Get a group of Australian nurserymen together and it’s not long before they’re comparing their Christiansen’s coefficients of uniformity, their scheduling coefficients and their distribution uniformity percentages — in the way UK growers banter about cuttings takes and crop wastage rates. Australia is a dry country and these key measures of irrigation efficiency really matter there.
But they matter increasingly here, too, thanks to the combination of climate change, environmental legislation and cost, which is forcing growers to be more aware of the need to avoid waste water and excessive run-off. While research demonstrates sub--irrigation through capillary mat or sand beds is more efficient, in reality most nurseries are faced with making the best use of existing infrastructure and that inevitably means inherently inefficient overhead sprinklers.
But there is plenty of scope to improve overhead sprinkler efficiency. The first step is to assess the performance of your current system using catch-cans (such as empty pot saucers) spaced evenly over a representative bed. The more you use, the better, making sure you sample right up to the edge of the bed. Run the irrigation so the containers catch enough water without causing any to overflow, then measure and record the volume of water in each container and the length of time the irrigation ran.
You can then use the results in one of the many irrigation-calculator software packages now available — such as the Australian industry’s Waterworks or the HDC Irrigation Calculator, which is supplied free to levy-payers with factsheet 16/05, explaining in detail the techniques for measuring the efficiency of overhead sprinklers and how to use the results.
When you feed the catch-can measurements into a calculator it will work out three key performance parameters:
• Mean application rate is the average irrigation rate over the sample area. Australian nurserymen aim for no more than 15mm per hour — the maximum amount of water most growing media can absorb. For small containers they aim for somewhere between 4mm and 10mm per hour.
• Christiansen’s coefficient of uniformity indicates how evenly the water is distributed over the bed. According to the Nursery Industry Association of Australia, this should be at least 85 per cent, which means the driest area is getting at least 85 per cent of the water received by the wettest part. Well-designed systems should be capable of 95 per cent in calm weather.
• The scheduling coefficient shows how much the irrigation time must be increased to ensure the driest areas receive the intended amount of water. Ideally this should be less than 1.5 — any more means water is being wasted and nutrients are leached.
An irrigation calculator will not only work out efficiency values from your measurements but will draw a 3D graph of the results to help identify problems, such as poor distribution along the edges of a bed. Calculators can also be used to illustrate the effects of changing sprinkler nozzle types or layouts to design a more efficient system.
These kinds of measurements are just as useful for the overhead systems used in polytunnels and glasshouses as those on outdoor beds. But as consultant and author of the HDC factsheet Chris Burgess points out, they won’t tell you how much water actually gets into a pot because some will bounce off the foliage canopy or miss the pot altogether, while some can be absorbed through the pot base. Pot weight is the most useful guide here — a 1g increase in weight after irrigation indicates 1mm of water absorbed by the media.
But overhead irrigation efficiency is about more than just sprinkler type and layout, as Australian nurseryman Garry Heyne told an International Plant Propagators Society (IPPS) conference last year. "I find the amount of water I apply is directly proportional to evaporation," he says. "You forget how much impact wind has on this."
Heyne installed 2.7m tall, 50 per cent mesh-net windbreaks to protect all the beds on his nursery in South Australia as part of a government grant-aided project to cut his water use. Not only has it reduced evaporation and transpiration from the crop but it also reduced wind-drift, which means Heyne can use lower-output sprinklers for a more even distribution.
"We also found that we could reduce the amount of water draining through freshly potted plants using a combination of pulsed irrigation applications and a wetting agent," he says. Heyne saw his uniformity coefficient increase from 68 per cent to 87 per cent just by changing sprinkler heads. Three years after the project began he is making a 20 per cent saving in his annual water use.
Sub-irrigation systems based on capillary mat or sand beds can be used on outdoor beds or for protected crops. In addition to consuming as little as 30 per cent of the water used by some overhead systems, other advantages include: reduced weed, moss and liverwort growth because the compost surface stays drier; more uniform crop growth thanks to more even water distribution; reduced disease and better leaf quality because the foliage stays dry; and better root growth. Although few nurseries can justify the cost of replacing overhead systems with sub-irrigation, it should be considered — along with rainwater collection and water recycling — if new beds are being installed.
Unfortunately, several irrigation development projects were disrupted when the -Efford experimental station closed in 2004. But East Malling Research’s (EMR’s) new water centre is now up and running, and is home to a major HDC-funded project demonstrating and promoting best-practice ideas in irrigating container stock. The centre has of a number of nursery beds irrigated by a range of systems — overhead, capillary matting and an Efford capillary sand bed.
Research has just started at EMR on different irrigation-scheduling techniques as part of a Horticulture LINK project that draws on both DEFRA and industry funding. According to EMR research scientist Olga Grant: "Good scheduling not only reduces water use, it can also result in improved plant quality and uniformity. And scheduling can also be used to control plant growth, -reducing the need for pruning."
