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Archive 2012

2012 in review Waiology Dec 21

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By Daniel Collins

Another year has come and gone, and with it more science of New Zealand water’s shared near and far. As I look back, with this handy Wordle below, it’s no surprise that “water” got mentioned a lot. Other common words in this year’s posts include “groundwater”, “Canterbury”, “flow”, “rainfall”, “recharge” and “snow”. All very important topics.

One of this year’s successes was the crowd-sourcing project we initiated to collect snow depth data on June 6. Waiology had more visitors that day than on any other. This says something about extreme events and public participation in science and should provide food for thought as we build bridges between the hydrological community and NZ at large.

Based on visits and social media mentions, the post on the water footprint of milk was also very popular. I put this down to the popularity and novelty of the “water footprint” concept as well as the significance of the dairy industry to New Zealand’s news cycle this year.

And along with other observations and inferences, I presented an assessment of hydrology, social media and public engagement at the American Geophysical Union conference in San Francisco in early December. That was a great opportunity to share what I have learned about communicating hydrology in NZ and also learn from other, better funded social media efforts.

Lastly, I should mention that while Waiology started life as a hydrology blog, it has now expanded to cover freshwater sciences more generally, as well as allied disciplines. All the better to serve NZ’s public and science communities. We’ve seen the topics expand already, and I look forward to greater diversity and more guest contributions next year.

In the meantime, enjoy your holidays, and spare a thought for water and all it brings as you pitch your tent, relax with a beer, or whatever the summer brings.


Dr Daniel Collins is a hydrologist and water resources scientist at NIWA.

What causes didymo blooms (“rock snot”) in NZ rivers? Waiology Dec 19

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By Cathy Kilroy

Ever since the alga Didymosphenia geminata was discovered growing prolifically in a South Island river, scientists have puzzled over how “blooms” (unusually large algal growths) of this alga can form. Rivers below lakes and dams in the South Island have been especially severely affected. Massive, persistent mats of didymo in these rivers have changed their character, reduced their appeal for angling, and have sometimes caused problems such as clogging irrigation intakes. So far didymo has not been detected in any North Island river.

Didymo in the upper Ohau River.

Didymo cells and stalks.

D. geminata (commonly known as didymo, or “rock snot”) is a stalked diatom. Cells colonise rivers by attaching to rocks or other surfaces and then growing out into the water column on stalks. The stalks are exuded from the cells and are made largely of carbohydrate. The cells divide while still attached to their stalks, forming thick mats of intertwined stalks with cells mostly at the surface.
Didymo is unusual because its blooms appear only in rivers with very low levels of nutrients. Often these are pristine rivers, like the Mararoa, Hurunui and Buller Rivers. But algal blooms are almost always linked to high nutrients. How does didymo do so well in low nutrients?

Early on in the New Zealand didymo outbreak we assumed that didymo possessed some special mechanism for sucking up nutrients. International researchers have proposed a couple of ways in which this might happen, both linked to processes on the stalks. For example, see here. However, research in New Zealand has revealed an alternative explanation.

The experimental channels set up at the confluence of the Otiake Spring Creek and a braid of the Waitaki River.

Since 2008 I have collaborated with Canadian scientist Dr Max Bothwell in running experiments in streamside mesocosms beside the didymo-affected Waitaki River. An early outcome of the research was development of a method to determine cell division rates in didymo. High division rates in most algal blooms lead to large numbers of cells, which form the blooms.

To our surprise, we found that cell division rates in didymo in the Waitaki River were extremely low. Nutrient addition experiments showed that the cells did not have enough phosphorus (P).

Further experiments demonstrated that stalk length in didymo was inversely related to cell division rates: the lower the cell division rates, the longer the stalks. This suggested that a well-documented property of diatoms was operating in didymo in low-nutrient rivers. The property is release of excess carbohydrates into the water when nutrients (especially P) are in short supply. In the case of didymo the carbohydrate is released as stalks.

In a survey of South Island rivers we found that cell division rates in didymo blooms were always very low. The blooms were present only in rivers where average dissolved P was very low. Didymo in higher nutrient waters had higher cell division rates, shorter stalks, and did not form blooms.

All this indicated that didymo blooms are not driven by any mechanism for obtaining additional nutrients. On the contrary, the blooms are caused by low nutrients in the overlying water, which promotes excessive stalk production. Subsequent surveys, experiments and observations in New Zealand have all been consistent with low nutrients (specifically low P) driving the blooms.

Explanations for didymo blooms involving additional supplies of P did not consider the responses of didymo cells and stalks. Furthermore, these explanations require water chemistry characteristics that are inconsistent with observed patterns of didymo bloom occurrence in New Zealand.


The New Zealand research has been published in three papers in international journals:

Bothwell, M.R.; Kilroy, C. (2011). Phosphorus limitation of the freshwater benthic diatom Didymosphenia geminata determined from the frequency of dividing cells. Freshwater Biology 56: 565-578.

