The Burdekin Groundwater Management Area (BGMA), is part of an extensive alluvial aquifer associated with the lower Burdekin River. It supports approximately 35,000ha of irrigated agriculture and drains to the Great Barrier Reef. Since the construction of the Burdekin Haughton Water Supply Scheme (WSS) in the late 1980’s, groundwater levels have risen by up to 10m. Water-tables within 1m of the land surface have been recorded in some areas, posing a significant threat to agricultural production. The WSS allows water to be pumped from the Burdekin River and delivered to farms via open channels, irrigation runoff is removed by a separate drainage network. Recharge to the aquifer is identified to be by rainfall, irrigation deep drainage, losing streams, and leakage from the channel supply system. Discharge mechanisms from the aquifer include coastal outflow, abstraction, gaining streams and evapotranspiration from groundwater dependent ecosystems (McMahon et al., 2012).
Hydrograph analysis (Ferdowsian & Pannell, 2009) was used to determine whether groundwater levels in the area have reached a new equilibrium. An investigative water balance approach was then used to determine why water levels have stabilised in some areas.
An equilibrium has been reached in the coastal sections of the BGMA, primarily through increased discharge, as the water-table has risen and intersected the surface water drainage. The drains act as either gaining or losing watercourses at times. While groundwater levels have stabilised to a height that may mitigate the risk of waterlogging during years of average rainfall, periods of above average rainfall and flooding would result in an exceptionally high water table, with major implications for agricultural production. Further, the quality of the groundwater entering the drainage system, and discharging to the Great Barrier Reef lagoon is of major concern (Vardy et al., 2015). The risks associated with these two issues suggest that additional management of the groundwater table is needed.
Water levels further inland in less transmissive sections of the aquifer continue to rise with increasing risk to agriculture. There is potential that the drainage network will have less of an impact in stabilising the water-table in this area due to lower transmissivities in the aquifer. Elevated water levels will result in increased salinisation and waterlogging which may have an irreversible impact on agricultural production.
These findings will guide future groundwater management decisions in the BGMA and may be transferrable to other irrigated agricultural areas.