The project is looking at two scheduling methods. One is based on a plant’s response to changing weather conditions. This uses an EvapoMeter developed at EMR and now produced by Skye Instruments to measure the evaporative demand of the air, which correlates with plant water use. "Reading the EvapoMeter each day indicates how much water the plants are using," says Grant.
The other scheduling method being studied at EMR is based on measuring the percentage of moisture in the growing media. "We use a system produced by Delta-T -Devices, which links the moisture readings with an irrigation timer, switching the irrigation off when the media is sufficiently wet," says Grant. "At EMR we are currently evaluating and improving both methods for use on nurseries. But they are already available to growers who want to optimise irrigation efficiency and take the guesswork and inaccuracy out of determining when and by how much to irrigate."
Many of the problems of overhead sprinkler systems stem from the difficulties of arranging circular sprinkler patterns to cover rectangular blocks of plants. Gantries solve the problem by moving a line of flat fan nozzles across the crop and are already in use in newer pot and bedding-plant glasshouses, where they lend themselves to automation.
Less-sophisticated gantry booms are also popular among nursery stock growers in Germany and Scandinavia, where wheeled booms are towed along nursery beds.
Although gantry irrigation has not caught on in the UK nursery stock sector, its time could yet come — and a gantry boom has been installed as part of the demonstration project at the EMR water centre.
Part of the Horticulture LINK research project is looking at developing more sophisticated methods of monitoring the precise water demands of a nursery stock crop and then delivering a measured dose of water to match — perhaps even on a plant-by-plant basis. Overhead gantries that can be -precisely directed are suited to such an -approach, says
Lancaster University researcher Russell Sharp. He told a recent IPPS conference: "Gantries can be more precisely controlled than overhead sprinklers. The gantry nozzles and winch can be linked through computer control to sensors monitoring plant demand."
Sharp believes that thermal imaging to measure leaf temperature is likely to be the most reliable way to measure plant water demand to control a gantry and its individual nozzles. "As the growing medium dries, the leaf stomata close and the reduced evaporation from the leaf raises its temperature. The difference between a drought-affected or well--watered plant may only be 1°C or 2°C, but this is clearly picked up if the sensors are calibrated correctly," he says. "The rise in temperature happens very quickly after the stomata close, so is one of the first indicators of drought stress, and it has the advantage that it can be picked up at a distance."
As species adapted to dry conditions close their stomata at much lower soil moistures than those requiring moist media, the system would also "automatically" compensate for the differences between different species’ water demand.
"Prices of thermal imaging cameras are dropping, making it economically feasible to use the technology on nurseries," says Sharp. "But one challenge is developing technology that will rely only on the foliage temperature and which can differentiate between sun and shade, plants and bare ground." The system also has to be able to cope with the effects of wind speed, water droplets on leaves and temperature effects caused by other stresses such as pest or disease attack.
More accurate and precise irrigation control will also enable growers to take advantage of techniques such as partial root-zone drying and controlled deficit irrigation to control growth and schedule crops without risking drought damage.
Sharp says: "The potential for improving crop quality could be just as important as saving water."
Calculating storage and collection needs
Nurseries that rely on mains or a borehole should aim to store at least 40 per cent of their summer peak weekly water demand (ie three days’ supply) as an insurance against supply failure, according to ADAS protected crops consultantDan Drakes.
Wherever possible, nurseries should expand storage capacity and collect water from greenhouse roofs and nursery run-off, bearing in mind that slow sand filters may also be needed to treat pathogens.
The amount of water likely to be available from a glasshouse roof is based on the roof area and the average rainfall. The rule-of-thumb calculation is: area of glass (acres) x rain-fall (mm) x 4 = volume of water you could collect (cubic metres).
For example, an acre (0.4ha) of glass in Yorkshire could yield 3,352cu m — significantly more than the average needed by a protected ornamentals crop (0.55cu m per sq m). In theory that would make the nursery self--sufficient but in practice it depends on storage capacity as rain does not fall at the time of year when irrigation demand is greatest.
For nurseries aiming to depend on collected rainwater as their main source, reservoirs should be as large as possible.
To calculate the capacity you will need to treat run-off if recycling, conservative estimates put the industry average for run-off on protected pot, bedding and nursery-stock crops at five to 10 per cent of the water -applied. Calculate this on your crop by measuring the difference in weight of your pot samples before and after -irrigation. The weight in grams is equivalent to the amount of water (ml) absorbed by the growing media. For outdoor nursery stock, treatment capacity will also need to account for rainfall.
An ADAS survey found that 20 per cent of nurseries never tested water quality despite the potential variability if relying on mains; and 70 per cent don’t test for plant pathogens — a big issue if recycling. Collecting from hard-standing areas and roads on site is a potential source of pollutants, while zinc contamination could also be a problem if collecting through galvanised gutters.