Returning Te Waihora/Lake Ellesmere to its pre-1940s condition was not pursued in restoration plans because of the "considerable" social disruption it would involve.
The fact that the Central Plains Water scheme, expected to lead to intensification of agriculture in the catchment and a higher nitrogen load, had already been granted resource consent and "this could not be overturned" was also a factor.
The Selwyn Te Waihora section of Environment Canterbury's Land and Water Regional Plan, which became operative last year, seeks to limit the amount of nitrogen reaching the lake to 4800 tonnes per year.
Before the 1940s, it is estimated the lake had about 800 tonnes of nitrogen reaching it per year. Since then, the nitrogen load has increased, accompanied by a deterioration in water quality.
An ECan staff report to today's Selwyn Waihora Zone Committee outlines the background to lake restoration measures as an update to new committee members.
The report said that achieving a nitrogen load of 800 tonnes per year would require retirement of intensive agriculture from the lake's catchment area, along with other lake and catchment interventions.
In agreeing on 4800 tonnes per year as the target, the zone committee had taken into account "the importance of agriculture to the Selwyn district economy and employment with the need to maintain farm financial viability" and that Central Plains Water irrigation "was consented and this could not be overturned" under provisions in the Resource Management Act.
Without the nutrient limits set in the plan, the amount of nitrogen reaching the lake would be an estimated 5600 tonnes per year.
The zone implementation programme for Selwyn Te Waihora states that none of the other lake mitigation options being employed were a substitute for nutrient load limits and active management to reduce nitrogen and phosphorus entering the lake.
"The lower the nutrient load limits set, the more effective the lake mitigation actions are likely to be and the better the outcomes for the lake. Conversely, the higher the nutrient load, the greater is the reliance on successful lake mitigations and the lesser are the lake outcomes that are possible."
ECan chief scientist Dr Tim Davie said the nitrogen load to the lake would be reduced by one-sixth and that was a large amount.
He also said water quality and lake health were not the same thing.
"Fishers now are likely to tell you it is very healthy, as healthy as it has been for a very long time. However, water quality is still an issue."