Irrigated horticulture in the Lower Murray (Riverland-Sunraysia) region has improved the water use efficiency over the past two decades from about 50% to about 80% as a result of improved irrigation practices. This has both reduced recharge into groundwater and increased on-farm water productivity.

But the improvements in irrigation management increase the risk of salt accumulation in the root zone, threatening the sustainability of the region. Irrigation management needs to account for both the crop's needs and additional water (the leaching fraction) to flush salt out of the root zone.

During summer precision irrigation management results in less than 10% of total applied water moving past the root zone as deep drainage. At 14 surveyed properties the water draining past the root zone had an average leaching efficiency (that is the amount of soil the water actually moves through) of only 65%. All this points to the potential for a build-up of salts in the root zone under the low winter rainfall conditions of the past few years.

For example, when the average river water salinity is 0.4 dS/m, root zone salinity with a 15% leaching fraction through the whole profile should be about 0.6 dS/m. However, field surveys indicate that the root zone salinity, though very variable, is often greater than 1.3 dS/m because of the inefficiency of salt leaching in these soils.

The build up of salts in the root zone was further explored using a model (LEACHM-TRANSMIT, a two dimensional solute transport mathematical model). Two simulations were run under the scenario of a drip irrigated vineyard with 10% deep drainage. First, when irrigation water salinity was 0.3 dS/m, the current river water salinity at Loxton, about 130 kg/ha of salt could accumulate in the root zone during one irrigation season. If irrigation water salinity was 0.8 dS/m, the Morgan benchmark, then 2,000 kg/ha of salt could accumulate. Under this scenario sign! ificant crop losses would be expected.

The Tristate Salinity project team is currently examining what management strategies can be used to minimise these potentially salt induced yield losses. A simple root zone monitoring toolkit is being developed for growers to track root zone salinity during the growing season. In addition, a modelling tool is being developed for water managers in the Lower Murray districts to better understand the impact of river salinity on root zone salt build up. In years of higher river water salinity and/or low winter rainfall it is likely that additional irrigation water will be required to leach the root zone or risk serious yield losses as a result of salt build up.