2 Environmental Impacts
From an environmental perspective the establishment of a further 25,000 hectares under irrigation within the upper Waitaki could have both a positive and negative impact on the environment. From a positive perspective, irrigation would reduce the erosion risk within the basin and also increase the opportunities for environmental enhancement. From a negative perspective, irrigation increases the risk of contamination of ground and surface waters and could adversely impact upon landscape values. All of these potential impacts are over and above any impacts that occur as a result of construction activities.
2.1 Soil degradation and erosion
Soil erosion, particularly wind erosion, has long been recognised as an important high country issue (Martin et al, 1994). The sparse vegetation on large areas of land in the Mackenzie Basin gives little protection to the shallow, friable soils which continue to be eroded by frost heave and westerly winds. Under the Rabbit and Land Management Programme, vegetation monitoring sites were established on properties in the Mackenzie Basin in 1991 and 1992 (Cuff, 1993). These sites have been assessed at mostly two-yearly intervals since their establishment. At the same time, the regional council established a number of sites in the area which have been assessed on a five-yearly basis. Results from these sites have helped identify the trends in vegetation cover through the basin. (See for example Cuff, 2001.)
Cuff (1994) used a mix of satellite imagery and field assessments to assess ground cover on 132,000 hectares in the middle of the Mackenzie Basin. Overall ground cover across the basin was assessed at 72.8 percent with large areas in the centre of the basin with more than 30 percent bare ground.
Basher (1996), using Caesium 137 techniques, found evidence for the widespread occurrence of erosion in the Mackenzie Basin. Variation in Caesium 137 levels in the soils through the basin appeared to be related to erosion. Highest levels of Caesium 137 were found on pedestalled,vegetated areas and lowest levels in bare, deflated areas. Soil losses of up to 67 millimetres were recorded. Mean soil losses over a 40-year period from bare sites ranged from 13-35 mm an overall mean loss of 25 mm. From pedestalled sites the loss was 4 mm and from fully vegetated sites there was no soil loss. The mean loss across all sites was 9 mm which equates to a mean soil loss of 0.22 mm/year or 2.2 tonnes of soil lost per year. Across the upper Mackenzie Basin this represents a significant soil loss over this time. However, it can not be assumed from this information that erosion rates will continue at this level. The information does confirm a strong relationship between the percentage of vegetation cover and erosion risk (Basher, personal communication).
The erosion problem has been compounded by the spread of flatweed, hieracium, through the 1990s. Fastier (2004) reported an increase in hieracium on the flat outwash soils on his property from 0.03 percent in 1991 to 35 percent in 2000. Similar trends have been reported on other properties throughout the basin particularly on the poorer soils with some areas approaching 50 percent bare ground (Shearer, personal communication). Espie (personal communication) has shown that the zones of bare soil around hieracium patches with winter frost lift increases susceptibility to wind erosion.
It is now generally accepted that any land management practice that increases vegetation cover and reduces the amount of bare ground will reduce the erosion risk in the Mackenzie Basin. Irrigation and the establishment of strong pasture swards falls into this category.
2.2 Water quality
The issue of poor water quality in areas of intensive agriculture has been highlighted in the recently released report by the Parliamentary Commissioner for Environment (PCE, 2004). Pollution of surface and ground water by intensive agriculture is a major indirect consequence of new forms of land use through irrigation. Because of the high capital costs of irrigation, plus high operational costs, farmers are inevitably drawn to higher yielding land uses, often requiring higher fertiliser and other inputs.
Environment Canterbury's Proposed Natural Resource Regional Plan (2003) recognises that increased nutrient loading in water in the upper Waitaki is an issue particularly given the high water quality that is there now. Loss of water quality in areas such as this has both regional and national costs through impacts on recreation and other amenity values, human health and vulnerable ecosystems.
A project which investigates current water quality (both groundwater and surface water) in the Waimate District, with particular emphasis on intensification and change in land-use, was initiated by Environment Canterbury in 1995 (Meredith, personal communication). The Mackenzie Basin was added to this project in 2002 following concerns expressed by a number of parties over the effects that intensification of agriculture in the basin may have on water quality. The approach was to sequentially develop background water quality of a number of streams through the basin. In addition Meridian Energy have continued some baseline and trend detection monitoring at four sites including the Twizel River, Ruataniwha Stream, and Lakes Ruataniwha and Benmore. Environment Canterbury undertook to annually report and interpret this data for Meridian Energy.
From an analysis of the results to date it appears streams through the upper Waitaki basin are only slightly nutrient enriched, and of very low nutrient status compared to other streams in Canterbury (Meredith and Hayward, 2002). Overall, there are no indications of appreciable degradation of water quality in the Mackenzie Basin monitored so far. However, there are indications of some catchments (eg, Mary Burn, Wairepo) showing significant changes (enrichment), such that trends and effects should continue to be scrutinised, particularly in those areas currently subject to land use changes (Meredith and Smith, 2004).
In an attempt to predict the impacts of future intensification of land use through the upper Waitaki, Meridian Energy has commissioned a modelling study on intensification and nutrient levels in the groundwater and lakes of the upper Waitaki (R Potts, personal communication). The study uses three scenarios: 10,000, 30,000 and 90,000 hectares under irrigation with a mix of land uses including some dairying, intensive sheep and beef and deer. The preliminary analysis has shown a significant tonnage of nitrogen entering groundwater, and ultimately the streams and lakes, under all scenarios tested. This study does not predict corresponding increases in phosphorus loadings via the same pathways, which implies that the effect of increased nitrogen may be limited by the availability of phosphorus. A full report on the Meridian work is due in February 2005.
