Understanding and managing groundwater and salinity in a tropical conjunctive water use irrigation district

Agricultural production around the world is increasingly being constrained by hydrological factors--such as over-extraction of groundwater in some locations, rising water tables in others, and worsening groundwater quality in general. One such area is the Lower Burdekin irrigation area in northern tropical Australia, where rising watertable levels and increasing salinity concentrations within alluvial deposits are causing concern. The aim of this study was to improve understanding of the processes driving trends in groundwater quantity and quality in Mona Park, a conjunctive water use irrigation district in the Lower Burdekin. The analysis is intended to enable land and water managers to explore alternative policy and management practices to help support the reversal in current trends, and to improve water table conditions in terms of both water quantity and quality. Key lessons that are applicable to the development of new irrigation schemes in wet-dry tropical regions elsewhere in the world are emphasised. This study demonstrated that simple qualitative methods that link historical developments and observed climatic and hydrological trends can support development of a robust understanding of groundwater behaviour. The results showed that to minimise groundwater accessions in wet-dry tropical regions, a large soil water deficit should be maintained in the unsaturated zone prior to the onset of the wet season to buffer against potentially large wet season recharge events, and that this strategy should be implemented from when irrigation is first commenced. It is very clear that groundwater systems under or down gradient from irrigated areas need to be managed adaptively, such that: (1) timely decisions are made in response to changes in watertable level and groundwater quality; and (2) suitable mechanisms are in place to ensure farmers have the financial incentives and flexibility to respond in the short-term. The work also demonstrated that the establishment of good baseline data prior to irrigation development, and long-term analysis (>30 years) involving various combinations of wet and dry periods, are required in order to build a comprehensive understanding of potential groundwater behaviour and adaptive management needs.