**6. Conclusions**

This paper has described the modelling of recharge from new irrigation developments, where there is the potential for perching over a deep vadose zone. The model relies on a surface water balance of the irrigation area and is intended to provide input to regional groundwater models. The model adapts algorithms for which there is no perching to those with perching, allowing a smooth transition through the parameter *A*.

The work shows that hydrological equilibrium between irrigation accession inputs and recharge to the water table can occur by increased hydraulic gradient through the clay and a greater area of ponded infiltration through the clay. Where the head of the perched water table is su fficiently close to the land surface, some water is returned to the land surface either through sub-surface drainage or by evapotranspiration. There is good agreemen<sup>t</sup> between estimates of recharge, ponded head and return to the surface from the semi-analytical PerTy3 model and those from the numerical FEFLOW model for one-dimensional situations, where recharge occurs beneath the irrigated fields. The perching leads to some proportion of the recharge occurring after a long time delay from the timing of the new development.

The model shows that for the parameter range chosen here the effect of two-dimensionality on the total recharge appears to be minor, whereas more detailed processes, such as the proportion of recharge that occurs external to the irrigation field and the ponded head, are sensitive. These latter lateral effects scale in a predictable fashion to a parameter *B*. Since recharge models usually only require the total recharge, the lack of sensitivity should simplify the modelling.

The modelling also shows that the recharge under brownfield developments, which are developed near pre-existing developments do not appear to be significantly different to that from greenfield developments. This allows the recharge under realistic and complex scenarios of irrigation development to be treated through superposition of recharge from individual fields.

The main input to PerTy3 is the irrigation accession under irrigation. The accuracy of the recharge outputs from PerTy3 is dependent on good information for irrigation accessions over the history of the irrigation development. This, in turn, is dependent on knowing how the irrigation area, and water-use efficiency has changed over time.

**Author Contributions:** Conceptualization, G.R.W., D.C. and T.S.; methodology, G.R.W., T.S.; software, G.R.W., T.S.; validation, G.R.W. and T.S.; formal analysis, G.R.W.; investigation, G.R.W. and T.S.; resources, D.C., G.R.W.; data curation, G.R.W., T.S. and D.C.; writing—original draft preparation, G.R.W. writing—review and editing, D.C.; project administration, D.C.; funding acquisition, D.C. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was partially funded by MURRAY-DARLING BASIN AUTHORITY, project number MD004683.

**Acknowledgments:** The authors would like to thank the members of the Technical Committee (Juliette Woods, Ray Evans, Emmanuel Xevi and Prathapar) and Hugh Middlemis for technical advice.

**Conflicts of Interest:** The authors declare no conflict of interest.
