Irrigating crops efficiently

Drip irrigation systems provide an efficient means of irrigating vineyards. As with all irrigation systems regular maintenance is needed to maintain this efficiency. Chlorination is a key factor in drip system maintenance. It prevents algal and fine clay sediment build up that will eventually cause blockages. Outlined below is a practical procedure for maintaining and chlorinating your drip system.

Flushing

Drip systems, including mains and submains, should be flushed completely before chlorinating. Most built-up organic matter will be removed this way. Flushing several times during the year should be a routine part of any irrigation system maintenance. Flush mains and submains first, then open the ends of the drip laterals and allow the water to flow until it runs clean. Your water quality will dictate how often laterals need flushing. Irrigators using Murray water should start with every four weeks; if there is no appreciable build up in this time extend the period between flushes.

Chlorination

Because each system is different and water quality varies, it is difficult to make strict recommendations on chlorinating drip systems. When chlorine is dissolved in water, hypochlorous acid (HOCI) and hypochlorite (OCI) are formed, which are together referred to as "Free Chlorine". Therefore the effectiveness of chlorination is highly dependent on the pH of the water. Chlorine is more potent in water with a low pH. It reacts strongly with iron, manganese and hydrogen sulphide producing insoluble compounds that may need removing by filtration and flushing. Chlorine can react with fertilisers particularly ammonia and nitrogen so the two MUST be applied separately.

Chemicals to use

The most common sources of chlorine are sodium hypochlorite (a liquid, pool chlorine), calcium hypochlorite (powder or granules) and chlorine gas (toxic).

Dose Rates

This will be dependent on water quality and site. The following guidelines minimise the growth of algae and build up of clay sediment. Three levels of treatment are suggested:

  • Continuous treatment. Chlorine is injected continuously to ensure a concentration of 1 to 2mg/L (ppm) of free chlorine at the farthest emitter.
  • Intermittent treatment. Inject at a rate of 10 to 20mg/L for 30 to 60 minutes, with a minimum of 20-30 minutes, after the chlorine has reached the last emitter. Chlorinate on a frequent basis, every two months depending on results and level of re-contamination. A low rate of re-contamination allows treatments to be spaced further apart. Irrigation systems are usually dosed twice per season.
  • Super - chlorination treatment - Inject at a high rate of up to 500mg/L and leave in the system for 24 hours before flushing. This method is used on an annual basis or when algal or sedimerits build up to severe levels.

Check concentration of chlorine at last emitter while dosing using a swimming pool chlorine test kit. Always thoroughly flush the irrigation system after dosing.

Calculating injection rate

To calculate the injection rate required to achieve a dose rate of 20mg/L, with a system flow rate of 50Lls, using 10% active sodium hypochlorite, use the following formula:

Injection Rate =

System flow (50L/s) x Dose rate (20mg/L) x 0.36

 

Active chlorine (10%)

=

             36L/h

If using calcium hypochlorite (powder) make up a solution of known chlorine concentration and use the formula above. Calcium hypochlorite is 60% active, hence 10kg of calcium hypochlorite in 100L of water produces a solution containing 6% concentration of active chlorine.

If using Chlorine gas, use the following formula:

Injection rate (kg/day) =

System flow rate (50L/s) X Dose rate 20mg/L x 0.0864

=

86.4kg/day

NOTE: special equipment and handling requirements apply to the use of Chlorine Gas. Contact supplier for details.

How to inject chlorine

Injecting liquid chlorine is carried out using an injection system made of non corrosive material. Injection should be carried out upstream of filters to help clean the filters. The same tank and pump used for fertilizer injection 1 can sometimes be used provided all the equipment has been thoroughly cleaned of residual fertiliser before chlorination.

There are many commercial injection products available on the market and these fall into the following categories:

  • a suction injector, usually a tank on the suction side of the pump.
  • pressure differential, such as a Massei Venturi Injector or pressure tank
  • pump injection, brands such as TMB, AMIAD or Dosatron

Whatever unit is purchased it should include a flow meter or some method of measuring and adjusting the injection rate to match the calculated rate of chlorine injection.

Hints

Monitor irrigation system flow rate using your irrigation flow meter. A slowing of flow rate can indicate partial blockage of emitters. Flush the entire system regularly usually twice per year depending on water quality. Chlorine is corrosive and poisonous - take care.

 

For further advice (on a fee for service basis) contact the ICMS on 08 8595 9138. 

 

Denis Sparrow, Irrigation Consultant, Irrigated Crop Management Service (ICMS), Loxton Centre

 

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