Selwyn water recharge project makes progress

This week marked the start of an exciting project to recharge groundwater in the Selwyn Waihora Zone.

Selwyn District Council, Environment Canterbury, Central Plains Water and DOC joined Te Taumutu in blessing the land before construction starts.

The project will use water from Selwyn District Council and Central Plains Water (CPW) consents. A large recharge basin near Hororata will be constructed.

How does it work?

An off-take from the recently completed stage 2 of the Central Plains Water scheme will pour up to 3.5 cumecs of Rakaia River water into the basin when groundwater levels are low.  This water will percolate into the groundwater system beneath, recharging the aquifer and feeding springs that are the source of streams where mudfish live.

As climate change takes hold, rainfall is becoming more variable and evapo-transpiration is increasing in Canterbury, reducing the amount of naturally occurring water going into waterways both above and below the surface.

Environment Canterbury project leader Dr Brett Painter began researching climate and water management in the catchment during the mid-1990s and is using the knowledge gained from this research in this project.

“During the last century, all major droughts in this catchment have occurred during extreme El Niño or La Niña climate events. These climate events have become more regular in recent decades and this trend is expected to continue,” Painter says.

All working together

The $2.8-million project is being funded by the Government’s Freshwater Improvement Fund and Environment Canterbury, with in-kind support provided by CPW.

Painter acknowledges the importance of the Canterbury Water Management Strategy and the Selwyn Waihora Zone Committee in enabling and supporting the project which should improve the health of the Selwyn River / Waikirikiri and its catchment. 

The basin will be in the middle of a pine plantation, purchased as part of the project.  An additional benefit is that this location has been assessed as ideal for enhanced lizard habitat and the re-establishment of native dryland species in clearings. 

Benefits of the project  

When completed, the system will only operate when the nearby Selwyn River / Waikirikiri is not flowing naturally and will not mix at the surface with natural river water.  It will primarily operate during winter to compensate for dry winters that result in low river flows the following summer. 

The recharged water will travel through local groundwater systems and support flows in the interconnected Hororata, Irwell / Waiwhio and lower Selwyn / Waikirikiri rivers.

While the project will recharge aquifers and in turn contribute to the small streams they feed, it is not expected to increase the length of time the Selwyn River / Waikirikiri flows under the SH1 bridge.

“This expectation is based on catchment research that identified the depth of gravel under the Selwyn (and nearby rivers), and the way the upper catchment rivers lose their water to underground river channels through the mid plains until the underground system is so full that water flows at the surface,” Painter says.

“Studying 70 years of nearby groundwater records, I found only a few short periods during large floods where the Selwyn flowed under SH1 without the groundwater system filling up first.”

An investigation into the viability of the project, as well as small-scale testing of the concept,  has been underway for six years in Boggy Creek and a tributary of the lower Selwyn River / Waikirikiri. 

Further knowledge was gained from a similar project in the Hinds River catchment where a smaller river recharge project has been constructed. 

Construction of the Hororata recharge basin is set for winter and spring this year, with full commissioning to follow once sufficient water and pipe capacity are available.