The rapid expansion of horticulture and in particular, avocado production on the Aupouri Peninsula (Fig.1) over the last several years has brought tremendous economic and social benefits to the region.
It has also brought increased demand for irrigation.
As a leading niche water consultancy, Williamson Water & Land Advisory (WWLA) has played a key role on behalf of a number of stakeholders, through the development of its proprietary three-dimensional Aupouri Aquifer Groundwater Model, or AAGWM (Fig.2).
The model covering an area of approximately 535 km² is being used as a critical tool in assessing the long-term sustainability of groundwater resources and determining aquifer conditions - based on current and proposed water use.
WWLA managing director Jon Williamson says the AAGWM was developed through collection and analysis of all available data within the area to characterise both catchment and aquifer physical conditions, climate and historical water use.
He said model calibration was achieved through the matching of groundwater monitoring observations to simulated groundwater levels at 56 locations throughout the aquifer. Of these, 17 monitoring bores were used for transient calibration dating back to as early as 1960. The accuracy of the model was evaluated at 5% of simulated heads across the model domain using the root mean square error method. A value less than 10% is considered accurate for regional groundwater analysis.
“These efforts have enabled a robust assessment of water resources on the Aupouri Peninsula and the likely environmental impacts from groundwater use under current and proposed pumping volumes.”
He said the model would not only be pivotal to assessing the environmental impacts for 23 pending consent applications for groundwater takes, but would also fundamentally improve the understanding of regional water resources and provide accurate estimates of proposed water takes on saline intrusion.
According to Mr. Williamson, groundwater takes that have been initiated in the last 10 years, for example at Valic Orchards and Sweetwater Farms, were important points of data collection because of ongoing groundwater monitoring in both deep and shallow bores.
Mr Williamson noted some key statistics from his company's work including that:
The Aupouri aquifer stores approximately 2,850 billion litres of water.
Rainfall over the aquifer averages approximately 687 billion litres per year, of which approximately 236 billion litres infiltrates into the soil to percolates downwards to recharge the aquifer.
The seasonal variation in aquifer storage amounts to approximately 60 billion litres, and varies naturally by up to 250 billion litres over consecutive wet or dry years.
The total of currently consented and pending groundwater consents is up to a maximum of 14.4 billion litres per annum, which is only required during the most severe droughts like that currently being experienced in early 2020.
Based on these numbers, the total level of proposed allocation, including current consents, represents only 6% of annual recharge and approximately 0.5% of the water stored in the aquifer, albeit not all water stored in the aquifer should be available for abstraction.
“While saline intrusion likely occurs under natural conditions in some locations, notably nearer the east coast where the basement rock is shallower, proposed groundwater takes are likely to have minimal impact in terms of increasing saline intrusion.”
He said it was also “clear that existing orchards had not depleted groundwater levels, as strong water level recovery from off-season recharge has been consistently observed in the monitoring record”.
"Overall it is a positive result, showing that existing natural resources can support proposed abstraction including those for orchard development alongside municipal and community needs, which will bring significant social and economic benefits to the region."