The Effect of Irrigation Water Salinity On the Saturated Hydraulic Conductivity of Sodic Clay Soils.

The Lower Burdekin is a major irrigation area in north Queensland, Australia.  In some areas of the Lower Burdekin, groundwater is being added to river water to increase the salinity of irrigation water and in so doing improve infiltration of local sodic clay soils. Rising groundwater levels in these areas however suggest that deep drainage has already been enhanced so further changes in soil hydraulic properties with increasing irrigation water salinity are causing concern. In this paper we discuss the use of a series of column experiments and modelling to explore the effect of irrigation water salinity on the saturated hydraulic conductivity of sodic soils and the potential for enhanced deep drainage. Mixed cation solutions of varying salinity were applied under constant head to columns of repacked soil. The solutions tested had salinities ranging from local river water to local saline groundwaters. Measured flow rates were found to increase rapidly after flow commenced then decrease gradually until flow rates became stable. The maximum hydraulic conductivity using the highest salinity water was approximately 100 times larger than the maximum hydraulic conductivity with the lowest salinity water.  Based on the results from these experiments, we are assessing the application of HYDRUS-1D, with soil water chemistry-dependant hydraulic conductivity, to modelling the response of these soils. Findings from the column experiments and HYDRUS-1D modelling are being used to improve understanding of the impact of long term treatment with saline irrigation water on these sodic soils. This is needed to test the validity of current practices in which water management in the lower Burdekin is being informed by regional hydrology modelling with groundwater recharge estimated using static soil hydraulic properties.