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Due to improved irrigation management and system upgrades, the field application efficiency (FAE) in the Lower Murray horticultural districts has risen from about 50 to 85% during the past 2 decades. Consequently, the drainage volumes have reduced from about 50 % to 15% of water applied. Under steady state conditions, the salinity in the root zone can be estimated from salinities of the irrigation water and the leachate (i.e. extra irrigation water applied which drains out of the base of the root zone).

For this estimate it is assumed that the leaching water uniformly displaces the (saline) soil water in a ‘piston’ flow manner. Despite the relatively low salinity of the River Murray flows in the past five years, growers practicing ‘precision’ irrigation expressed concern about the gradual, but visual accumulation of salinity in the root zone of drip irrigated vineyards in particular.

A ‘Tri-State Salinity’ syndicate of Government agencies from western NSW, Victoria and South Australia (with support from federal agencies) was formed to generate new knowledge for managing root zone salinity hazards by undertaking a 3 year laboratory and field scale studies. Besides assessing the risk of salinity accumulation under precision irrigation, the project team is working on strategies for minimising production losses. Field data from conventional drip in the Riverland and Sunraysia regions showed that only less than 10% of applied water was found to leave the root zone during the grape growing season, which resulted in salt build up in root zone.

This is particularly of concern when the average leaching efficiency at the 14 surveyed properties was 63% and where only <5% of total applied water is flushing the root zone during the irrigation season. Using the data from a drip irrigation vineyard at Loxton in Bookpurnong Lock-4 district, the output of a two-dimensional numerical flow/transport model (LEACHM-TRANSMIT) showed that during summer about 2 t of salt /ha would accumulate in the root zone if the River water salinity is 0.8 dS/m. However, even at the currently River salinity of 0.3 dS/m and 95% FAE, crop losses due to gradual salinity build up may be inevitable in the Riverland/Sunraysia districts.