Kilroy, C.; Bothwell, M.L. (2011). Environmental control of stalk length in the bloom-forming, freshwater benthic diatom Didymosphenia geminata. Journal of Phycology 47: 981-989.

Kilroy, C.; Bothwell, M.L. (2012). Didymosphenia geminata growth rates and bloom formation in relation to ambient dissolved phosphorus concentration. Freshwater Biology 57: 641-653.

A further paper examines the evidence for one of the alternative explanations for didymo blooms:

Bothwell, M.L.; Kilroy, C.; Taylor, B.W.; Ellison, E.T.; James, D.A.; Gillis, C-A.; Bladon, K.D.; Silins, K.D. (2012). Iron is not responsible for Didymosphenia geminata bloom formation in phosphorus-poor rivers. Canadian Journal of Fisheries and Aquatic Sciences 69: 1723-1727.

This research has been funded by NIWA capability funds/core funding, Department of Conservation, NZ Fish & Game, and Meridian Energy.


Dr Cathy Kilroy is a freshwater ecologist specialising in freshwater algae.

Phreatogammarus fragilis: The fragile well shrimp Waiology Dec 06

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By Daniel Collins

Phreatogammarus fragilis is an endemic New Zealand crustacean that lives in aquifers. It is an amphipod (a relative of the sand hopper), and is one of the largest (commonly up to 25 mm excl. antennae) and strongest swimming of NZ’s stygofaunal* crustaceans. Because it is so rarely observed, it does not have a common name; the best translations are ‘fragile well shrimp’ or ‘fragile groundwater lobster’, ‘fragile’ probably because its appendages broke off when early specimens were being identified and preserved.

The individual below is a 12 mm-long female with a brood pouch beneath the abdomen. It is white and translucent because there is no point in investing in pigments if it’s too dark to see or if there’s no risk of sunburn. This individual was caught in a trap in a 6 m-deep well beside the Selwyn River in Canterbury by Nelson Bousted and identified by Graham Fenwick. It was photographed live in water in a custom-built aquarium with several off-camera flashes.

Photo credit: Nelson Boustead

Waiology will have some more in-depth science of stygofauna in a future post.

* Stygofauna: animals that live in groundwaters, named after the river in Greek mythology, the Styx, which separated the Earth from the Underworld.


Dr Daniel Collins is a hydrologist and water resources scientist at NIWA.

Map: Projected effects of climate change on New Zealand freshwaters Waiology Nov 27

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By Daniel Collins

Maps are helpful tools in communicating and understanding the potential implications of climate change. We have national maps of projected changes in temperature that show faster warming in the north, and in precipitation that show more rain in the south and west and less in the north and east. We also have national maps of projected changes in drought, that show much of the country is likely to experience more severe droughts.

Now, I am able to give you a map of the potential freshwater changes across New Zealand. This includes changes in snow, ice, river flow, groundwater, aquatic ecology, geomorphology, and water use/management.

This is an important step in synthesising and understanding climate change impacts, drawn from existing case studies across the country. Projections are pin-pointed on the map below; in some cases they are more national in scope (e.g., salinisation of coastal groundwater).

This illustrates quite a complex picture. Retreating snow and ice. More flow in Alps-fed rivers, less flow in others. Higher lake levels and lower lake levels. More water demand from both agriculture and city. Higher erosion as well as channel aggradation. Higher lake nutrient levels and more frequent algal blooms.

There is a lot we know but also a lot we don’t know. As yet, we cannot provide a complete national assessment for river flows, nor for groundwater recharge. And very little research has connected the dots between climate change and aquatic ecology. But as new studies are carried out this map will be expanded and the gaps filled in.

In the near future I will describe the projected changes in more detail, so stay tuned.


Dr Daniel Collins is a hydrologist and water resources scientist at NIWA.

Waiology moving from water cycle to freshwater sciences Waiology Nov 26

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By Daniel Collins

After over a year serving New Zealand as a source of information and discussion on hydrology and the water cycle, I am pleased to announce that Waiology is expanding its scope to freshwater sciences and allied disciplines more generally. This includes all things hydrological, as before, but also aquatic ecology and chemistry, fluvial geomorphology, hydraulic engineering, and related policy and management.

Waiology will continue to serve as a conduit among scientists, professionals and the public, and guest posts will continue to appear from time to time. The pool of contributions will now grow, and no doubt the relevance to both science and the country.

The reason for the evolution is simple. Research and management of freshwater is better if working across disciplinary boundaries. Our challenges are not about the quantity of water alone, nor quality or hazards, but many related issues that must be managed in unison. Hence a science blog that treats them in unison.

I look forward to continue serving you as editor of Waiology, and bringing many more minds to this forum.