Recent monitoring of Mackenzie Basin streams by Environment Canterbury has shown that the ratios of both soluble and total N:P nutrient ratios are very low (generally <10) indicating that, although both nutrients are in low concentrations, the streams (and therefore groundwater) are more nitrogen-limited than phosphorus-limited (Meredith and Smith, 2004). With the optimum nitrogen to phosphorus ratio for algae and macrophyte growth being around 15-20:1, any increase in nitrogen loading increases the risk of increased or nuisance levels of plant and algal growth caused by contamination. Water bodies with the lowest N:P ratios and therefore most likely to be immediately at risk are the smaller streams and rivers and their recreational and biodiversity values (Meredith, personal communication).
A cautious approach to future irrigation development is warranted given that adverse trends have been recorded in other regions of similar pristine water quality and climatic environment (Otago Regional Council, 2003). From an intensive study over a 15-month period on the Taieri River in the Maniototo, with sampling sites above, below and through the main irrigation area, a significant decline in a number of nutrient and bacterial water quality parameters was shown. While not specifically identified, it is likely that the decline was due to a combination of factors including stock access to waterways, irrigation bywash, increased stocking rates and dairy effluent irrigation. What the Otago study shows is that often it is not a single factor that causes the problem but rather a combination of management activities. It also clearly demonstrates that an integrated irrigation/land management programme is essential if future problems are to be avoided.
All the evidence points to an enhanced risk of ground and surface water contamination in the upper Waitaki Catchment as a result of intensification through irrigation. This risk may be partly offset by the method of irrigation used although considerable care will still be needed to avoid adverse impacts. It is expected that almost all future irrigation in the upper Waitaki will involve spray rather than flood irrigation methods (Shearer, personal communication). Spray irrigation gives greater control over application rates and allows surface runoff to be avoided. Electronic soil moisture measurements allow application rates to be matched to crop requirements and weather forecasts to take advantage of rainfall and avoid over watering.
2.3 Landscape
The Mackenzie Basin is described in a report by Boffa Miskell (1992) as a vast open landscape where landforms are often huge and vistas are wide and uncluttered. The low form of the vegetation allows the landform to dominate the landscape and define the horizon.
Irrigation of the dry Mackenzie Basin to create greener landscapes may be seen by some as detracting from the general appearance and visual character of a nationally significant landscape. The issue is mentioned in landscape assessment undertaken by Densem (2004) and Boffa Miskell (1992); however, neither provides specific information on the impacts of irrigation development on landscape values in the upper Waitaki.
In the Mackenzie the major landscape disturbance has already occurred with the completion of storage and supply structures from the Upper Waitaki Power Scheme. In addition hieracium invasion has reduced tussock density and visual appeal in many areas. Irrigation will undoubtedly further change the visual appearance of some areas within the upper Waitaki. Not all of these areas will be visible from the main highways thus lessening the impact on visitors to the region.
2.4 Biodiversity
The ecological character of the upper Waitaki has been well documented (Knox, 1969; O'Connor, 1976; Espie et al, 1984) and specific assessments have been undertaken on those properties involved in tenure review. No specific studies have been reported on the impacts of intensification on biodiversity values.
The conversion of semi-natural grasslands to improved irrigated pastures will significantly reduce indigenous plant and invertebrate biodiversity. Espie (2004) suggests that representative examples of these systems need to be protected. He submits that these have already been identified through the Protected Natural Areas Programme and that a variety of protection mechanisms are available, with potentially large areas becoming available through the tenure review process of Crown pastoral lease land.
Espie notes that extensive areas of basin floor tussock grasslands are already in full Crown ownership for conservation management after completion of tenure reviews in the Mackenzie. Other areas with significant conservation values have been protected through reservation or voluntarily through Queen Elizabeth II covenants (eg, Tekapo Scientific Reserve, Omarama Station, Glenmore, Ohau Station QE II covenant and so on).
Urquhart (2004) notes that irrigation development provides an opportunity to enhance ecological values. He suggests as an example that wetlands for wading birds could be included in the irrigation design. Similarly, Espie notes that centre pivot irrigation is restricted by its radial design to use only part of the available area, leaving considerable extents of surrounding land as de facto biodiversity preservation under existing land uses.
2.5 Biota effects
There is a reasonable amount of data on the native and introduced fisheries within the Mackenzie Basin. Most indicates a generally healthy resource (M Webb, personal communication; Meredith, personal communication). The two main concerns relating to the protection of this resource are:
- abstraction and direct effects on flows in the rivers
- water contamination from intensification and its effect on aquatic life.
There are currently 3,300 hectares consented for irrigation in the upper Waitaki east of the Ohau river. Water for this area is drawn from both river sources and hydro canals with approximately two-thirds currently drawn from river sources. Any additional extraction from the streams and rivers will put further pressure on these waterways and the biota within them (Shearer, personal communication).
In some cases existing irrigation takes from streams could potentially be replaced with hydro canal water to allow natural streams to be maintained over the summer irrigation season. For example, this could be readily achieved at the Mary Burn where providing access to irrigation water from the canal would allow natural flows to be maintained in the stream (Shearer and Miller, 2004).
The impacts of intensification of land use on water quality have been discussed previously in section 2.2. There is a significant risk, particularly to the fisheries on smaller streams, and planning for this risk is required during the development phase of any irrigation schemes.
