The Waimea Plains are situated in the Tasman District, southwest of Nelson, and are a valuable source of groundwater for the district. The hydrogeology of the plains consists of three key aquifers separated by aquitards. Tasman District Council (TDC) engaged us to develop an integrated surface water-groundwater flow model of the system and evaluate various water management strategies.

We have developed the Waimea Plains model collaboratively with TDC over multiple stages spanning several years. The model’s primary objective is to simulate transient (time-varying) groundwater levels and river flows to enable informed decision-making on future water management strategies. Key aspects of the model development included:

• Model Development

  We developed a three-dimensional integrated surface water-groundwater MODFLOW model (with a 100 m x 100 m grid resolution) that incorporated spatially and temporally-variable groundwater takes, land surface recharge, surface waters, and the interface with the Waimea Inlet.

• Calibration

  The latest version of the model simulates the period 2000-2020. We calibrated this against groundwater level data from 40 monitoring bores and also river flows and gains/losses. Replicating low flows at TDC’s monitoring site on the Waimea River was a particular focus.

• Scenario testing

  Using the model, we ran multiple management and future-use scenarios, testing the impact of groundwater abstraction, river augmentation, land-use changes and regulatory policies.

The study underscores the need for adaptive groundwater and surface water management to protect the Waimea Plains' freshwater resources. Key findings include:

• Groundwater Abstraction Impact

  Water abstraction influences groundwater levels, particularly in coastal areas. Reduced groundwater levels increase the risk of saltwater intrusion, affecting freshwater resources.

• Groundwater-Surface Water Connections

  Groundwater abstraction leads to lower river flows, particularly at TDC’s Nursery monitoring site, highlighting the importance of effective allocation strategies.

• Management Strategies

  TDC’s water take restriction regime has successfully maintained groundwater levels higher during dry periods, prolonging river flow sustainability during extreme droughts.

• Future Water Demand

  Simulations indicate that growing urban, industrial, and irrigation demands and future climate change will require enhanced management approaches, including optimised augmentation of river flows from the Waimea Community Dam.

The study is ongoing. However, the model was used as part of a joint technical review (with Landcare Research) of the science of supporting the Waimea Community Dam, the report from which is available to download from TDC.