Dr Daniel Collins is a hydrologist and water resources scientist at NIWA.

Water allocation and limit-setting in a changing climate Waiology Nov 20

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By Daniel Collins

Last week, the Land and Water Forum released its third and final report on water management in New Zealand. It is a substantial piece of collaborative work with 67 recommendations. Number 29 is that allocation limits be set by taking into account “any flow and water level fluctuations caused by seasonal or other climate variations”. While this primarily refers to natural variability, such as the Interdecadal Pacific Oscillation, it’s also important to consider climate change. And along the same lines, last year’s National Policy Statement for Freshwater Management stated the need to account for the “foreseeable impacts” of climate change.

This is an important issue, as climate change is expected to bring about a raft of changes to New Zealand’s freshwaters (more details on that soon). Among these changes are reductions or increases in the amount of water available for use. Also importantly, climate change makes assessments of future water resources less certain.

So how should resource managers set allocation limits for long-term consents in the context of climate change, accounting for both a change in supply and an increase in uncertainty?

To explore this issue, I propose the following method. It is still in its formative stages, so feedback is welcome.

Let’s start by considering a hypothetical New Zealand river. Its allocation limit is currently set at 40 m3/s. And let’s put aside any complications like priority rights.

Now suppose that results from a climate change impact assessment indicate that allocable flow will reduce by 7% by 2050. This is a middle-of-the road projection, associated with a moderate greenhouse gas emissions scenario and using the median result from 12 global climate models (GCMs). But if you account for the uncertainty of the scenarios, the GCMs and the hydrological models that convert climate changes into runoff changes, then the impact could be anywhere between a 4% and a 12% reduction. That is, it is almost certain that the allocable flow will drop by 4%, it will likely drop by a further 3%, and it might drop by another 5% again.

To set a conservative new allocation limit, first reduce the existing limit by 12% to 35.2 m3/s You can be pretty sure that this water will still be available in 2050 and so you should have no qualms about allocating it for the longest possible duration under the RMA of 35 years (2012 + 35 = 2047). This gives water users the confidence to invest in long-term infrastructure, and it will mean that over-allocation is unlikely to occur.

Second, take an additional 5% of the water (2 m3/s) and allocate this for a shorter period of time, say 10-15 years. It is likely that this water will also be available in the future, but we can’t be as sure. For those water users who are willing to accept the higher risk, they should be allowed to, thus making better use of the available resource.

(If the climate change projections were for an increase in water availability, the same method applies, but the numbers are shifted in the opposite direction.)

Every few decades or so, the long-term allocation limit is re-assessed and changed as needed. Every 10-15 years or so, the short-term allocation limit is also re-assessed and changed as needed.

This allocation scheme meets users’ needs for long-term consents for most of the water (the “certain” water), giving them the confidence to invest in long-term infrastructure, while also allowing them to seek additional water if they are not too risk averse. The scheme also allows the limits to be managed adaptively as new information comes to light – new data on water availability or better climate change projections. And finally, it means that the social, cultural and environmental limits are met whatever happens with climate change, and that the detrimental effects of over-allocation are avoided.

In terms of climate change adaptation, the scheme ticks the boxes of adaptive management and balanced risk-based assessment, and is robust to uncertainties in climate change. As far as I can tell, it also meets the different stakeholders’ needs while accounting for the realities of climate change (that is, change plus uncertainty).

But what do you think? Your feedback would be appreciated in refining this time-dependent allocation scheme.

Canterbury does not have 70% of New Zealand’s freshwater, it has 12% Waiology Oct 29

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By Daniel Collins

In the Christchurch Press today there is an article about the race to irrigate Canterbury. In it is the following statement:

“Canterbury had 70 per cent of New Zealand’s fresh water resource, and 34 per cent of its hydro-generation capacity.”

Unfortunately the 70% is an error that has appeared in the past and it is timely to make a correction and provide some other statistics that describe water resources in Canterbury more generally.

Canterbury has about 12% of New Zealand’s freshwater. This number is obtained from a Statistics NZ report, written by NIWA, on the water stock accounts of New Zealand.

To check for yourself, download the two Excel files from the Statistics NZ website, calculate the annual average of “Outflow to sea and net abstraction” for both Canterbury and the country as a whole, and divide one by the other.

This error has already been noted by the Christchurch Press in an article from March 2010.

