm can be withheld before water infiltrates into the soil or is lost by run-off. The gradient really only influences infiltration and run-off. Depending on the surface condition of the green, run-off usually begins in less than 10 minutes of irrigation and water moves from the high to the low parts of the green. To avoid this run-off repeat cycles of shorter duration are the only way to get penetration. If there is ‘dry patch’ and the rootzone has become hydrophobic the tendency for run-off is greater.
Storage and daily evapotranspiration losses
Once water penetration into the soil or rootzone is attained adequate storage to the full depth of the roots is important. Below the root depth water accumulation is no real value to the grass cover. Movement of water upwards by capillary action is slow and is not a consideration in judgement of the water available to the grass turf. Available water varies essentially with the texture of the soil and its physical condition – and it is the distribution of the different particle sizes that determine the porosity – or measurement of air and water spaces. Sandy root zone material with root depth to 50mm has storage of not more than about 6mm of available water. In other words when this amount is lost by root uptake the root zone begins to dry out.
Water loss from grass turf has been the subject of much research. Weather stations can calculate this daily loss but is helpful to realise what contributes to this loss termed evapotranspiration (ETo) which is a combination of plant transpiration losses and evaporation from the soil and plant surface. With the main factors being temperature, wind, humidity and solar radiation and the Penman-Monteith formula adopted internationally, basic ETo losses can be easily calculated and adjusted for the grass turf. In an effort to produce a practical table incorporating the main factors with normal variable measurements experienced in the summer, the following are taken into account:
Assuming hot clear days have maximum solar radiation, warm part cloudy days have intermediate solar radiation and milder overcast days have the minimum solar radiation , the formula has been used with the above data to create a representative number of typical ET values on windy and still days in humid or dry conditions for each of the four months in the following diagrams:
Note: The above calculations for ETo have been made with allowance for latitude 51 (south England) and 67m elevation and adjusted to ETc with the crop coefficient of .8 used for cool season turf grass. With experience a measurement can be adjusted up or down where weather measurements differ markedly those given before the tables above. For example on May 13 2005 in partly cloudy conditions, maximum temperature 17°c and minimum 2°c with average wind at 3m/second the ETc was 1.9mm compared with the 2.5mm entered in the table. Wind is the major influencing factor and can increase the ETc by at least .5mm in a day.
Balance of water supply
With intermittent irrigation and rainfall and daily water losses it seems logical to be aware of the ever changing balance of water supply in the soil over the critical dry periods. Like the importance of monitoring a bank balance, keeping track of the water supply available on a daily basis would logically help in planning an irrigation programme. A simple daily calculation will give peace of mind – especially on a golf green as it is the water storage in a restricted root depth that will determine the onset of acute water stress or ‘dry patch’. A little stress is better than too much water – more air is available and the grass is less succulent. Most will agree that an ideal putting surface comes a few days after a penetrating rain – with the surface drying out and all other factors optimum. Sure, probing gives a good idea of the variation in water supply in the different locations and aspects and a powdery dry condition should be never allowed to develop. Selective hand watering is often the best solution where run-off has developed wetter and drier conditions on a green. On a sports pitch with deeper root depth, water storage is greater and conditions are more uniform. Still, with the quantity of water needed the judicious application would be cost effective.
It is interesting to note that ETc values are calculated on the condition that water supply to the roots is adequate. In reality, as a degree of stress approaches the actual daily ETc will reduce every day with no further water supply. This must be expected as the grass plant starts to restrict water loss by closing stomata when the water supply becomes limiting. With experience restricting watering can be taken to the stress level where the quality of grass cover is not lost. A simple water balance table in a golf green revealing a little water stress could take the following form (remembering that ETc values inserted do not allow for a reducing water supply which would be happening in this instance).
Note: fc is short for field capacity – the maximum this depth can hold against gravity.
There can be no doubt that over the 18 odd weeks when water supply can be critical the sound programming of water supply can be as beneficial to the playing surface as it can be to the cash budget.