4. Overview of requirements for water
Water plays various roles in sustaining the natural and social values of the catchment. Each role requires differing amounts, timing and patterns of water. There is potential for increased demand for recreation and tourism, for new hydro-electricity and increased abstraction for town/municipal supplies and irrigation.
Physical requirements of river and wetland systems for water
The braided river system of the catchment has evolved to carry the high flows of water and gravels produced in the mountains. A key role of water is the maintenance of the braided river system itself, including the main braided channel, slow-moving backwaters, riparian wetlands, and gravel islands. The presence and connection between these various habitats is important to the flora and fauna that inhabit the rivers. The river's braiding pattern, depth, width, bed material, bank stability and the functioning of the river mouth depend on the flow regime in the river. Floods are the major natural channel-forming mechanism, reshaping and refreshing the bed and riparian areas, to establish the bed material and braiding pattern. Lower flows provide important physical conditions, particularly depth and width, and maintain connections between the main channel and adjacent habitats. The number of braids can reduce if there is a lack of flows of sufficient magnitude to transport sediment, remove vegetation and form channels.
Wetland ecosystems contain plants adapted to various water levels experienced over a year. Some plants require year-round water whereas others require fluctuating water levels. The diverse vegetation around the margins of wetlands is attributed to the fluctuation in water levels.
Variability of flows over different time-scales is important for ecological functioning with different species and communities adapted to different types of variability.
Ecological requirements for water
Macrophytes and periphyton
Periphyton require relatively stable areas in river riffles, but can form excessive growths if there are insufficient high flows to cause abrasion or prevent accumulation of fine silts.
Macrophytes generally require still water areas, and different species and communities are adapted to different flow regimes and water levels.
Invertebrates
The Waitaki catchment supports a diverse range of aquatic invertebrates. The composition of invertebrate communities depends on local conditions, most importantly the stability of flows and substrate material. Some species prefer high velocities whereas others prefer the lower flows of margins and side channels. Most invertebrates feed on periphyton and play a role in controlling organic material within a stream ecosystem. They are an important food source for birds and fish.
Indigenous fish
While some indigenous fish exist throughout the catchment, most are found only in habitats that meet their specific feeding and spawning requirements. Indigenous fish need water flow and levels that provide:
- habitat conditions suitable for freshwater insects and access to terrestrial insects as a food source;
- preferred velocities and depths (ranging from low-flowing swampy streams for the Canterbury mudfish, to bouldery rapids in sub-alpine streams for the alpine galaxias) for feeding and spawning;
- suitable turbidity, temperature and oxygen levels;
- riparian vegetation, woody debris and/or large rocks for cover;
- flat surfaces, vegetation or river bed substrate suitable for spawning;
- exclusion of predators, particularly salmonids;
- refuges during high flow events;
- stable water levels to prevent eggs drying up or being disturbed once laid;
- prevention of unnaturally high levels of plant growth; and
- passage between rivers, lakes and wetlands, and to and from the sea for diadromous species. Migration times vary but at least one species of fish is migrating up or down river at any one time.
Salmonids
Trout and salmon need water flows and levels that provide:
- conditions suitable for freshwater insects and small fish as a food source for trout;
- their preferred depths, velocities and substrate for feeding, migration and spawning;
- for adult and juvenile fish passage to and from the sea/lakes (adult salmon enter the river from November to March, adult rainbow trout spawn in late winter or early spring in tributaries, and brown trout spawn in mid-winter in main channels and tributaries); and
- river bank vegetation to provide shelter.
Birds
The three major types of habitats for river and wetland birds are the braided river bed, the lakes and wetlands. Birds require flows and levels that provide:
- conditions suitable for freshwater insects and small fish as a food source;
- their preferred velocities and depths for feeding, ranging from shallow riffles for most braided river species to torrenting mountain streams for the blue duck;
- for braided riverbed birds clean gravels for nesting preferably on predator, vegetation, and flood-free areas (islands between braids are preferred; breeding season is from September to early January);
- vegetation along lake edges for nesting of lake birds, (for example, the southern crested grebe breed from November to March); and
- wetlands and shallow margins along lakes for winter feeding.
Tāngata whenua cultural requirements for water [Drawn from Te Rūnanga o Ngāi Tahu Freshwater Policy.]
Sustaining the mauri of a water body requires management of water bodies that:
- protects the water's capacity to renew its groundwater and surface water flows and stocks;
- sustains habitats, breeding, food sources and migratory requirements of mahinga kai species such as eels, flax and watercress, in their freshwater and coastal environments;
- provides seasonal flow variability via a range of flows including seasonal floods of different magnitudes;
- protects the exchange of freshwater and seawater at the mouth, maintains freshwater flows in estuaries, and prevents the unnatural closing of a river mouth;
- enables the longstanding histories and traditional and cultural uses to be maintained; and
- prohibits the unnatural mixing of water from different bodies.
