You will get the best possible yields by maintaining uniform soil moisture in the root zone during the growing season.

By applying only the amount of water required by the crop, savings can be made on pumping, fertigation and water costs and reduce run-off problems.

Right rate and amount of water

Knowing the application rate will help you apply the right amount of water to your crop.

Measure how many litres of water your irrigation system emits in one hour. This is called the discharge rate.

For example, with sprinkler irrigation:

• discharge rate = the number of litres emitted per minute (as collected in a measuring cylinder) per sprinkler (in the sprayline) × 60 minutes

The space between two sprinklers along a sprayline and between adjacent spraylines (lateral space) is called the effective area.

For sprinkler irrigation:

• the effective area = sprinkler spacing (m) × lateral spacing (m)

Dividing the discharge rate by the effective area calculates the application rate.

Thus, for sprinkler irrigation

• the application rate (mm/hr) = discharge rate/effective area

Applying irrigations

• Do not base the length of irrigation applications on time alone.
• Do not apply the water too fast or at too high a pressure as it results in run-off.
• Find out how your soils react to the application of water. The application rate should not exceed the soil's ability to absorb the water being applied or its infiltration rate. The infiltration rate of a soil can be determined by field tests. As a guide, the application rate for slopes up to 7° should not exceed 40 mm/hour for deep sandy soils, 20 mm/hour for sandy loams and 3 mm/hour for heavy clay soils.
• Do not water for too long. This results in too much water being applied. Excess water will leach natural mineral nutrients and added fertilisers, and create subsoil drainage problems.Once the soil has absorbed all the water it can, further application will only result in waterlogging and then surface run-off. Too much water also encourages plant diseases.
• Do not apply too little water. The plants will become stressed and your yield will suffer, no matter how much water you apply in the future. If the subsoil becomes too dry then irrigation water may be drawn down from the soil above, especially the crop root zone.

Uniform water distribution

The uniformity of water application is important, so you need to check your irrigation equipment each season. Poor uniformity will affect crop growth as some plants will receive more water than others.

Application uniformity is related to pressure, nozzle size, sprinkler spacing and wind. You can get a good idea of the uniformity at each sprinkler along the sprayline with a pitot tube which measures sprinkler pressure. A pressure variation greater than about 15% along the sprayline produces poor, uneven water emission and distribution from the sprayline's sprinklers.

Irrigation systems

If irrigating with town water, use mini or low pressure sprinklers. These sprinklers have a lower application rate which better matches the soil infiltration rate, producing less run-off and allowing improved crop access for the continuity of farm activities such as harvest and spray operations.

An alternative irrigation system is sub-irrigation which uses drip irrigation lines and applies water to the plant from below the soil surface. Advantages include improved soil aeration and reduced surface soil crusting.

Irrigation scheduling

To grow the best crops you need to apply the right amount of water at the right time. This is called irrigation scheduling.

Use a rain gauge so you can record the amount of rain water that falls.

One aid to applying the right amount of water is a tensiometer. A tensiometer is one type of irrigation scheduling device which is fairly cheap (about $100) and easy to use. Usually two tensiometers are used together, at different depths in the soil. The shallow one will show you when to irrigate and the deeper one will show you how much water to apply.

Other tips

Ensure you have enough water to irrigate all the area you are farming. Planning, design and construction information is available from NSW Agriculture's Irrigation Officers. They can calculate if your dam is big enough and if the flow rate from the mains pipe will be adequate.

Wind speed is often lower at night, so irrigating then can result in better water distribution and less evaporation.

Frequently check the irrigation water quality. Vegetable crops vary in their sensitivity to salinity which can reduce plant growth and yields. Water which is termed 'hard' can produce white deposits that block pipes and sprinklers. Water that has a low pH is acidic and corrodes pumps, pipes and tanks. Soluble iron stains leaves and blocks sprinklers. Suspended solids such as algae, can also block pipes and sprinklers, and stain crops.

Keep the irrigation system in good working condition. Repair leaking pipes, regularly clean filters and service the pump.

Keep your dam in good condition. Seal leaks as soon as possible, control weeds and keep a protective cover growing on the dam wall.

Further information

Further advice on the advantages and disadvantages of different irrigation systems is available from Irrigation Officers of NSW DPI.

You may also wish to consult the Agfact 'Water quality for vegetables' (Agfact AC.23).

These points are taken from Best Practice Guidelines for Growing Vegetables by Leigh James, Jim Murison, Bill Yiasoumi and Ashley Senn.