Groundwater management under instationarity: scenario simulations for the Wairau Aquifer, New Zealand

Groundwater resources are fully allocated in many coastal aquifers in New Zealand. External forces such as a reduction in recharge and climate change add additional pressure for a sustainable management of the resource. Groundwater levels in the unconfined Wairau Aquifer (Marlborough, New Zealand) have been declining for decades due to both natural and human-made reasons and are  superimposed by strong seasonal variability and increasingly also climate-change effects. Groundwater is abstracted mainly for irrigation (viticulture) but also for municipal and industrial uses. There are growing concerns that current management limits and thresholds in the regional water plan are not sustainable.

A detailed 3D surface water-groundwater flow model (MODFLOW) is used to investigate the effect of different groundwater allocation scenarios on groundwater storage and the flow of low-land springs which possess high cultural and recreational values for the community. An earlier version of the model (Wöhling et al. 2018, Groundwater, doi:10.1111/gwat.12625) has been recently extended and updated with an improved conceptualization. The regional-scale model was calibrated using 2.5 years of data and evaluated on more than 20 years which include two major flood events (18/07/2021 & 21/8/2022) that rank among the highest on record. Uncertainty of model simulations are estimated using Null-Space Monte Carlo simulations. The regional-scale model performs well with respect to observed groundwater heads, spring flows and river-groundwater exchange flows and generally agrees well to the data, even for the flood events in 2021 and 2022.

Current groundwater management regulations are cut-off limits at four goundwater observation wells and at a major spring as well as a total abstraction volume of 73 million m³ per year. Under current conditions, groundwater abstraction for irrigation can vary widely between years, while industrial and municipal water demands remain relatively constant. The actual groundwater abstraction is on average only 30% of the permitted annual allocation limit. However, the cut-off limits for groundwater levels and spring flows have been approached and exceeded frequently in recent years. This occurs in the summer months, when irrigation demand is high and river recharge and groundwater storage are on a seasonal low.

The simulations show a strong impact of irrigation water takes on groundwater depletion in summer. Compared to current conditions, a scenario with the full permitted annual abstration leads to significantly lower groundwater levels, aquifer storage and spring flows which would lead to continuous cut-offs given current regulations in the regional management plan. Under past climatic conditions, a strong increase in carry-over effects of storage depletion to consecutive years is not evident. But it has been shown previously that prolonged summer low-flow periods lead to low groundwater storage that may take several wet years to recover.

The scenario simulations suggest that the hard cut-off levels in the current management plan are not suitable for the future groundwater mangement of the Wairau Aquifer. A lowering of the annual allocation limit for irrigation to 20-25 M m³/a seems appropriate for the near-future and would not impose severe restrictions on farmers under current land-use practice.