2.3.5.1 - What is the frequency of my irrigations?

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Optimum irrigation frequency is important for efficient irrigation. Early irrigations can increase the number of irrigations in a season and can increase the total runoff and deep drainage. Late irrigations at higher irrigation deficit will increase crop water stress and can reduce yield and quality.

Irrigation aims to prevent plant water stress. The required frequency of irrigation and the volume of water applied will depend on climatic conditions and the amount of water your soil can store. Soil water content and cumulative evaporation less rainfall (E-R) measurements are the best guides for the timing of irrigation.

As a rule of thumb for Tatura, northern Victoria, when pasture plants deplete soil water moisture by 40 mm since the previous irrigation, this will result in pasture water stress. In practice this equates to an irrigation interval for perennial ryegrass/white clover based pastures of about 7 days during the December - February period. This 7 day interval typically corresponds to the optimum E-R interval of 50 mm but a shorter interval may be required under extremely hot conditions. Obviously you may need to extend the irrigation interval following rainfall. When evaporation rates are lower in spring and autumn, the irrigation interval needs to be longer.

The number of irrigations required to prevent the soil water deficit exceeding 40 mm is shown in Table 2.5. Areas with high irrigation requirements (see Table 2.3) need a greater number of irrigations.

Note - E-R is pan evaporation less rainfall. 50 mm of E-R is the recommended irrigation interval over summer for border-check irrigated pasture, and is approximately equivalent to 40 mm of pasture water use.

Table 2.5 – Number of irrigations required per year to prevent water stress

Site	Average	01/02	02/03	03/04	04/05	05/06	10/11	11/12
Tatura	19	21	24	20	18	19	12	18
Cobram	19	20	25	19	18	19	14	18
Echuca	20	21	23	20	19	19
Swan Hill	23	22	27	23	20	22	19	25