Impacts of past and present irrigation practices on River Murray salinity is a priority for river management in the Mallee region. A 2017 review examined modelling approaches to estimate salinity impacts from irrigation and found they could introduce biases. Additionally, there is no direct modelling of the unsaturated zone to account for perching on clays which has a significant influence on the magnitude and timing of salt loads to the river. These limitations create potential for bias when linking on-ground actions (such as irrigation efficiency improvements) to salt loads in the river but could be addressed by (1) developing unsaturated zone models to deal with perching; and (2) incorporating and calibrating such models within an accredited groundwater model. This talk addresses the latter step.
A pilot trial was conducted for the Loxton-Bookpurnong irrigation districts of South Australia, comparing two modelling approaches. The first was that of a 2011 groundwater model, where some assumptions were made about hydraulic conductivity, and then the recharge was inferred, via an inverse method. An agronomic water balance was used as a check, but was not used to estimate the recharge.
The second approach was developed and calibrated during the pilot trial. An agronomic water balance was integrated with a semi-analytic unsaturated zone model and a numerical groundwater model. The integrated model used a recharge time series generated by the water balance and unsaturated zone model. This was applied to distinct recharge zones, based on lithology, date of irrigation development and drainage practices. The outputs, especially recharge and salt loads to river, were compared to those with the 2011 accredited model. , in which an integrated agronomic water balance-unsaturated zone-groundwater model was developed and calibrated; and the modelling outputs, especially salt loads to river, were compared to those with the 2011 accredited model.
The integrated model was fit-for-purpose in that it enabled estimation of salt loads for various scenarios. Irrigation actions were able to be more directly linked to groundwater responses and salt loads than with the inverse method. Drainage information was important to constrain soil properties and vertical fluxes.
The conclusions from the study are
The results of the pilot trial are being used to develop recommendations for modelling salinity impacts for Mallee irrigation districts.