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 0.9mm 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
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
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
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
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