Minimum Flows for Ecosystem Health in the Whakapipi Stream (Pukekohe)

TR 2007/28
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Report: TR 2007/28

Author: Thomas K. Wilding (NIWA)

Abstract

Managing the water resources in the Waikato region requires information on instream flow requirements. This report addresses the flow requirements for aquatic ecosystems of the Whakapipi Stream. Abstraction pressure here is high, supplying one of New Zealand’s most intensive market gardening areas. Fish habitat and water quality in the lower reaches were the focus of investigations.

The Whakapipi Stream flows into the Waikato River and RHYHABSIM was used to model habitat for fish and other biota in the lowland reach (below State Highway 22). The lowland reach provides pool and sluggish run habitat that supports prolific plant growth. Oxygen was also monitored in this reach to calibrate a model of the effect of flow changes on oxygen concentrations (using WAIORA).

Fish diversity and abundance are higher in the lowland reach, so maintaining adequate flow for habitat is expected to have the greatest benefit there, compared to inland reaches where fewer fish have access. Invertebrate sampling did not reveal communities of greater significance in the upper catchment. So in terms of habitat at least, it is reasonable to base flow requirements on the lower catchment. Flow requirements for fish habitat in the lowland reach were estimated at 0.050 m3/s (Table 1).

Low oxygen concentrations are stressful to aquatic life and reduced flows have the potential to exacerbate this. With a low stream-gradient and prolific aquatic plant growth, dissolved oxygen concentrations in the lowland reach fluctuated between morning and afternoon (2.7 to 9.7 g/m3 on average). A flow requirement of 0.083 m3/s was predicted to achieve a 24-hour oxygen minimum of 4 g/m3 in the lowland reach, and is expected to be adequate in maintaining the existing aquatic ecosystem. The short duration of diurnal oxygen minima may allow a lower oxygen standard to be adopted without significant impacts on the receiving aquatic ecosystem (flow of 0.071 m3/s required to maintain 3 g/m3 of oxygen). Some reaches further upstream were smothered by aquatic plants, and experienced low oxygen conditions as a result (for example, Barnaby Road). Further investigation may be required to determine flow requirements for these reaches, or other management options explored to provide for aquatic ecosystems.

Minimum Flows for Ecosystem Health in the Whakapipi Stream (Pukekohe) [PDF, 1.3 MB]