This report addresses options available to Horizons Regional Council for setting defensible minimum flows for rivers in its jurisdiction.  Horizons is currently considering water allocationoptions for the various water management zones within its region, and an appropriate minimum flow is recognised as a key part of the water allocation framework.  However, minimum flow setting is often subject to much debate in policy and resource consent decision making.  At present Horizons have minimum flows set in regional plans for some of its rivers, but aims to incorporate specified minimum flows in the new regional plan (“The One Plan”) to enable debate around minimum flows to occur at the policy level, as opposed to a consent-by-consent basis.

Like many other regional councils, Horizons have used the Instream Flow Incremental Methodology (IFIM) habitat modelling to inform minimum flow setting.  However, this methodology is relatively expensive, time consuming, and relies on extensive and carefully timed field work during low flow periods.  Horizons have an established programme of IFIM surveys, but are limited to completing two of these surveys in any one financial year.  Consequently, Horizons have sought advice on alternative methods to inform interim minimum flow setting.

This report was commissioned to:

  1. Summarise methods available for setting minimum flows.
  2. Suggest which of these methods (or which combination of methods) could be applied to minimum flow setting within the Horizons region; including verification of the applicability of generalised habitat models to the Manawatu/Wanganui Region (by comparing predictions with existing IFIM predictions from the region).



Flow decisions should be science-based, but the effort put into the science ought to reflect the values of the instream resources.  The values need to be weighed against the risk, and Consequences of error in predictions based on the science.  

Based on this premise we suggest a tiered approach to instream flow assessment and minimum flow setting.  This approach consists of four methods that can be employed depending on the level of demand for water abstraction and the significance of instream values, these are:

  1. Historical flow methods, where the minimum flow can be set according to historical flow statistics (e.g. the MALF or a proportion of it) if the total abstraction demand is a small proportion of river flow (e.g. <10% of the mean annual low flow, MALF) at any downstream point in the catchment; 
  2. Application of generalised habitat models, requiring a minimum of site investigation in cases where the total abstraction demand is moderate (e.g. <30% of MALF), or where the instream values are low;
  3. Detailed site instream habitat analysis (e.g. IFIM) and consideration of effects where abstraction demand is high (e.g. >30% of MALF) and where the instream values are high;
  4. The use of WAIORA to set flow requirements for small streams dominated by macrophytes, where dissolved oxygen concentration is a limiting factor.  Note that this may have to be combined with other technical methods, for example groundwater modelling if drying of spring-fed streams is perceived as an issue.

Associated Models

  • Water Allocation Impacts on River Attributes (WAIORA)

    WAIORA is a decision support system, developed by NIWA, with particular utility in cases where changes to a flow regime are considered likely to impact on water quality.  It uses information on stream morphology, either from simple measurements at two flows or from a RHYHABSIM dataset, to predict how instream...

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Instream Flow Assessment Options for Horizons Regional Council

Hay, J and Hayes John; 2006.

Prepared for Horizons Regional Council by Cawthron Institute – Report No. 1242