Abstract

The Whenuakite River flows into the upper Whitianga Harbour (Coromandel Peninsula) from native forest hill-country and pastoral lowlands. Abstraction pressure on the Whenuakite River is increasing with demand for irrigation of kiwifruit and other horticulture. Existing and proposed consents are spread throughout the lower catchment (downstream of Boat Harbour Road). Reduced flows from abstraction can affect stream ecosystems. Potential issues raised for the Whenuakite catchment include fish and invertebrate habitat, growth of nuisance aquatic plants, water quality and whitebait spawning, with the relative importance of each issue expected to vary along the length of the river. Environment Waikato asked NIWA to investigate the flow requirements for these specific issues. The river was divided up into reaches that have common issues and appropriate methods were chosen to investigate each issue, including WAIORA for oxygen modelling and RHYHABSIM for habitat and temperature modelling.

The reaches assessed and issues investigated are summarised in Table 1. Natural low flow conditions were found to maintain adequate habitat and water quality in the lowland and upland reaches. However, dissolved oxygen levels in the tidal reach drop below acute limits during natural low flow conditions because of oxygen depletion by aquatic plants. Abstracting water will further reduce dissolved oxygen concentrations during low flow conditions and extend the time that the river experiences low oxygen concentrations. This suggests the critical issue for setting allocation limits from the Whenuakite is oxygen depletion in the tidal reach.

A minimum flow of 0.205 m3/s is recommended for the tidal reach (based on the moderate impairment oxygen standard of 4 g/m3). This is considered adequate to maintain the resident aquatic ecosystem, which is likely to be impaired by existing low oxygen conditions. To achieve regional plan objectives for a net improvement in water quality, it is recommended that allocations be made subject to mitigation (e.g., avoid abstraction during times of daily oxygen minima). In addition to the resident fish, the tidal reach is also the migratory pathway for all diadromous native fish living in the greater Whenuakite catchment. These migrating fish will include more sensitive species that inhabit the forested tributaries. Therefore, a higher minimum flow of 0.26 m3/s is recommended during the whitebait migration season (1 August to 30 November).

Flow Requirements for Ecosystem Health in the Whenuakite River (Coromandel) [PDF, 1 MB]