Imminent water restrictions offer new challenges

Article from Pitchcare, July 2006

The need to conserve water now becomes a reality – shortly we could face drastic water restrictions and rising costs as a result of low rainfall this winter. Capacities of stored water reserves are much reduced and restricted pumping rights from rivers will present the same concern. On the other hand there is the challenge to reduce our water losses from run-off and control our water use. Monitoring will become a necessity. It is in times like these we need to become more informed of actual water losses, the rainfall that is effective and the minimum irrigation need. In essence, keeping track of the balance of available water supply in the root zone seems logical yet vital.

Water losses

With water now critically scarce and rising in cost it will become clearly evident how much water is squandered at present. Looking for a moment at how water is lost daily, there are three main ways.

  • Evapotransporation (ET) losses every day vary with the weather. Furthermore, ET values emerging from sophisticated irrigation installation and weather stations are open to question.  Measured on the basis of continual adequate water supply these values do not account for reducing water loss every day after rain or irrigation has ceased (Penman,1948).  Nor do they allow for the restrictive capability of the grass plant in controlling the rate of water loss through the stomata in the leaves (Beard,1973).  However, in order to create a simple yet representative range of typical ET values under a variety of conditions over the summer season, calculations were made using the internationally accepted Penman-Monteith formula.  In brief, ET losses range from .9mm per day on a mild, cloudy and still humid day in May to 4.8mm on a clear, sunny, hot and windy day with low humidity in July.
  • Losses from surface drainage can be significantly underestimated.  Generally low rainfalls of in the region of 2 to 6 mm per day can be penetrating with little water lost as run-off.  Once more than 10mm is recorded in a day there are often bursts of high intensity rain included and run-off can amount to 20%.  Short storm downfalls of 10 to 20 minutes duration can generate up to 80% run off.  With this insight and experience, a good idea of ‘effective’rainfall can be judged.
  • Water beyond root depth lost by percolation is lost to the plant.  The myth of greater water need in sand root zones is incorrect.  Sure, sands hold less water than loams but surplus water simply percolates out of reach into the drainage system below.  Seldom are water applications made in accordance with the degree of water penetration and the water holding capacity.

Rainfall and irrigation

It is the intensity and duration of rainfall that is most important.  Furthermore, not all rainfall is beneficial.  Average rainfall intensities in south east Britain are in the region of 5mm per hour and in a ten year study it was found that over 90% of rainfall is less than 10mm per day with 60% of that amount being less than 2mm per day (Royal Horticultural Society,2004).

In order to get the best value from irrigation systems it is vital they apply water evenly.  A measurement of the uniformity of a spaced sprinkler spacing design is the first prerequisite for conserving applied water.  Secondly, measuring the actual precipitation in millimetres is logical if we are to balance water lost by ET with water falling as effective rainfall.  Ensuring penetration must be a main objective but the decision when to irrigate and with how much brings a further challenge.  Water barely sufficient to overcome daily ET losses can be water wasted if the water content below is not monitored.  Short duration repeat irrigation cycles are not commonly applied and yet thatch, compaction and gradient make this form of scheduling essential if water is not to be lost as run-off.  Four to five part-circle sprinklers around a golf green apply 2mm in about 5 minutes and being able to apply water at 25mm per hour water, operation for more than 8 to 10 minutes usually results in run-off.

The storage

The water gains and losses are ultimately dependant on the amount of short term storage within the zone of root growth.  The shallow root system of fine grass –  generally not more than 50mm – poses a problem in storage as not much more than 6mm of water is available at this depth (the amount between field capacity and wilting point).  With ET rates up to at least 3mm per day in the summer this seems hopeless.  Yet there is a strange anomaly in the fact that somehow it is possible to hold off irrigation in the summer for at least four days after a good rainfall of 10 mm in a day.  The reason for this is the time-lag the upper root zone takes to reach field capacity after saturation together with the plant’s ability to slow down the rate of loss through the stomata in the leaves.

