Author(s)

Bryan R. Jenkins

Abstract

When sustainability limits for resource use or adverse effects from multiple users are reached, there is a need to manage for the cumulative impacts of all users as well as the impacts of individual users. Ir- rigation expansion in Canterbury led to sustainability limits for water availability for further irrigation and water quality decline from land use intensification. This paper illustrates some of the key lessons for management of cumulative impacts from Canterbury examples. In the Rakaia-Selwyn Groundwater Zone, the spatial scale of management shifted from managing the drawdown impacts on adjacent bores to managing the total extraction from the groundwater zone and the effects on flows in groundwater-fed streams. The management of nitrate contamination from land use intensification across the Canterbury Plains required analysis of the cumulative effects of current and future developments on drinking wa- ter quality: this also led to requirements for increased controls on existing users and raised the issue of affordability of additional controls. As illustrated in water quality management for Lake Benmore, modelling of cumulative effects is more complex than modelling the effects of individual projects. Uncertainties in predicting cumulative effects can be greater than the contributions of individual projects. This creates difficulties not only in setting overall limits for managing cumulative effects but also in determining whether impacts from project proposals can be accommodated. The setting of nitrogen discharge limits on land users in the catchment of Wainono Lagoon demonstrates the issue of equity considerations in allocation of discharge allowances. This is not only for the allocation among existing users but also between existing dischargers and potential new dischargers. The management of salinisa- tion of the Woolston-Heathcote groundwater supply is an example of the need to manage the collective contributions of those responsible for adverse effects not just the individual users. The greater complexity of monitoring for the management of cumulative impacts is illustrated in the management of environmental flows in the Te Ngawai River. This required integrating the measurements of individual extractions and the combined effect on river flow in real time.

Keywords

complex models, cumulative effects, equitable allocation, monitoring, sustainability limits, water management