Water conservation

The need to manage water efficiently is a major concern in the use of scarce resources. Responsible action is increasingly supported by legislation and the higher price of using water. Clearly the major factors that determine the level of water use are related to the choice of plant species to be grown and the reasons for growing. The selection of drought-tolerant rather than water intensive plantings is fundamental. Some growing systems are inherently less water intensive, but in most of them there are many ways in which water use can be reduced if certain principles are kept in mind and acted upon appropriately:

• Whenever possible, the use of artificial water should be avoided.
• Use recycled water. The recycling of water and the capture of rainwater are important considerations in the choice of water source.
• Minimize evaporation of water. This is best achieved by not spraying water into the air and by minimizing the time when the soil surface is moist. When water does have to be applied overhead, this should be undertaken in cool periods.
• Increase the water reservoir of the soil. The application of water can be reduced by increasing the growing medium’s water holding capacity; most soils can be improved by the addition of suitable organic matter.
• Encourage plant root systems. Plants should be encouraged to establish as quickly as possible but, after the initial watering-in, infrequent applications will encourage the plant to put down deep roots by searching for water. Most importantly, soil pans should be eliminated and good soil structure maintained to increase the rooting depth.
• Minimize water lost through drainage. Where application is partially controlled, the correct relationship between water applied and water holding (see water-holding capacity) helps to prevent leaching, which leads to nutrient loss (see nitrogen), as well as wasting water. Thus returning an outdoor soil to less than field capacity helps avoid losses by drainage or run-off in the event of unexpected rainfall.

Water is lost more rapidly from a moist than from a dry soil surface. After just 10 mm of water has been lost from the surface, the rate of evaporation falls significantly. Infrequent application thus helps, but even more effective is the delivery of water to specific spots next to plants (see trickle lines) or from below through pipes to the rooting zone. Avoid bringing moist soil to the surface. If hoeing is undertaken it should be confined to the very top layers; this also reduces the risk of root damage. Losses from the surface can be reduced considerably by plant cover and almost eliminated by the use of mulches. Loss of water from the plants themselves is reduced when they are grouped together rather than spaced out.

Unless maximum growth rates are the main consideration, reduced application saves water, money and staff time without detriment to most plantings. In production horticulture, the introduction of sophisticated moisture-sensing equipment and computer controls has enabled water to be delivered more precisely when and where it is needed. This has led to considerable reduction in water use.

Nutrient loss and run-off from overhead watering used in container nursery stock production can be minimized by matching application to rainfall, growing medium, container size, plant species, stage of growth and time of year. Nozzles should be maintained to ensure even water application. Loss from sub-irrigated capillary sand beds tends to be lower. Recirculation (closed) systems should be considered in new developments. The quantity of water required for flooding soils in protected culture (e.g. to remove excess nutrients when a crop sensitive to high salt levels, such as lettuce, is to be grown after a tolerant one, such as tomatoes), can be reduced by discontinuing the liquid feeding of the previous crop as soon as possible.
In non-recirculating (open) hydroponics systems excessive water waste should be avoided by using fl ow meters to measure the quantity of run-off and comparing it with standard figures for the growing system used. A run-off of over 30 per cent is usually considered to be excessive and the amount and frequency of nutrient applications delivered by the nozzles or drippers should be reviewed. Closed systems (see NFT) recirculate the nutrient solution, but this is not always practical. Where they are used, the system must not be emptied illegally into watercourses or soakaways. It is recommended that the volume in the system be run down before discharge and the waste nutrient solution be sprayed on to crops during the growing season. Permission to empty into public sewers might be granted, but it is usually subject to a charge depending on volume and contamination level.