Different parts of a golf green or sports pitch vary in their ability to hold water.  A simple probe is the only practical way of gauging the amount of available water.  If the soil probed to 75mm wets the hands there is too much water present but if the soil flows like salt it is probably already too late and grass cover has shown signs of deterioration.  Good management can prolong the water storage and the water use rate in turfgrass can be enhanced by the degree of maintenance of the grassed area.  Deep tine aeration to create a porous root zone and deeper roots will mean that more water can be stored to a greater depth.  Good nutrition ensuring adequate potassium levels also promotes better durability at the time of water stress and increasing the interval between mowing reduces the rate of water loss (Kneebone,1992).

Determining the water need

A simple water balance table revealing a little water stress could take the following form (remembering that after good penetrating rain there is a time-lag before field capacity is reached and ETc values will lessen daily when no water is added).  This understanding gives comfort in judging the water available but nevertheless the table only gives a ‘guide’ condition.  Only by probing can we judge the variation in water supply in the different locations and aspects and a powdery dry condition should be never be allowed to develop.  Selective hand watering is often the best solution where run-off has developed wetter and drier conditions on a golf green.  On a sports pitch with deeper root depth, water storage is greater and conditions are more uniform.

 

Date

Day

Reservoir in 50mm   mm

Rainfall in 24hrs mm

Effective rainfall mm (r)

Irrigation effective mm (i)

ET crop

mm (e)

Gain/Loss r+i-e mm

Balance reservoir c/f mm

18.4.05

1

6max (fc)

 

 

4

2.6

1.4

6.0 max

19.4.05

2

6

3.0

-3.0

3.0

20.04.05

3

3

 

2.8

-2.8

0.2

21.04.05

4

0.2

2.6

-2.6

-2.4

22.04.05

5

-2.4

15

11

1.5

9.5

6.0 max

23.04.05

6

6.0

2.8

-2.8

4.2

24.04.05

7

4.2

2.5

-2.5

1.7

25.04.05

8

1.7

2.3

-2.3

-0.6

(Note: fc is short for field capacity – the maximum water this depth can hold against gravity.)

 

Interestingly, the turfgrass plant shows little visual loss in quality with carefully managed deficit irrigation (Kneebone,1992).  This simply means applying less water than the theoretical calculated requirement.  In practice, aiming to ensure the available water content between field capacity and wilting point is the main consideration and a temporary shortfall causing a little stress is better than too much water – more air is available in the root zone, roots will endeavor go deeper and the grass will be less succulent and develop greater hardiness.

Duncan Kelsoe, golf course director at Kingshill Golf Club, in maintaining a weather station able to record daily ET losses, plans to apply 50% of these losses at intervals of two to three days allowing for expected rainfall.  There is no doubt that with experience, maintaining a tight rain on water application and continuing with a sound maintenance programme, the water use rate of turfgrass cover can be controlled and even reduced.  Restricted watering can be taken to a stress level where grass cover is not significantly impaired.

It now must be evident that as we become more aware of the water losses and effectiveness of rainfall and irrigation we can, with sound management, use water more wisely and produce hardier turfgrass growth that still meets our requirements.

References

Beard, J. B. 1973.  Turfgrass Science and Culture.  Prentice-Hall, Englewood Cliffs, NJ.  Pp 261-303.

Kneebone, W. R., Kopee, D.M., and Mancino, C.F., 1992. Agron. Mon. No 32. Am. Soc. Of Agron., Inc., Maddison, Wisconsin, USA.  pp441-467.

Penman, H. L. 1948.  Natural evaporation from open water, bare soil and grass.             Proc. of the Royal Soc. London. Ser. A193: pp120-145.

Royal Horticultural Society. 2004.  Ten year study of daily rainfall. Wisley Library, Woking, Surrey.

Gordon Jaaback

March 29 2006

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