Lake Benmore water quality: A modelling method to assist with setting limits for sustainable irrigation development

While irrigation can bring many beenfits to communities, intensifying land use also causes more contaminants such as nutrients, to enter rivers, lakes and groundwater. Unless managed carefully, this can result in cumulative adverse effects on water quality and environmental values. For lakes, such as Lake Benmore, this could mean irreversible degradation of clarity, colour and biological processes, as well as effects on ecological, recreational, commercial and tourism values. Defining the capacity of waterways to assimilate contaminants and establishing sustainable limits for water quality are recognised as key requirements for effective management (e.g., Government's New Start for Freshwater). We undertook a modelling study to provide statutory water managers Environment Canterbury with a series for options for: i) measurable planning objectives for water quality in Lake Benmore, and ii) the associated catchment nutrient load limits needed to achieve each of the objective options. The modelling study allowed science to inform the decision-making process by describing the environmental consequences of a series of options. The choice between options is beyond the role of science because it requires a value judgement that takes into account social, cultural and economic, as well as environmental consequences. However once a decision is made, setting objectives and nutrient load limits for Lake Benmore could establish a scientifically robust 'line in the sand' for managing cumulative water quality effects and allowing sustainable irrigation in the upper Waitaki catchment.