The full story
Changing irrigation practices, over-abstraction of groundwater and piping of irrigation and stock-water distribution races had contributed to declining groundwater levels on the Canterbury Plains in the Hinds/Hekeao catchment.
Apart from a sophisticated tool for modelling groundwater and surface water, Environment Canterbury was also looking to build capacity in their organisation so that they would not have to outsource in the future.
Bounded by the Rangitata River to the south and the Ashburton River to the north, the Hinds Plains sub-regional catchment covers an area of approximately 1375 km2 © ECAN/DHI
Establishing a Hinds/Hekeao catchment technical team
Historically a vast wetland, the farming practices in the Hinds River/Hekeao catchment have changed the catchment significantly. In recent years, modified irrigation methods had led to increased groundwater use and increased application efficiency from surface water-sourced irrigation schemes. There were areas that the groundwater and surface-water were so interconnected they were unable to be modelled separately, which led our client to seek out a way to couple the modelling. After surveying the market, MIKE SHE was determined to be the best tool for integrated modelling.
Addressing the entire hydrological resource, there was a focus on setting limits for both quality and quantity for the groundwater and surface water resources. To address this and to support a limit setting process, Environment Canterbury established a Hinds Plains Technical Team that includes water quantity and water quality scientists, ecologists, and numerical modellers.
We implemented the solution using MIKE SHE, a flexible modelling framework that includes a range of numerical methods for each hydrological process. It has an advanced, conceptual, model-independent user interface with full water balance accounting for all hydrological processes.
MIKE SHE can be used for the analysis, planning and management of a wide range of water resources and environmental problems related to surface water and groundwater. It is especially helpful in determining surface-water impact from groundwater withdrawal and conjunctive use of groundwater and surface water, which was instrumental in achieving the goals set for Environment Canterbury.
As the application of choice, it was selected to quantitatively assess resource management options and ultimately aid in establishing best catchment management.
Specifically, it was used for simulating the relative change between land use scenarios, rivers and lowland stream flows, irrigation demand and application efficiency as well as groundwater levels in a range of deep and shallow wells across the catchment.
MIKE SHE allowed for:
- a coupled groundwater-surface-water model where interactions are defined
- a fully distributed spatial discretisation
- a sensitive model that can show measurable response to various land use changes
The components of MIKE SHE that were utilised included: an evapotranspiration (climate) model, an unsaturated zone (UZ) model, an overland flow model, a river model and a saturated zone (SZ) model.
Surface water near the Lynnford Hall in the Hinds/Hekeao catchment, © DHI
Effective management of the water resources of the Hinds/Hekeao catchment was dependent on a detailed understanding of the biophysical processes. Following the project completion, the client gained a robust model that could be used to support local authority hearings.
The client was able to create a coupled groundwater/surface-water model that simulated groundwater inputs into coastal streams, a fully spatially distributed grid to allow modelling of relative change between land use scenarios and a model sensitive enough to show the expected differences between various land management scenarios.
Critically, the model was used to show that Managed Aquifer Recharge could be used within the Hinds/Hekeao catchment to remediate groundwater declines and improve spring-fed stream flows.
The modelling showed that one of the primary drivers for the increased seasonal water level fluctuations seen in recent years was groundwater pumping. The modelling work undertaken suggested that the declining groundwater levels and consequently declining spring-fed waterway flows were heavily influenced by the:
- shift from border-dyke irrigation to spray irrigation practices
- piping of local irrigation scheme
- increased groundwater abstraction