Assessment of the natural values at the proposed centre pivot irrigation site and surrounds on Ben Ohau Station, Mackenzie Basin, New Zealand August 2016 Abstract The natural values of the area proposed for centre pivot irrigation south of the Pukaki Ohau canal on Ben Ohau Station, have been largely modified, however the integrity of the geomorphology and some small modified natural remnants could be integrated into the proposed irrigation site and surrounding areas. Te Manahuna Consulting ryoung@temanahuna.co.nz
The author: My name is Rob Young and I am the sole consultant and director of Te Manahuna Consulting, established in 2013 and based in Twizel. I have a Bachelor of Science degree from the University of Canterbury. Te Manahuna Consulting (www.temanahuna.co.nz) undertakes conservation and environmental work for a variety of clients. Prior to establishing this consultancy, I was employed by the Department of Conservation as the Area Manager, Twizel for 24 years. Previous to that I worked as a ranger both seasonally and in permanent roles over a (Wrightson, PGG, 2014)20-year period. During this career with the Department of Conservation, National Park Service and NZ Forest Service I was involved in numerous vegetation, bird and fauna surveys. Te Manahuna Consulting. Unless permitted by contract or law, no part of this publication may be reproduced or stored in a retrieval system in any form or by any means without the prior written permission of Te Manahuna Consulting. Disclaimer - Te Manahuna Consulting does not accept any responsibility or liability whatsoever whether in contract, tort, or otherwise for any action or omission taken as a result of reading or relying on any information or documentation or part thereof supplied by Te Manahuna Consulting.
1 P a g e Background Te Manahuna Consulting was engaged by Simon and Priscilla Cameron to undertake a survey and a report on the values of the area proposed for centre pivot irrigation south of the Pukaki Ohau canal. The water take for this irrigation has been granted under Resource Consent, CRC100234. (Environment Canterbury, 2016) Methodology I visited the site on two occasions in August 2016, and familiarised myself with the area features by walking and driving over the site and undertook a rapid survey of the key areas of native vegetation. Ben Ohau Station, the site and surrounding area description Ben Ohau Station is a freehold property. The property completed tenure review in 2003. The command area for the 4 centre pivots (the site ) which will cover an irrigated area of 400ha is located on the south side of the Pukaki Ohau Canal between the Twizel River and Fraser Stream. The site has been highly modified by both the construction of the canal system in the 1970 s and the pastoral modification of the grasslands. The canal works included the highly modified area of the haul road which is about 50-100 m wide immediately below the canal embankment and on the northern edge of the site. This disturbed area is a visual prominent when viewing the site from the canal. The canal construction also resulted in the diversion of two streams which once flowed across this site. The initial grassland modification occurred with oversowing and topdressing starting in the 1960 s and more recently through the drilling of most of the site with ryecorn and barley. Figure 1: Hauler road to north of site which has been significantly disturbed with loss of geomorphological features.
2 P a g e Values of the area Geomorphology The geomorphology of the area is largely a Holocene alluvial plain (<11,700yo) with the active river plain of Dry Stream bisecting it (Figure 8). The active river plain of the Twizel River borders the area affected by the irrigation to the east. Despite the changes induced by pastoralism and cultivation the significant geomorphological features of the alluvial plain are still evident. The geomorphological features of the river plains have also remained unscathed by the changes and are also very evident across the landscape. The dry riverbeds of the two diverted streams are still apparent. Vegetation Outside of the areas that have recently been cultivated with ryecorn and barley the grasslands are relatively depleted and largely composed of exotic grasses, especially browntop and sweet vernal. There are some perimeter areas with sparse distribution of fescue tussocks, Hymenathera spp. and native broom. There is one significant area (8.4ha) of grey shrubland on the river plain of a creek which is now dry within the pivot 2 area. This shrubland is dominated by matagouri with some native broom and Hymenathera spp. Rosehip is also present. The matagouri is up to 4m tall and appears to be in healthy condition. Fauna No bird breeding or nesting sites were found within the site. The only birds sighted or heard during the survey were skylarks in the open grassland and silvereyes in the grey shrublands. Other species are likely to be present from time to time in the shrublands including grey warblers and fantail. Common skinks are likely to be found within the site but due to the seasonal conditions were not seen during this survey. Within the site (in the environs of Dry Stream) native fish (both long finned eels and upland bullies) have been noted