SciBlogs

Archive November 2011

Where does NZ take its water from? Waiology Nov 28

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By Daniel Collins

I mentioned previously how much water we are allowed to use in NZ. The amount varies markedly from region to region, and is growing over time, with Canterbury and Otago accounting for well over half of the consumptive takes (excluding the Manapouri hydro scheme). But seeing as Kiwis seem to know less about our aquifers than our rivers, I’d like to turn now to the issue of where we can take our water from — rivers, lakes, aquifers or reservoirs (or storage lakes).

SWGWAllocation

About two thirds of the water NZers are allowed to take is from surface water — rivers and lakes. This is for all non-hydro schemes and other non-consumptive uses; the data come from a 2010 report from Aqualinc Research for MfE. About a third comes from groundwater. Five percent comes from reservoirs, which are largely fed by rivers.

Looking across New Zealand, we see that the relative importance of surface water, groundwater and reservoirs differs among regions. In Otago, surface waters are relied upon for 84% of the allocated water supplies; in Auckland, it’s 4%. In Hawke’s Bay, groundwater accounts for 74%; in Otago and Taranaki it’s 7%. And in Auckland, reservoirs account for 74%; in Waikato and Manawatu-Wanganui it’s zero.

So why do these proportions vary so much?

There are three key factors behind this: whether there’s enough water in the rivers; whether there are accessible and productive aquifers; and how important reliability of supply is to the water user.

Otago, for example, only has a smattering of significant aquifers, so they get most of their water from rivers. Hawke’s Bay, Southland and Tasman each have productive aquifer systems available. In the Auckland region, where over a third of NZ’s population needs to be provided with a reliable supply of drinking water, they turn to reservoirs and more recently to a reliable supply from the Waikato River via a tunnel. On the Canterbury plains, water from rivers and aquifers are both readily accessible and both highly used, though it is typically easier and cheaper to take water from rivers, even if the reliability of supply isn’t as high. You can see maps of NZ’s key aquifer systems here.

So we can see, yet again, that the physical environment has a huge effect on the availability and reliability of water supply in New Zealand. In posts to come, we’ll see what exactly this water is used for.

Recent NZ research from climate change to tussock | Journal of Hydrology (NZ): 50(2) Waiology Nov 24

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By Daniel Collins

For those of you who don’t receive New Zealand’s own hydrology journal, or for those who want to save some time, here’s an overview of this month’s edition — the journal’s 100th.

1. Barry Fahey (Landcare Research) et al. use their water balance model, WATYIELD, to assess whether tussock in Otago’s uplands can really intercept appreciable amounts of fog, turning it into runoff. Their conclusion: no. This is actually one in a long line of studies that have considered the same question, a question that has turned out to be a veritable controversy, with different papers firmly coming down on opposing sides.

2. Suzanne Poyck (formerly NIWA) et al. use their catchment hydrology model, TopNet, to forecast the impacts of climate change on the Clutha River basin, with a particular focus on changes to snow. Annual precipitation is forecast to increase, as is streamflow (mainly in winter and spring), while the role of snow diminishes.

3. Michael Stewart (Aquifer Dynamics and GNS Science) et al. use isotopic analysis to identify the sources and ages of nitrate in the Waimea Plains, near Nelson. Two kinds of contamination were identified: diffuse contamination from inorganic fertilizers and manure, and point source contamination from a large piggery (now closed). This has been a problem because Ministry of Health guidelines for drinking water have been exceeded for some years. And while input of nitrogen has been decreasing, best practices and nutrient budgeting are still encouraged.

4. Luke Sutherland-Stacey (University of Auckland) et al. test a mobile rain radar device in Tokoroa, central North Island, during 2008 and 2009. Their X-band radar system was able to make observations with high spatial (~100 m) and temporal (~15 s) resolutions, which helps resolve rainfall patterns during convective weather systems at least compared with existing rainfall monitoring systems. But as accuracy declined with distance, particularly over 15 km, the device is best suited for small study areas.

5. Tim Kerr (NIWA) et al. develop a new map of mean annual precipitation for the Lake Pukaki catchment, which includes Aoraki/Mt Cook, using data from 1971-2000. The catchment average is 3.4 m/yr, with over 15 m/yr falling in the north west of the catchment.

6. Clare Sims (BECA) et al. study the dynamics of snowmelt in the Pisa Range, Central Otago. Their focus was how the meteorological conditions that develop over fault-block mountain ranges in the region affect snowmelt. They showed that net radiation (basically sunlight) was slightly more important than sensible heat flux (basically wind), resulting in a sustained pulse of meltwater. They went on to suggest that changes in winter snow could have a significant effect on summer river flows.

