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Farmnote 23/1990 [Reviewed August 2006]
By Greg Luke, Irrigation Research Officer, Division of Resource Management, South Perth
Irrigation aims to replace the water used by a crop, or lost by evaporation or drainage through the soil. The objects are:
- to produce the optimum yield, and
- to apply water efficiently.
The efficiency of water application is controlled by the design of the irrigation system, and the way in which the watering programme is scheduled.
Scheduling, or deciding how frequently to irrigate and how much water to apply, usually relies on using one or more of three different approaches:
- experience,
- replacing evaporation, and
- monitoring soil moisture.
Experience
Most growers rely on years of experience to develop irrigation schedules that produce good crops.
Agriculture Western Australia trials have shown that many of these schedules, which were developed to maximise yields rather than save water, tend to over water. Water savings of up to 70 per cent have been made by introducing an alternative scheduling technique.
Over watering causes many problems including:
- waterlogging and yield losses on heavy soils;
- wasted water and unnecessarily high pumping costs; and
- inefficient use of applied nutrients by plants, resulting in leaching of nutrients into the groundwater and wetlands adjacent to sandy soils.
The Environmental Protection Authority requires that new horticultural enterprises be non-polluting. As a result, more reliable scheduling techniques must be used.
The need to upgrade the scheduling of irrigation systems therefore stems from the joint needs to conserve water, reduce costs, and preserve the environment.
Evaporation replacement
Irrigation can be scheduled effectively by replacing the amount of water transpired by the crop. A crop's transpiration rate can be estimated from local evaporation rates, adjusted using a different Crop Factor for each crop.
Crop Factors have been determined from Agriculture Western Australia's field trials form any different crops - see Farmnote No. 66/95 'Watering requirements of vegetables grown on sandy soils'(Agdex 250/560), Farmnote No. 42/88 'Irrigation requirements of avocados' (Agdex 235/561) and Farmnote No. 114/88 'Growing cucumbers' (Agdex 264/11).
The evaporation data can be obtained by measuring the amount of water lost from an open water body, in an evaporimeter. Data are also available from local meteorological stations, Agriculture Western Australia Research Stations, or by constructing an evaporimeter on site.
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The technique for constructing an evaporimeter and for using the data for scheduling irrigation is outlined in Farmnote No. 22/90 'Scheduling for trickle, sprinkler and flood irrigation' (Agdex 561).
Soil moisture monitoring
Using either experience or evaporation factors, a grower only estimates how much water a crop is using. One way of determining more accurately the amount of water a crop is using is to measure the soil moisture content on a regular basis,
There are many different pieces of equipment that can be used to monitor soil moisture. They each have their advantages and disadvantages. See Farmnote No. 26/90 'Soil moisture monitoring equipment' (Agdex 561), Farmnote No. 25/90 'Tensiometers - preparation and installation' (Agdex 561) and Farmnote No. 24/90 'Interpreting tensiometers readings' (Agdex 561).
By using gypsum or ceramic blocks, tensiometers or neutron or capacitance probes, a grower can accurately determine not only how much water has been used by the crop, but how frequently it should be irrigated.
Combination methods
Information on the use of tensiometers for a number of specific crops has been developed. These guidelines give a good indication of the usefulness of soil moisture monitoring equipment for irrigation scheduling.
See Farmnote No. 27/90 'Efficient irrigation for determinate tomatoes in the Gascoyne River Area' (Agdex 262/561)and Farmnote No. 9/93 'Using tensiometers for effective banana irrigation in Carnarvon' (Agdex 231/561).
By listing both evaporation data and soil moisture readings, a grower can become extremely efficient at irrigation scheduling. Both systems have weaknesses. In combination they provide useful cross checks.
For example, the Crop Factors published in the Farmnotes are average figures. Local conditions such as exposure, windbreaks, soil types and cultural practices can change the amount of irrigation required. The factors can be adjusted to local conditions by monitoring soil moisture levels.
The result of using a combination of techniques is to make both methods more effective and reliable.