Science Centres: Freshwater

Towards better irrigation practice

 

Irrigating near Luggate, Central Otago.(Photo: Alan Porteous, NIWA)

As the area of irrigated land in New Zealand continues to rise, the ability to predict effects on stream flows becomes increasingly important. NIWA has been using a hydrological model to investigate the implications of current and future irrigation scenarios. The model can simulate irrigation requirements under various land-use scenarios, and predict impacts on in-stream flows throughout a catchment.

The model has recently been used in the Manuherikia catchment, Central Otago, where five scenarios were simulated: historical pre-development conditions, dryland farming, a mix of current flood and spray irrigation practices, and two scenarios incorporating efficiency increases through the use of spray irrigation and lining of water races.

The study indicates that:

• partial replacement of flood irrigation by spray irrigation, and partial upgrade of water races, could reduce water requirements, increase mean river flows by 7%, and increase summer low flows by 25%.
• a full upgrade to spray irrigation and lined water races could increase mean flows by 11% and low flows by 40%.
• under both pre-development and dryland farming scenarios, rivers could have 50–60% higher mean flows, and low flows six times higher than under current conditions. Inevitably there would be social and economic costs under these scenarios.

NIWA is working in a Foundation for Research, Science & Technology-funded project with Aqualinc, Otago Regional Council, and other agencies in similar studies in various catchments in Otago and Canterbury.