Sea level fall and floods due to massive evaporation Waiology Nov 21

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By Daniel Collins

On average, about 87% of the Earth’s evaporation takes place over the oceans. 9% of this water then makes it over the land and falls as precipitation, the rest falls back to the sea. But this is just an average. Over much of 2010, there was so much evaporation from the oceans that the global average sea level actually dropped 6 mm.

GlobalSeaLevel

With more water circulating in the atmosphere, some parts of the planet received much heavier rainfalls, triggering the floods in Australia, Pakistan and Venezuela. This extra water can be detected by its effect on the gravitational pull around the Earth, as measured by GRACE (Gravity Recovery and Climate Experiment). (GRACE figure courtesy of NASA/JPL-Caltech.)

GRACE2010

These changes were associated with a dramatic shift from El Nino to La Nina conditions. For New Zealand, La Nina conditions tend to bring more rain to the north-east of the North Island and less to the south and south-west of the South Island, but we’ll talk about this more in the future.

Of course, as the extra water on the land eventually flows to the sea, we can expect a similarly abrupt rise in sea level, on top of the trend associated with global warming.

(H/T: Hot Topic)

Long-term fluctuations in river flow conditions linked to the Interdecadal Pacific Oscillation Waiology Nov 17

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By Ross Woods

Within the Waterscape research programme, we’re doing some case studies on the potential effects of climate change on water resources, in water-limited parts of New Zealand, such as Canterbury and Hawkes Bay. In effect, we’re asking how these water resources might look in the future. In another post, I’ll look into the results of a couple of the climate change studies we’ve done that relate to water resources in Canterbury, as part of wider team efforts.

Before thinking about climate change, it’s a good idea to understand the variations in water resources that are happening already. The weather is different from one year to the next, and one decade to the next, so water resources also vary on these various time scales. Understanding those variations will let us put any potential future changes in the context of the changes that water managers already deal with.

Here I’ll look specifically at decadal-scale variability in river flows. This is a peek at some new work that isn’t published yet — I’ll be presenting a fuller story at the Hydrological Society’s Symposium in Wellington in December. Understanding decadal variability in streamflow can be critical for the design of water infrastructure for hydropower, irrigation and water supply. Without this understanding, it is difficult to use river flow data from the past as a guide to the future.

IPO? The Interdecadal Pacific Oscillation (IPO) is one of the causes of decadal-scale variability in New Zealand streamflow statistics, for some parts of New Zealand. The IPO is a cyclical change in the Pacific ocean-atmosphere system. A characteristic circulation pattern predominates for a 20-30 year period, and then the system changes to having a different characteristic circulation pattern. These patterns are known as phases of the IPO; the IPO was in a negative phase from 1945-77 and in a positive phase from 1978-99. During the positive phase, El Nino events and westerly winds are more frequent than usual, and rainfall in the west and south of the South Island is higher than usual. The opposite applies during the negative phase.

Does the IPO affect river flows? McKerchar and Henderson (2003) (PDF) showed that there was a significant difference between streamflow statistics for the periods 1945-77 and 1978-99, especially in the west and south of the South Island. For example, the mean flow in the Clutha River at Balclutha over 1978-99 was 14% larger than during 1945-77.

So why look at it again? In 2000, the IPO changed back to the negative phase (i.e., like 1945-78), though the magnitude of the recent IPO is closer to zero (neutral). So now I’m asking, in the ten years since 2000, have the river flow statistics changed back to the values they had for the negative phase (1945-77)?

Analysis. I looked at long river flow records for 35 sites across New Zealand. I calculated annual values of the mean flow, maximum flow, and 7-day low flow. The figure below shows an example of the results, for floods, mean flows and low flows in the Buller River at Te Kuha, on the West Coast of the South Island. The mean annual flood over the period 2000-09 was 17% lower than the corresponding value for 1978-99. Mean flows and mean annual low flows were also lower in 2000-09 than in 1978-99, by about 10%.

IPOFlowVariability
Results for 15 other rivers in the South Island also showed this general result: that all of these three flow statistics were lower in 2000-09 than they had been in 1978-99. Not all the differences were statistically significant, but in almost every case the flows for 2000-09 were lower than for 1978-99. Results for North Island rivers were more mixed, and didn’t display a consistent pattern of change. I’ll need to investigate more to understand the reasons behind both results.

