Across New Zealand, and in particular the South Island, as the summer bites, many rivers are struggling to supply enough water to meet demand. A near complete lack of rain in many parts across February and below average rainfall in January has left West Coast, Canterbury, Nelson, Tasman among others facing increasing restrictions as rivers fall below their minimum flows – the point at which those with consents to take must cease.
In Nelson, the Waimea River has virtually stopped flowing and a bund has been built across its riverbed in the lower reaches to stop salt water intrusion by make freshwater pressure build up. With high demand still being placed on it to provide water for fighting the fires that broke out over a week ago. And despite a tropical cyclone tracking towards New Zealand possibly bringing rain, only a prolonged period of wet weather will be sufficient to recharge what are now depleted aquifers.
Similarly the famously wet West Coast is drying out as well. Whilst occasional heavy rain warnings are being issued, for a province that has rain recorders known to receive in one instance over 500mm on rare occasions, the amount that has fallen is well below normal. The Whataroa River is normally about 1.00 metre deep where it passes the flow gauge. As of 18 February it’s depth was 0.45 metres; at the same time the Grey River at Dobson which can get to as low as 0.20 is about 0.04 metres, and thereby probably unreliable.
The river catchments in Canterbury that have their headwaters in the Southern Alps are also heavily affected. Many are now on minimum flow restrictions, meaning because the flow in the river at a particular point is below a minimum level set, irrigation and other water takes must cease.
One example is the Waimakariri River, which has a minimum flow of 37m³/s-¹, but which is (at the time of writing this article)currently running at 31m³/s-¹ at the Old Highway Bridge and 45m³/s-¹ at Otarama, upstream of Waimakariri Gorge. Because the Old Highway Bridge is in a tidal zone recording station it is not subject to water take restrictions as the water is not fresh.
Upstream though, at sites on tributaries of the river, the water takes tied to those sites are subject to restrictions. The restrictions depend on how close the flow at that site is to the trigger level, with some being on partial restriction and others being on full restriction. The Waimakariri is not the only river with partial or full restrictions placed on water takes from it. More information is available at the Environment Canterbury website irrigation page.
In a rapidly changing environment, where water supplies are fully allocated in many catchments, issues with supply are only going to get worse during future summers. Some of the flow sites monitored by Environment Canterbury are currently recording levels below the trigger level at which consents are triggered. They therefore have restrictions imposed, and more will join them if there is not significant rainfall in the near future.