Adaptive management of groundwater in the Rakaia-Selwyn Groundwater Allocation Zone: technical and implementation issues

This report recommends that a recharge-based groundwater management method be implemented. We describe a groundwater management method based on annual supply of recharge. The method varies the quantum of groundwater abstraction in the Rakaia-Selwyn Groundwater Allocation Zone to correspond with the state of the resource as measured by the land surface recharge to groundwater. We suggest an allocation volume held by consent holders be made up of a base (fixed) entitlement and an adaptive (variable) entitlement. We recommend use of this method over those involving groundwater trigger levels alone because: • it avoids the complications of localised interference effects; • it may be adapted to deal with climate change effects; • it allows prediction of environmental outcomes and may be tailored or adapted to achieve these outcomes. The recharge-based management method, using climate data for the period from 1960 to 2008, indicates the following indicative reliabilities as a percentage of full allocation that is available for abstraction in a year: • 100% in 20 years out of 49; • greater than 90% in 32 years out of 49; • greater than 75% in 41 years out of 49; and • between 60% and 75% in 8 years out of 49. Restrictions based on antecedent recharge will not necessarily occur at times of high demand. Comparison of calculated demand and with restrictions based on antecedent recharge indicates that in some years entitlement is greater than the demand, and as a result no practical restriction would have been experienced by users (e.g. 1990-91). In some years demand is higher than entitlement, meaning that real constraints would have applied (e.g. 2001-2008). We recommend July 1st as the primary date when the adaptive entitlement should be assessed. We have modelled the relationship between restrictions in water use resulting from the rechargebased method, and the corresponding increase in discharge from the aquifer system. Using one of the spring-fed streams, Harts Creek, as the indicator of the health of the discharge from the system, we have modelled an indicative increase in flow at Harts Creek of the order of 200 L/s between managed and un-managed abstractions during periods of low flow. Robust water use data will improve certainty with which we predict environmental outcomes resulting from the implementation of the recharge-based method. The recommended, recharge-based method allows prediction of environmental outcomes and to be effective within the framework of the Proposed Natural Resources Regional Plan Variation 1 and the proposed Restorative Programme for Lowland Streams it needs to be: • based on precise data that are easy to measure; • be robust and straightforward, in order that it can be understood; • produce technically correct verifiable results straightforward enough to be communicated to nontechnical user groups; • a potentially equitable solution to the management of cumulative effects; and • able to predict environmental outcomes in the form of surface flows; these have been achieved. Monitoring data requirements are largely in place but further monitoring and analysis are recommended, such as: • telemetry of monitored groundwater level data from four existing multi-level piezometer wells to further our knowledge of the dynamics of groundwater flow through the aquifer system; • monitoring of daily mean flows in more spring-fed streams (River Irwell, Boggy Creek) is undertaken to allow for measurement of the short- and long-term dynamics of discharge from the aquifer system. Such monitoring will allow verification of the choice of indicator site, hydraulicconnection effects and provide valuable input to the proposed management method; • better definition of environmental outcome. In this report, the minimum flow at Harts Creek has been chosen; • that a programme of analysis of climatic data in association with the data derived from the metering of all consented takes, be initiated in order that the relationship between climate and water use, between use and effects, and between soil and use are better understood. These data will strengthen the technical justification, community acceptance and eventual operation of the adaptive management mechanism.