For a fuller picture, here are some more useful statistics, from the Ministry of the Environment’s 2010 water allocation snapshot compiled by Aqualinc Research:

    Canterbury’s percent of national water consents (number): 30%
    Canterbury’s percent of national irrigation consents (number): 35%
    Canterbury’s percent of national annual consumptive allocation (m3/yr): 19%
    Canterbury’s percent of national annual consumptive allocation, discounting Southland’s Manapouri hydro scheme (m3/yr): 46%
    Canterbury’s percent of national irrigation allocation (m3/yr): 62%
    Canterbury’s percent of national consented irrigated area (ha): 63%

And from the Canterbury Water Management Strategy:

    Canterbury’s percent of national water allocation (m3/yr): 58%
    Canterbury’s percent of national irrigated area (ha): 70%

The rise and fall of the Heathcote River Waiology Aug 17

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By Daniel Collins

It would have been hard to miss the deluge that washed over the country this past week. Otago and Canterbury were particularly affected, with flood damage in Waitaki District estimated at about $1 million. Minor flooding also occurred around Christchurch, with the lower reaches of the Avon, Heathcote and Styx Rivers overflowing their banks.

To put these high Christchurch flows into context, on late Tuesday afternoon I drove out to two flow gauging sites on the Heathcote River, one at Ferniehurst St (managed by Christchurch City Council) and one downstream at Buxton Tce (managed by Environment Canterbury and Christchurch City Council) and filmed the flows. See the footage below (top = Ferniehurst; bottom = Buxton). In many places, the water levels reached just shy of the top of the bank or beyond; a portion of the road near Buxton Tce had been flooded. There’s some background material on Heathcote River flooding here.

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The river continued to rise until about 9 pm, when it surpassed 8000 L/s at the Ferniehurst site and nearly reached 20000 L/s at the Buxton site. During the 28 seconds I was filming at Buxton, almost eight 40-foot shipping containers’ worth of water passed under the footbridge.

The hydrographs below (data thanks to CCC and ECan) show how the discharge in the two Heathcote River sites rose over the over the week prior to the 14th, and how these peaks compare with historical data. Flows this high don’t last for long. For only 0.02% of the record time have flows ever been higher at Buxton. In the last 20 years, there have been five other flood peaks of this size or larger at Buxton Tce.

You might have noticed that the Buxton Tce site has more than twice the peak flow of Ferniehurst, which responds quite gradually. The Buxton Tce site is a few km downstream of Ferniehurst; Bowenvale Stream joins the Heathcote River between them, and is the main source of extra flood water. Bowenvale Stream drains a steep Port Hills valley that reaches up to the Summit Road and includes part of Victoria Park. Flood waters run off rapidly from the Port Hills, and cause most of the rapid rise and fall at Buxton Tce.

The big hydrological OE in New Zealand Waiology Aug 10

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Guest post By Jasper Hoeve, visiting student from the University of Twente, Netherlands

I am a third year Civil Engineering student at the University of Twente, the Netherlands. As part of our studies, we have to do an internship at a company relevant to our field. I thought this was a good excuse to travel to the other side of the world. My family did not share my enthusiasm but I went anyway. NIWA invited me for a 10 week long internship to develop and apply a methodology for high river flow estimation under El Niño Southern Oscillation (ENSO), Interdecadal Pacific Oscillation (IPO) and climate change under the supervision of Roddy Henderson.

After some research on the internet I found out that NIWA is actually a rather big company. I was expecting that a company of this size would have a formal interaction culture. However, this was not the case. I could just walk into the office of my supervisor to ask any question. He would always have time to answer my questions or discuss some results. When my supervisor did not know the answer to a question, he would send me to one of his colleagues. I could simply enter their office as well, introduce myself and then ask my question. Everyone had time for me; it was a really nice working environment. The informal working environment resulted in me telling about my weekend, what parties I visited and what trips I undertook to my colleagues and supervisor. This was all received by them with a lot of enthusiasm and interest. It was not like what I expected when I started planning this internship.

Predicting high flows and flooding in catchments relies on historical runoff data. A stationary climate, one where there are fluctuations but no long-term trend, is assumed when calculating, for example, the 100-year return period floods. However, changes in climate can distort these calculated high flow events. This may cause errors within the assumed level of security against floods. This is the reason why I studied the effects of ENSO, IPO and climate change on the high flows in New Zealand rivers.

The figure below is an example of my work. Flood frequency curves for the Ahuriri River are plotted for different scenarios of climate change in 2090. As you can see, floods are generally expected to become more extreme. The most important conclusions from my work were that a lot of catchments in New Zealand are affected by the IPO and ENSO phenomena and 20 years of flow data is not enough time to discern the effects of climate change.

I really enjoyed working here and I hope I can visit this beautiful country another time, preferably summertime.

Climate change and NZ’s freshwaters: NIWA presentation, Friday Waiology Jul 26

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By Daniel Collins

For those of you in or around Ōtautahi/Christchurch, I will be giving a presentation on climate change and the future of New Zealand’s freshwaters on Friday (tomorrow), 3:30 pm, at NIWA’s Kyle St site. I’ll talk about what we know and what we don’t know about the potential implications for the freshwater system, including water quantity and quality, ecology, and management. I’ll also discuss the more pressing avenues and what adaptation options different stakeholders may adopt. Ka kite ano!

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