Results. Flow data from 2000 onwards in the South Island support the notion that flows in those rivers are lower during the negative phase of the IPO. The data suggest that the post-2000 reduction in flow has been of the order 10%. We don’t know how long the IPO will remain negative for, but previous IPO phases have lasted 20-30 years, so the current negative phase may last another 10-20 years. Similarly, we don’t know whether the observed correlation between flow and IPO will continue.

Implications. If we look at this from a water resource manager’s point of view, it’s probably a good idea to treat South Island flow data from the period 1978-99 as being slightly higher than the long-term average. So when making forecasts of future water resources for planning purposes, it’s sensible to make allowance for the possibility that flows for the next 10-20 years could be lower than the long-term average. Towards the end of that time horizon (i.e. 2020-2030), we must begin to consider the potential effects of climate change, which is expected to increase precipitation in the South and West of the South Island. But that’s another story …

The importance of groundwater Waiology Nov 14

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By Tiejun Wang

Groundwater is one of the most important natural resources in New Zealand, which by one estimate accounts for about 80% of the water present at any one time across and beneath the country. According to a 2010 review of water allocation [2], the number of groundwater consents accounts for 68% of all national consents. In terms of the allocated volume for consumptive use, groundwater allocation is about 12% (3.3×109 m3/year; equivalent to 6% of the water in Lake Taupo) of the total annual allocation (26.9×109 m3/year). Over the last decade, the demands for groundwater have increased dramatically in New Zealand, which is manifested in the volume of groundwater allocated and the percentage of groundwater allocation in the total allocation [2]. Most of the groundwater resources have been allocated to irrigation purposes in NZ. For example, in the Canterbury region alone, 55% of the agricultural lands are irrigated by groundwater.

tsingtao_logoCompared to surface water, groundwater possesses some unique features. For example, groundwater is typically a more reliable resource, particularly in dry regions, and sometime has higher quality (e.g., temperature and minerals). My hometown (Qing Dao) is famous among other things for its beer (Tsingtao beer). The beer is made of groundwater that is brought up to the surface by springs that are rich in minerals. In fact, it’s the number one branded consumer product exported from China, which you can occasionally find here in NZ. However, the growing demand for groundwater resources in China has raised concerns about the sustainability of both agriculture and the economy. The overdraft of groundwater not only has economic consequences, but also may pose serious threats to the environment.

To start off, dropping groundwater tables, for example due to pumping, can significantly increase the cost of irrigation because the energy required to bring groundwater up to the surface increases with the depth to the groundwater. Secondly, there are numerous cases regarding the adverse impacts of overdraft of groundwater on the environment, which we should learn from. To name one example, the excessive pumping of groundwater, particularly in arid and semi-arid regions, can produce massive depression cones (e.g., in the North China Plain, more than 20 depression cones stretch over 50,000 km2), which can cause the Earth’s surface to subside just like earthquakes (most of the time it is an irreversible process!!), and intensify groundwater contamination.

The water problems that New Zealand is facing right now require us as hydrologists to take immediate actions on how to sustainably manage our water resources. As Ross mentioned earlier, groundwater and surface water are tightly coupled. Therefore, in order to more effectively manage water resources, hydrologists from different sub-disciplines (e.g., surface water hydrology, groundwater hydrology, and ecohydrology) should work together to tackle the problems. This is one of the major goals for the Waterscape project.

The low down on groundwater Waiology Nov 10

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By Daniel Collins

Several posts back I reported on a public perceptions survey that showed that New Zealanders seem to know less about groundwater and wetlands than about rivers and lakes. (Hydrogeologist Michael Campana, OSU, lamented a similar bias towards surface waters among US and international water resource management circles.) I expect this is because rivers and lakes are much more in the public eye — more noticeable when driving around the landscape or on a map, or easier to cover on prime time news. So I’d like to start shedding some light on a part of the water cycle that is basically always in the dark. This should put readers in good stead when reading about groundwater on Waiology or seeing it in the news.

So, groundwater — what is it? The name is almost a dead giveaway really. It’s water contained in the gaps in rock, soil or some other geological layer. But there’s a catch: to count as groundwater and not as soil moisture, all the gaps must be filled with water — that is, the geological material must be saturated.

And where does this water come from? Groundwater is just another part of the water cycle, recharged by water percolating from the soil, rock or rivers above. Over time, groundwater can return to the surface when the water table reaches the ground surface, when it discharges into a surface water body, or when tapped by plant roots or wells.

