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:
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:
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