HARVESTING HIGH FLOWS

This project aimed to improve knowledge of potential options for environmentally sustainable high flow harvesting that is practical for users while safeguarding the environment.   

Harvesting high flows involves:

• drawing water from a river or stream or from an unconfined shallow aquifer in periods of high groundwater levels

• storing the water off stream

• using the water during low flow periods 

 

The water that is stored can be used directly from the water storage or used to augment flows during low flow periods, when water from the river or stream is being most heavily utilised. In this way, harvesting high flows can improve the overall environmental sustainability of surface water utilisation, by reducing the take from the river flow during low flow periods.  High flow harvesting can also contribute to more effective water management in the face of climate change.  It can enable regions to smooth out access to water during the more extreme weather patterns that climate change brings.  

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Alongside the potential benefits from harvesting high flow harvesting, the impacts that it could have on instream water quality values need to be taken into account in determining whether to harvest high flows, and how much.  Drawing water off a river during high flows can alter the behaviour of the river and its surrounds.  High flows provide waterbodies with benefits to their ongoing health and vitality, such as flushing of sediment.  In addition to ecological factors, human needs and values, such as Māori cultural beliefs and practices, need to be considered when making decisions about harvesting and use of high flows.

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Figure 1: Diagrammatic representation of how high-flow harvesting relates to various components of river ecosystems.

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For these reasons, regional councils want to better understand the consequences of high flow harvesting in their regions before any further harvesting activity is undertaken.  Councils currently set their policies on harvesting high flows in relation to national level requirements under the NPS-FM.  However, these nationally set requirements do not take into account the local characteristics of waterbodies within individual regions.  Climatic and hydrological characteristics and instream values will differ across regions, as do Māori kaitiaki requirements.  Regions also face different levels of water over-allocation, water demands and water storage availability.

 

AIA commissioned NIWA to pull together what is currently known about harvesting high flows and to consider what would be involved in developing regionally tailored approaches for sustainably harvesting high flow.  NIWA are undertaking the following activities:

• Stage 1: Reviewing domestic and international literature on the use and impacts of high flow harvesting. 

• Stage 2: Developing improved guidance, methods and allocation tools for Northland Regional Council and Gisborne District Council that are consistent with Te Mana o Te Wai.

 

Northland Regional Council set its high flow allocation policy based on research undertaken on flow harvesting impacts in other regions, which have different climactic and surface water characteristics.  This project enables the council to look at the potential impacts of high flow harvesting in the Northland context to inform high flow management decisions in the future.  Northland has a complex hydrological system, reflecting the region’s variable geology, soils, topography, land uses and climate. Northland’s river system consists of a very dense network resulting in relatively short river courses and many outlets flowing to the sea. Climatic and geological conditions in the region result in many streams with relatively low flow magnitudes during dry periods, but where higher flashy flows can occur at any time of year after intense rainfall events.

 

Northland Regional Council anticipates increased demand for abstraction of water at higher river flows as a consequence of a combination of their current water allocation policies, increasing water demand, and anticipated climate change. Where water availability and reliability are limited during prolonged dry periods, high flow harvesting and water storage may be a viable option for providing access to water for out-of-stream uses such as irrigation whilst minimising the risk of producing detrimental effects on in-stream values.   

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Next steps: Northland and Gisborne continue to work with NIWA to progress the modelling undertaken into a tool that can be used for allocation purposes.

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How to find out more

• Email us: info@aquaintel.co.nz

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