Challenges in irrigation plus addendum
(The Groundsman, May 2008)
Can we benefit from using less water?
Warm temperatures are at last with us and the need to irrigate will soon arise. Monitoring use is logical but it becomes critical at times of restrictive use. Daily evapotranspiration (ET) losses question the value of light daily sprinkling and much rainfall can be lost as run-off and drainage beyond the reach of roots. Can we check this loss and how can we better conserve water and still achieve healthier grassed surfaces?
Temperature, wind, humidity and solar radiation measurements collectively influence ET but values emerging from weather stations are open to question. No account is taken of the fact that there is a reducing water supply every day without further applications and then there is the well documented phenomenon that a plant can restrict water loss to a degree with stomata control. Nevertheless in order to create a representative range of typical ET values under a variety of conditions over the summer season, calculations were made incorporating a range of each of these weather measurements. In brief ET losses range from .9mm per day on a mild, cloudy and still day in May to 4.8mm on a clear, sunny, hot and windy day with low humidity in July.
Losses from surface drainage run-off and penetration beyond the depth of the roots can accentuate the need for water supply. Run-off is significant after sharp showers and porous sandy rootzones convey water after heavy rain or extended irrigation beyond the depth of the roots where the water is lost.
Rainfall and irrigation
Contrary to general opinion there is a pretty even distribution of rainfall in Britain and in a 10 year study rain can be expected on average 12 days a month during the summer season. Yet not all rainfall is beneficial. . A light shower offers little other than cooling if hot sunshine follows. In practical terms it is the intensity and duration of rainfall that is most important. 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. Simply put the following gives a good guide to the measurement of effective rainfall.
- generally 4 to 6mm in a day is effective and penetrates
- with nearer 10mm per day there could be up to 20% loss by run-off
- quick 10 to 25 minute storms can generate up to 80% run-off
Irrigating is often ill-judged. Weather station ET values are generally over-estimated or decision to irrigate is made based on the prevailing temperatures rather than the actual water content in the root zone and the real need. Ensuring penetration must be the main objective. Water barely sufficient to overcome ET losses is water wasted. Short duration repeat cycles are not common and yet thatch, compaction and the gradient make this approach essential if water is not to be lost as run-off. In addition the even distribution of sprinklers around a green is often overlooked – this should be checked annually. Furthermore, if ET water losses are measured in millimetres, irrigation should be recorded accordingly. 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. Judging the rate of infiltration will help determine the duration of an irrigation cycle.