For New Zealand to maximise future agriculture and horticulture production, in many areas, reliable irrigation supply will be essential. In Canterbury, we're working with multiple stakeholders to utilise and integrate the natural capacity of Lake Coleridge - while maintaining our generation capability and environmental commitments.
Reliable Irrigation Supply is Fundamental
Irrigation for agriculture and horticulture is a major user of New Zealand's fresh water. So clearly, irrigation users have a responsibility to manage water as efficiently as possible, and minimise the adverse effects.
Efficient use depends on reliable supply, because reliability allows users to target use against demand, and avoid waste. But across much of New Zealand, rivers and streams are under pressure. Irrigation schemes, particularly new ones, are unlikely to achieve the level of reliability needed to maximise efficiency and support sustainable economic value. The solution? To incorporate storage into irrigation schemes, ideally close to, and higher than, the demand area.
Utilising Lake Coleridge for Canterbury Agriculture
Canterbury agriculture contributes over $1.7 billion to the economy each year, much of it relying on irrigation. Further, the region's future development of high value agriculture, critical to New Zealand's growth, will depend on reliable irrigation.
That's where we turn to Lake Coleridge, a natural lake west of Christchurch that is managed by TrustPower. The lake and associated inflow feeds the Coleridge Power Station, supplying power to the Canterbury region. It is also located near to, and above, some of the largest potential, developing and established irrigation schemes in mid-Canterbury, serving over 160,000ha of farmland.
In 2012, TrustPower secured a variation to the existing Water Conservation Order on the Rakaia River, under which the Coleridge Station operates. The variation did not seek to change the water allocation criteria, as these protect environmental aspects of the river system. Instead, the change simply allows water to be stored in Lake Coleridge, as at present, but now for irrigation supply as well as for hydro-generation.
Lake Coleridge storage, in conjunction with river take, can provide high levels of reliability for up to 65,000ha of irrigation.
Delivering Reliable Supply to Two Significant Irrigation Schemes
Over the last two years, TrustPower has worked closely with two large irrigation schemes in mid-Canterbury, namely Barhill Chertsey Irrigation (BCI) and Central Plains Water (CPW), to determine the viability of supporting irrigation reliability and security of supply via Lake Coleridge.
The Barhill Chertsey Irrigation Scheme has been operating for three seasons. It currently supplies approximately 25 per cent of the ultimate potential supply area of 40,000ha south of the Rakaia River. TrustPower's Highbank Pump Station is the primary supply system to the irrigation scheme.
CPW intends to develop a new irrigation scheme, on the north bank of the Rakaia River. The potential supply area is some 60,000ha. The scheme accesses river flow with low reliability, so storage from Lake Coleridge is fundamental to CPW's long-term viability.
Working For an Integrated Future
Of even greater significance for efficient water use is the potential integration of Lake Coleridge into the wider Canterbury water infrastructure. This could encompass both existing infrastructure such as the Rangitata Diversion Race, supplying three existing irrigation schemes totalling 64,000ha, and future infrastructure, such as additional storage facilities and supply systems.
TrustPower is working with Environment Canterbury and irrigation users to develop infrastructure plans that would allow water to be conveyed and supplied over a much greater area of Mid- and South-Canterbury. A key focus is progressive sustainable development, over a number of years.
Once coupled with integrated infrastructure, Lake Coleridge would be fundamental to opening new irrigation areas, allowing water to be allocated to new users without negative impact on existing users. Integration means water could be traded amongst users, ensuring it is applied to the highest value use.
Integrated storage and supply would also enhance the opportunity to incorporate hydro-generation facilities. These facilities would typically operate in-sync with water demand, so partially match energy demand from agricultural processes.