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IRRIGATION efficiency is a well understood concept among irrigators, representing the proportion of irrigation water applied that is retained in the rootzone for use by the crop.

In an attempt to minimise environmental impacts from irrigation, high irrigation efficiency (>85%) has been promoted.

Irrigation efficiency is related to leaching fraction, in that leaching fraction is a major component of the “inefficiency” proportion of irrigation.

So for example, an irrigation efficiency of 85% implies that approximately 15% (100% – 85%) of the applied water is not stored in the rootzone for crop use, and the majority of this water is assumed to drain below the rootzone, leaching salt with it.

The concept of leaching efficiency is not nearly so well understood.

Leaching efficiency is a measure of the proportion of the water supposedly dedicated to leaching salt from the rootzone, that effectively removes salt.

It is often assumed that this figure is 100%, that is all the excess water travels uniformly down through the rootzone, collecting salt as it goes, and flushing it out of the base of the rootzone.

Unfortunately, this is not the case.

Water will take the path of least resistance, which may be along soil cracks in heavy soil, or through dead root channels.

This water does not interact fully with the rootzone soil, and consequently does not carry its full complement of salt.

As a result, leaching is less effective (or efficient) than intended, and salt accumulates unnoticed in the rootzone.

With higher irrigation efficiency being pursued by irrigators, the problem of low leaching efficiency can result in significant salinity build-up in vineyard rootzones.

Consequently it is important that this concept is understood, and taken into account in irrigation management.

Research carried out by SARDI researchers in the Riverland found that leaching efficiency of individual irrigation events in a drip irrigated vineyard ranged from 48 to 85%.

Data from a survey of multiple vineyards in the Riverland, with a range of irrigation system types, indicated that average season leaching efficiencies as low as 40% were common.

At these levels of leaching efficiency, combined with high irrigation efficiency, rootzone salinity is close to levels likely to cause yield loss, even with relatively low irrigation water salinity (400 µS/cm).

If river salinity rises in the future, these vineyards are likely to suffer yield loss unless leaching efficiency can be increased, or irrigation efficiency is decreased (that is, more water is flushed through the rootzone).

Given the impacts on the environment from lower irrigation efficiency (in particular drainage flows causing increased flows of saline groundwater to the river), the preferred option is to maximise leaching efficiency, or find other ways to cope with salt build-up.

Options include:

  • Plant vines on salt tolerant rootstocks, which allow rootzone salinity to rise to higher levels without any impact on the vines;
  • Increase leaching efficiency by applying water intermittently (e.g. pulsed irrigations), to reduce the hydraulic pressure forcing soil water down through the rootzone, thus facilitating greater lateral movement of water, and resulting in greater mixing between applied water and water already present in the soil;
  • Leach during winter and in conjunction with rainfall events, to reduce the amount of irrigation (and therefore salt) applied to achieve leaching, and to take advantage of the more even distribution of water achieved by rainfall.
  • Deficit irrigation strategies (e.g. Regulated Deficit Irrigation) rely on maintaining reduced water content in the rootzone for a prolonged period. As a result, irrigations applied during the deficit period are small, and will not result in leaching, leading to salt accumulation. It is therefore critical that special care is taken to adequately leach the rootzone immediately following the end of the deficit period.
  • Under drip irrigation, irrigating during significant rainfall events to ensure that salt is moved away from the rootzone, rather than being mobilised by rain and moving back into the rootzone.

This article was written by Mark Skewes, SARDI Research Scientist, Loxton Research Centre and Rob Stevens, formerly SARDI, Waite Precinct.

Full story in the August 2014 issue of Grapegrower & Winemaker.

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