The assessment of the impact of Mallee irrigation on River Murray salinity is a management priority for the MDBA Salinity Strategy. A 2017 review identified two different modelling approaches for the treatment of irrigation recharge, both of which could introduce consistent biases and hence affect salinity management. This issue could be addressed by (1) developing unsaturated zone models (UZMs) that can represent perching; and (2) incorporating and calibrating such models within an accredited groundwater model. This talk addresses the first step.
Previously, simple UZMs were used to estimate time lags between irrigation changes and recharge changes. These models are not appropriate for low conductivity soil layers that cause perching. Perching leads to lateral movement away from the irrigation field and possibly water returning to the land surface, affecting both timing and magnitude of recharge. The complexity of historical irrigation developments and soil properties challenge the development of UZMs that are simple enough for groundwater modelling. This talk describes the development, testing and application of appropriate UZMs. The outputs from these models for a range of ‘benchmarking’ studies have been successfully compared with those from FEFLOW.
Several steps are required to apply UZMs: (1) districts are disaggregated into recharge zones based on soil properties and date of initial irrigation; (2) an agronomic water balance is developed to represent irrigation history; (3) run the models for each zone using agronomic water balance as input; (4) use drainage output to partially calibrate soil properties; and (5) calibrate irrigation efficiency parameters alongside hydrogeological parameters to best match groundwater data.
The applicability of these models has been trialled for the Loxton-Bookpurnong irrigation districts. Modelled recharge and drainage volumes were found to be sensitive to perching, emphasizing the importance of perching for salinity assessment modelling. As well, the approach provides for a holistic calibration process to constrain salt load estimates.