By Daniel Collins
Following a recent Timaru Herald article (3 February, 2015), I learned of a claim that 98% of NZ’s rainfall is left to flow out to sea, and that we only capture the other 2%.
‘‘This country doesn’t have a water shortage issue. What it has is a water storage issue. We capture a mere 2 per cent of our country’s total rainfall, the rest pours out to sea!’’ – Waitaki MP Jacqui Dean’s office.
‘‘It is wasteful that we only capture around 2 per cent of rainfall in New Zealand, with the rest roaring out to sea.’’ – Minister for Primary Industries Nathan Guy, in a speech to Crown Irrigation Investment Ltd.
These statements aren’t quite right, but because the topic is of vital importance, it is worth commenting on what is actually happening. Some of the rain evaporates before it can reach the sea or get used by us, and the “2%” isn’t actually how much we capture anyway.
Evaporation is an important component of the water cycle
The amount of water that falls on New Zealand during an average year is 610,000 million m3. That’s 10 times the volume of Lake Taupo. Roughly 20% of this evaporates before reaching the coast, leaving us with a freshwater resource flowing down our rivers and through our aquifers of 490,000 million m3 per year.
Of this freshwater flow, 27,000 million m3 was consented for abstraction for consumptive uses over 2009-2010. This equates to 5.4% of the total freshwater resource. Most of this is for the Manapouri hydropower scheme, which diverts water from Lake Manapouri through a mountain to Doubtful Sound. Because we typically think of hydropower schemes as “non-consumptive”, removing this allocation from the total leaves 11,000 million m3, or 2.2% of the total freshwater resource. If we were to express this value in terms of national rainfall, it would be 1.8%, the difference relating to water lost to evaporation.
It would make sense to round both the 2.2% and 1.8% to 2%. After all, they are estimates and the actual values vary from year to year, but that is a slippery slope to confusion. Because councils grant consents to take terrestrial freshwater not rain water, 2.2% is more meaningful. In either case, this quantity is apparently the source of the “2%” used in the above quotes.
So by saying that all the rain water that we don’t capture flows to the sea, we neglect the rainfall that evaporates before it can be considered a ‘water resource’.
We capture much more than 2% of the water resource
As explained above, 2.2% of New Zealand’s freshwater supply was allocated for consumptive uses in 2009-2010, excluding the Manapouri scheme. This is not the amount of actual water used or actual water captured, it’s a legal entitlement subject to consent conditions. A very rough estimate of actual use for non-hydropower users is about 50% of the total allocation, but as data are coming in now we should get a more accurate estimate this year or the next.
But we shouldn’t actually exclude Manapouri from this calculation. There is little difference between the hydrological effect of the Manapouri scheme and the hydrological effects of municipal and industrial water use, even if it’s conceptually convenient to separate them. Most if not all of the water used in these different applications is returned as liquid water to a river, lake, or the coast. Even a small portion of irrigation and stock water will likewise return to a water body before flowing to the sea.
It is thus more accurate to say that, as of 2009-2010, we were allowed to abstract 5.4% of the nation’s freshwater supply.
Lastly, there is a difference between “abstract” and “capture”. To abstract water, it must be captured, but water that is impounded in run-of-river dams is also captured while not abstracted, and so is a portion of the rainfall that falls directly on crops.
The water captured by run-of-river dams is substantial – the Clyde Dam, the various dams along the Waitaki and the Waikato, etc – even if you avoid double-counting water that is impounded multiple times. While the exact numbers aren’t immediately available, a quick estimate puts the amount of freshwater that we were consented to capture in 2009-2010, and did actually capture one way or another, well above 10% of the country’s average annual freshwater supply.
Direct rainfall can also not be overlooked. All crops get a free supply of water this way – “green water” in water footprinting parlance. Any irrigation, which is part of our “blue water” footprint, is added on top of this. How much this captured green water amounts to has not been calculated, and therefore we cannot say with much certainty at all how much rain water we capture.
We use even more
The last point to make is that we use water in more ways than are encapsulated above, so it would be inaccurate to say that any uncaptured water is necessarily wasted. In addition to water used for hydropower, irrigation, stock watering, municipal supply, and industrial operations (all consentable volumes), we also use water for recreation (swimming, kayaking), for provision of habitat for fish and mahinga kai, as a conveyor belt for sediment that we extract, and also as a spiritual, cultural, or aesthetic resource. All of these are what we term “ecosystem services” – goods or services provided to us by the water, whether yielding economic, societal, or cultural benefit.
By the numbers
- 610,000 million m3 of water falls on New Zealand in an average year.
- At least 20% of this evaporates.
- This leaves 490,000 million m3 of freshwater to flow via rivers or aquifers to the coast.
- In 2009-2010, 26,000 million m3 of freshwater was allocated for abstraction, equating to 5.4% of the freshwater supply, most of this would have been used.
- In 2009-2010, over 10% of New Zealand’s freshwater supply was captured for one use or another.
 This may be on the low side, but it is very difficult to be sure.
 “Consumptive” water uses are those that move water from one point in a water body and deliver it somewhere else. Some or all of this water may return to the original water body. “Non-consumptive” water uses simply interrupt the flow of water along its normal course.
Dr Daniel Collins is a hydrologist at NIWA.