Environmental Benefits
- An estimated 50% of farmers who are Central Plains Water shareholders currently pump water from underground aquifers (some of which are now suffering from over-abstraction). By instead delivering water direct to farmers’ gates, the groundwater system will return towards its natural state. It is estimated that some of the recharged aquifers will rise by as much as 20m.
Steady state change in groundwater levels resulting from the CPW scheme
- Irrigated scheme water that is not used by plants, animals or lost via evaporation, will ultimately drain into the groundwater system that flows towards and beneath Te Waihora-Lake Ellesmere, rather than towards Christchurch city.
Modelling studies show that the scheme will result in a small but helpful increase in flow in Christchurch’s Avon and Heathcote Rivers. Some of these increased flows will be due to a pressure effect related to the generalised increase in groundwater levels, whereas some may come directly from the scheme (however this will be a very small proportion).
It is also expected that there will be a significant rise in the volume of water flowing in the Selwyn and Irwell Rivers, the upper Hanmer Road Drain, Birdlings Brook and Wakanui Creek. Less significant increases in baseflow are expected in the Avon, Heathcote, Halswell, and LII Rivers; Boggy, Waikekewai, and Harts Creeks; and Tent Burn.
In turn, the increase in these lowland stream flows will provide more water into Te Waihora / Lake Ellesmere. - Because of the direction of groundwater flow, the main aquifer that Christchurch draws its drinking water from will be unaffected by the scheme. However, the West Melton, Templeton, and Halswell areas may experience small water quality issues with respect to nitrogen concentrations in shallow bores.
- Parts of the scheme will be developed as ‘vegetation corridors’ that connect pockets of native plants that remain on the central Canterbury Plains. The corridors will encourage the movement of native bird species and therefore, ultimately, a permanent natural means of increasing native plant life on the plains, from the mountains to the sea. This design feature has attracted positive support from the Department of Conservation.
- In recent years, many Canterbury sheep farms have been developed for more intensive farming practices, particularly dairying. Not only has this increased demand for surface and groundwater abstraction, it has increased the problem of managing water contamination (especially nitrates from fertiliser and animal effluent) of aquifers and streams.
In contrast, the CPW scheme requires farmers to commit to a Sustainable Management Agreement, whereby their farming practices are monitored to ensure they do not over-water or over-fertilise their land. In effect, the environment will be better off with the Central Plains Water scheme than without.
CPWL is also actively supporting a national Sustainable Farming Fund project entitled "Development of an Irrigation Scheme Sustainability Code of Practice”.
All water supplied to farms by the CPW scheme will be subject to a levy collected on behalf of the CPW Trust. Called the Environmental Trust Fund and amounting to around $60,000 a year, the fund will be used to support and encourage various environmental enhancement initiatives (eg. Riparian planting and habitat restoration) within and ‘downstream’ of the CPW scheme area. - Technical studies indicate that careful management of irrigation and fertiliser application will, over time, improve the depth and organic content of soil within the scheme area.
- Prior to the development of irrigation on the plains, wind erosion from dryland soils was considered to be a problem of national significance. With the development of irrigation, soils are much more resistant to wind erosion.
- As well as better quality soils providing better productivity, greater organic content creates a natural biological filter to prevent water draining so quickly – this improves nitrogen uptake by plants and minimises the potential for it to leach into groundwater systems.
Earthworms are a good indicator of the organic content and general health of soil. A recent study undertaken by Crop and Food Research at Winchmore Irrigation Station counted earthworms in different soils. While 250 was a typical number of worms in a dry unfertilised square metre of soil, irrigating the same soil would raise the count to around 500 worms per square metre and applying the correct amount of water and fertiliser could raise the worm count again to 800/m².