in previous surveys (Department of Conservation, 2002). Land Environments of New Zealand The site is described as LENZ N6.1.a which is Critically Underprotected (Leathwick, 2003). While the area of LENZ within the command irrigation area will effectively be highly modified, the areas on the perimeter and between the pivots will provide unmodified remnants if they are not disturbed by cultivation. Current Protection Measures within the site Ben Ohau is part of the Pukaki Ecological District and was surveyed as part of the Mackenzie Protected Natural Area Programme in 1983. While two RAPs were identified on the property neither of them are within or adjacent to this site. As noted, Ben Ohau completed tenure review in 2003. No significant inherent values were identified within this site during tenure review and as a result no areas were protected other than the Dry Stream which has a marginal strip of
3 P a g e 20m laid off (Department of Conservation, 2002). This marginal strip will be included within the expanded 50m buffer margin as proposed in the consent. Figure 2: The irrigation command area looking east Dry Stream in foreground. Discussion and recommendations for management Across the irrigation command site which is largely an alluvial plain, minimising the earthworks disturbance and modifications to the geomorphological features will retain these features for their landscape value, especially if the sharp defining features of the swales are not rounded off and the swales are not infilled. Further to this four Natural Areas have been identified which if left undisturbed by cultivation will provide remnants of the remaining natural features and from a landscape perspective will serve to break up the irrigation command site. The removal of the exotic rosehip from these sites would increase their naturalness. Natural Area 1 - This is the area surrounding and including Dry Stream with a total area of 27ha between Pivot 1 and 2. Some of this area is included in the proposed 50m extended riparian zones but by expanding this to follow the natural and centre pivot perimeter boundaries the area will be more intact and functional. The dry river plain of Dry Stream is an important geomorphological feature. Natural Area 2 This area of 8.4ha is within the command area of Pivot 2. The swales in this old creek bed are up to 2m deep. The intactness of the matagouri shrubland could be maintained by providing narrow tracks through it for the centre pivot wheels at 50m intervals and having the centre pivot irrigation turned off while it traverses the shrubland. Not applying water will ensure the area will retain its naturalness and the site does not become rank with exotic grasses. (pers. comms. S. Walker). Some of the old man matagouri may need to be trimmed to allow the centre pivots to pass over them.
4 P a g e Figure 3: Dry Stream and riparian environs. Pivot 1 will be on right and Pivot 2 on left of photo. Figure 4: Matagouri area Natural Areas 3 (18.5ha) and 4 (6.4ha) both of these areas are outside of the centre pivots but while they are they dominated by exotic grasses they still retain some native species like fescue tussock, matagouri, native broom and Hymenathera spp. They could provide buffers and breakups to the irrigation command area if left in their current state. Potentially both of these areas could lend themselves to enhancement with native species if desired.
5 P a g e Figure 5: Exotic grasses and a mixture of native shrubs on site between pivots (Natural Area 3). Figure 6: Perimeter area of fescue tussock and exotic grassland which could be potentially used as a landscape buffer (Natural Area 4). Conclusion The command irrigation area is largely highly modified from a fauna and flora perspective however the area still retains the significant geomorphological features of the recent (<11,700y.o.) alluvial plain formation. Sensitive cultivation and earthworks will retain these features from a landscape perspective particularly when observed from the canal road to the north. Careful management of the four small natural areas identified (especially areas 1 and 2) will retain the existing natural features, provide some linear connection for the Dry Stream and provide some natural landscape buffer to the command irrigation area.
6 P a g e Natural Area 3: Exotic grassland Natural Area 2: Matagouri Shrubland Natural Area 1: Riparian zone and Dry Stream Natural Area 4: Fescue Figure 7: Centre pivots and natural areas
7 P a g e River plain Alluvial plain River plain Figure 8: Geomorphological features showing alluvial plain and river plains
8 P a g e References Department of Conservation. (2002). Crown Pastoral Land Tenure Review: Conservation Resources Report. LINZ. Environment Canterbury. (2016). Resource Consent CRC100234. Environment Canterbury. Leathwick, J. e. (2003). Land Environments of New Zealand Nga Taiao o Aotearoa. David Bateman. Mackenzie District Council. (2015). Mackenzie District Plan. Walker, S. et al. (2007). Guide for Users of the Threatened Environment Classification. Landcare Research. Walker, S. (January 2012). Potential for biodiversity protection and restoration on the Upper Waitaki Basin floor: What do we know? Wrightson, PGG. (2014). Proposed Irrigation Design. PGG Wrightson.