Climate and Water Resource Availability in Canterbury

October 2006
NIWA collects data on river flows, rainfall etc throughout the country. In Canterbury it has been studying the effects of various weather events on ground and surface water levels, river flows, aquatic and riparian life, and determining the effects of variations in climate. This is medium- to long-term work that will allow better forecasting of water supplies over the next few decades important in terms of ability of the water system to support various forms of agriculture, and the huge investments being proposed and made in water storage, irrigation, dairying, water demanding crops, etc.

The possibility of a moderate El Nio pattern this summer has just been raised by NIWA. El Nio and La Nia are important weather patterns that regularly impact our climate. So too do the Interdecadal Pacific Oscillation (IPO) and general climate change (global warming). Predicting how they will interact to determine our weather, and hence rainfall distribution, snow, and the availability of river and ground water, is essential to determining what forms of agriculture will be sustainable long term and what farmers should invest in to secure their livelihoods.

NIWA records and analyses information from a monitoring network consisting of several hundred river flow recording stations, rainfall stations, and soil moisture monitoring stations throughout NZ. Regional councils also operate monitoring stations and share information and facilities.

NIWA has developed computer simulation models that use climate data for forecasting water movements. They can put forecasted weather into the model and it will predict what will happen e.g. if it rains heavily or snows in the high country, what will happen in the rivers and on the plains, what will happen to the water table, and so on. It can be used to predict flooding, topping up of aquifers etc.

Selwyn River project

The Selwyn river is flowing in its middle reaches at present, which is quite unusual, and that is because we had a particularly wet winter. It has been particularly low in the last few years down in its lower reaches, and there are lots of questions about whether that is because the weather has been dry or because farmers have been taking more water out of the ground water system there are connections between how much water is in the river and how much there is in the ground water system. Take too much out of the wrong ground water aquifer and the water starts to disappear from the river.

Weve been looking at what happens seasonally to the water in the river. When the weather is very dry the middle reaches of the river are completely dry and when the weather gets wetter then the river starts to flow over more and more of its length, says Ross.

Weve been doing detailed studies of how that changes and what stream communities are affected by that. Why does the river wet up and dry out like it does? Whats controlling that? It isnt always the same. It isnt always predictable. So far we have some ideas and half an explanation of whats going on.

The object of the exercise is to be able to say with some certainty whether it is normal for this sort of fluctuation in water level to happen and whether or not removal of water by farmers is having any important effect on the river ecosystem. If it is, at what level will it be necessary to curb draw-off of water?

Understanding the effects of fluctuating weather conditions on surface and ground flows is obviously important in the short term, just as climate change is important in the longer term.


NIWA has just recently that it believes there is a 50% chance of an El Nio weather pattern developing over summer. This generally means wetter weather in the west, drier in the east. However, predictions over the next 15 to 20 years are that there will be more frequent La Nia events and fewer El Nio. In the 20 years to 1999 there were more El Nio events, and if people are making decisions based on their experiences over the past 20 years they could come unstuck.

Add to that the effect of the Interdecadal Pacific Oscillation (IPO) which NIWA believes will also cause it to be drier in the mountains, and you have significant change that will impact investment decisions by farmers.

Farmers are used to the weather varying from year to year, but they need to understand the differences that are occurring and take that into account in their planning.

Over the next 15 to 20 years, while it will be drier in the mountains it will be wetter on the plains, so its not all bad news. Long term outlook for climate change is for things to get slowly wetter on the mountains and slowly drier on the plains, which is against the IPO trend, so for that period they will work against each other, says Ross.

Beyond that they will probably start to work together in the period 2040 to 2060. It is important that this information is available in particular to people creating very large irrigation schemes in the South Island, e.g. the Central Plains Irrigation Scheme in central Canterbury, which is looking at taking water from the Waimakariri, the Rakaia, and a number of tributaries in between, and putting that water in a big reservoir and using it to irrigate around 60,000ha.

This project is worth hundreds of millions of dollars, and you would want to know that you were making it the right size and whether you would get sufficiently reliable irrigation water out of it.