Town and community requirements for water
Water from both surface water and groundwater is required for the supply to towns and communities, and for domestic supply to individual users where no community supply exists.
Increased demand for town and community water within the catchment is likely to be driven by increased subdivision and lifestyle development near Lake Tekapo, Lake Ruataniwha, Twizel and Otematata. Out-of-catchment demand for water is expected to grow significantly, particularly from settlements along the dry North Otago coast that have little or no access to groundwater. Interest has been expressed in taking Waitaki River water as far south as Dunedin.
Recreation and tourism requirements for water
Recreational uses of water require flows and levels that provide for:
- access for recreational users;
- stable water levels for rowing and lake boating;
- flow conditions suitable for target species - fish or game birds;
- suitable velocities, clarity and depths;
- a large enough area of suitable river or lake to prevent conflict; and
- the taking of water for tourism and recreational facilities, including snow making.
For many recreational users and tourists who do not directly use water, the main role of water bodies in the catchment is to provide a backdrop or setting to the activity. The most significant effect of changing water use is likely to be on experiences, perceptions of landscape amenity and the naturalness of the environment.
There is considerable potential for growth in active and passive recreation and tourism within the catchment both from New Zealand and overseas. This growth will require greater use of water for drinking/domestic use. Water use for these activities is usually provided by town supplies, and is small compared to the flow required for other uses such as hydro-electricity and irrigation. As an example, Aoraki/Mt Cook village has consent to use 0.03 cubic metres per second for these purposes.
Hydro-electricity
Hydro-electricity uses the greatest amount of water, but returns water to the system so that other activities can use or abstract the water downstream. Because the highest inflows to the Waitaki system occur in spring and summer, and peak electricity demand is generally in winter, storage provides a critical ability to match supply and demand. Because of the importance of the storage in Lakes Tekapo and Pūkaki to the New Zealand electricity system, water in these lakes has the highest value for hydro-electricity generation within the Waitaki scheme. Water from Lake Tekapo flows through eight power stations, and water from Lake Pūkaki through five stations. The flow through Benmore Power Station reaches a maximum of 660 cubic metres per second and uses, on average, 11,000 million cubic metres a year.
Compared to other sources of electricity, the output from a hydro-electricity generator can be ramped up and down quickly. Flexibility in the rate of taking water is critical to the ability to match the daily demand peaks and maintain the stability of the transmission system.
Potential new hydro-electric generation includes large or small scale schemes on the Lower Waitaki River using canal or canal/tunnel options, micro hydro-electricity and hydro-electricity generation in combination with irrigation.
Agriculture and horticulture
Existing resource consents that take water upstream of Waitaki Dam provide water for 12,600 hectares of irrigation using a peak rate of 15.5 cubic metres per second and an estimated annual demand of 96 million cubic metres. Estimates of the potentially irrigable land upstream of Waitaki Dam reach as high as 80,000 hectares, representing an eight-fold increase in water demand.
There are currently about 46,000 hectares of land irrigated with water taken from the lower catchment, although some of this is outside the catchment boundary. Some remaining pockets of land within the lower catchment could benefit from irrigation, the largest of which is in the Hakataramea catchment.
The Waitaki catchment provides a possible source of water for irrigation in the dry parts of South Canterbury and North Otago. Significant additional irrigation demand is likely to come from these areas. Known proposals include taking water from Lake Tekapo over Burkes Pass into South Canterbury, and taking water from the Lower Waitaki River into both South Canterbury and North Otago. A resource consent has recently been obtained for eight cubic metres per second to irrigate 20,000 hectares of land outside the catchment in the North Otago downlands, and Stage 1 of this scheme, to service 10,000 hectares, is already under construction.
Existing resource consents for irrigation and stock drinking-water in the lower catchment require an estimated peak flow of around 60 cubic metres per second (90 percent of which is taken directly from the Lower Waitaki River) and a seasonal volume of 900 million cubic metres. Estimates of the peak demand to provide for the approximately 74,000 hectares identified as potentially irrigable in the remainder of the catchment, South Canterbury and North Otago, amount to a 70 percent increase in the peak flow of water required.
Pastoral farming also requires stock drinking-water. The existing allocation to stock drinking-water in the catchment exceeds reasonable use estimates, even with full irrigation development assumed. Therefore, additional stock drinking-water needs can be met by efficiency improvements or provided in combination with irrigation development.
Groundwater maintains soil moisture levels in some unirrigated areas of the lower Waitaki valley where groundwater is shallow and clayey soils exist.
Commercial and industrial
There is not a great demand for water in the catchment for commercial and industrial purposes currently, although there is likely to be some out-of-catchment demand through the Oamaru and Waimate town supplies. The only major existing industry is the Pukeuri freezing works. Increased agricultural and horticultural production is likely to require additional processing capacity either within or near the catchment.
There are three salmon farms on the hydro-electricity canal system in the Mackenzie Basin.
