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The Loam Ranger - Regulated deficit irrigation

 

Dear Loam Ranger,

The drought is worsening and I recently read that irrigated rice planting in Australia had dropped by 93%. So how do we make our limited water resources go further?

Working smarter rather than harder can help. By scheduling irrigation to the periods when plants actually need the water, and by restricting the amount applied to less than luxury amounts, we can achieve the same yields with savings of up to 30% water. This practice is called regulated deficit irrigation, or RDI.

The CRC for Viticulture, in Adelaide, defines RDI as the use of irrigation to maintain plant water status within prescribed limits of deficit for part of the growth cycle of the plant. Mild to moderate plant moisture stress is allowed to develop at growth stages at which it has little or no effects on production. The ideal aim is to restrict plant growth while increasing harvestable yield, at the same time as improving water use efficiency.

The frequency of irrigation is determined by the decrease in plant water potential below a specific limit that depends on the stage of crop growth. Ideally this is measured on the plant with a leaf moisture meter, but for convenience it can be inferred from soil moisture content or estimates of plant water use from a reference evaporation pan.

RDI was developed in the 1970s at Tatura, in Victoria, as a way to cap early-season shoot growth, when fruit growth is slow, and thus reserve water for later in the season when fruit growth is critical. It was later extended to grapevines as a way to increase berry sugar content. But the concept can be applied to any managed planting.

In practical use, plants are kept short of water when growth is not critical, but are given sufficient water when growth is critical.

Research shows that fruit yield can be increased, that sugars are increased, and that any production losses are much smaller than the water savings; for example, a 40% reduction in water use reduced growth by only 8%.

Further reading

FAO. 2002. Deficit Irrigation Practices. FAO Water Report 22. Food and Agriculture Organization, Rome.

Goodwin I. 2000. Irrigation scheduling for regulated deficit irrigation. Agriculture Notes AG0299. Department of Primary Industries, Victoria.

Kriedemann PE, Goodwin I. 2004. Regulated Deficit Irrigation and Partial Rootzone Drying. Irrigation Insights 4, Land & Water Australia, Canberra.

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