The Central Otago community faces a challenge after an "eye-opener" report showed a significant loss from irrigation schemes to groundwater, Otago Regional Council chairman Stephen Woodhead says.
An investigation of the Dunstan Flats, Earnscleugh Terrace, Manuherikia claybound and Manuherikia alluvium aquifers showed rainfall was a minor source of recharge. Instead, "pervasive" surface irrigation systems and tributary rivers sustained their water balances, groundwater scientist Jens Rekker told a recent natural resources committee.
"Stony soils, wild flood irrigation or leaky irrigation races were all conditions that give rise to high-intensity recharge. The Fraser River in particular was found to recharge the underlying aquifer with substantial volumes of water."
The aquifers, important for domestic and stock water supply, irrigation bores and frost water bores, were not fully allocated but the data from the study would be used to develop future allocation policy.
Mr Woodhead said the policy drive needed to be towards a more efficient use of water.
"Significant loss from some irrigation systems to groundwater is very large in some cases, which is not efficient at all."
Losses from water races in Dunstan showed nearly 70% went straight through to the Clutha River.
Mr Rekker said if the leakage from irrigation was removed from the Dunstan Flat system, it would be "a pale reflection of its current state".
"It is an artificially recharged system in its current state."
Another report to the meeting on groundwater rainfall recharge across Otago gave the council's science team more accurate data to manage the system. Thirteen aquifers were studied.
It found rainfall was one of the primary sources of replenishment for Otago aquifers, with recharge depths ranging from 25mm per year (5% of rainfall) in semi-arid basins to 212mm per year (22% of rainfall) in the wetter basins of South Otago, groundwater scientist Scott Wilson said in the report.
Cr Michael Deaker said the report showed that while the council was committed to efficient use of water, there was "not an awful lot of recharge out of the sky".
Chief executive Graeme Martin said for many of those areas, the development of irrigation systems and how groundwater was treated would be a major challenge.
Cr Doug Brown said it was important the council defined its water resources so people were aware of how those resources interacted.