If a geological layer is particularly good at containing and conveying groundwater, it’s called an ‘aquifer’, from the Latin ‘aqui-‘ (water) and ‘ferre’ (to bear or convey). Its first known occurrence in the English language was in a 1901 article in the journal Science (PDF):

‘The artesian system shows four or five aquifers, or water-bearing strata, more or less completely separated from one another.’

In French, ‘aquifere’ seems to have been used nearly 100 years earlier by the biologist Lamarck when distinguishing between air-conducting passages and water-conducting passages in the body. It attained its hydrogeological meaning by 1835, when Arago described the physics of natural springs.

For a geological layer to be capable of bearing an appreciable amount of water, it must have enough gaps or fractures in it to allow water to flow. If there isn’t much space for water, or if it can only flow very slowly, the layer is called an aquiclude.

How conductive the layer is depends on how it was formed, geologically, and with what material. Gravel deposited by ancient rivers or glaciers tends to form good aquifers; fine sediments deposited beneath lakes or estuaries tend to become aquicludes. In some regions, like the coastal parts of the Canterbury Plains, you can actually find alternating layers of geological material: aquifer, aquiclude, aquifer, aquiclude, etc. In Canterbury’s case, this came about by sequential glacial advances and sea level rises over geological time.
Aquifer2
The upper-most aquifer, when not disconnected from the soil by an aquiclude, is termed an ‘unconfined aquifer’. Aquifers that lie beneath aquicludes are ‘confined’. In some instances, the water in confined aquifers can be under such pressure that they give rise to artesian springs or wells (named after the region in France, Artois) — where water flows unaided above the top of the aquifer or even the ground surface. And as for the water table, this is the depth where the ground switches from being partly saturated to fully saturated, and only applies to unconfined aquifers.

In aquifers, groundwater flows from gap to gap, or from fracture to fracture. While water on the Earth’s surface flows downhill, water in aquifers flows down the pressure gradient which could be upwards. Compared with rivers, however, groundwater flow is much, much slower. It might take decades or centuries for water to cover the distance in an aquifer that river water might cover in a day.

So there you have it – the lowdown on groundwater. It’s an important resource, so we had better understand how it works.

Water storage is not a panacea Waiology Nov 07

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By Daniel Collins

Federated Farmers released their election year manifesto last week. In the press release, President Bruce Wills said:

“Water storage represents New Zealand’s strategic ace. Water storage removes the annual lottery all New Zealanders face from La Niña, El Niño or a changing climate. Our rainfall is plentiful but we miss opportunities with most of it washing out to sea.’

The ‘annual lottery’ relates to the chance that any particular year or growing season turns into a drought*. This risk is heightened during La Nina or El Nino, depending on where in NZ you are, and is likely to heighten further as the climate warms.

Water storage can help alleviate these costly shortfalls in supply, but it is no panacea.

Water storage (i.e., using a reservoir) helps by storing up water throughout the year, or perhaps just the wet season, making it available for use in the growing season. With 80% of NZ’s precipitation flowing to the sea, there may seem to be ample water. But this water is not equally available around NZ. Where water is plentiful, such as on the West Coast, irrigation typically isn’t in as much demand.

Furthermore, just as droughts come with a cost, so too do reservoirs. Large reservoirs cost a lot to build, and while small farm-scale dams may be cheaper they are actually more expensive to develop, in terms of $/m3. Moreover, changes to a river’s flow means that costs may also be incurred by the aquatic biodiversity, recreational opportunities, natural character, and cultural assets of the river system and even the coast.

As David Schiel (University of Canterbury) and Clive Howard-Williams (NIWA) wrote last December:

‘… there are no free lunches when it comes to major extractions of water: benefits in one place have downstream consequences somewhere else.’

So while water storage can help agricultural production it can also hinder other aspects of NZ life, and is thus not a panacea for water resource management. Where the balance is to be struck is a value judgment (not a scientific one), which is made all the more difficult by the diversity of opinions NZers have about their freshwaters.

* Droughts are actually more predictable than the lottery.

Te Radar wades into water Waiology Nov 03

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By Daniel Collins

Soon after ‘Ever Wondered?’ screened its episode on water back in September, TV1 viewers were treated to another installment on water resources, this time by Kiwi entertainer Te Radar on ‘Global Radar’. He set the scene thus:

‘It’s an incredible resource, water, and one that I think we take for granted.’

While the ‘Ever Wondered?’ episode focused on the science behind water resources, Global Radar shifted the focus towards its economic value. Rain, or precipitation more generally, powers so much of NZ’s economy, but the value of this water is a matter of perception.

Consider bottled water. We’re presented with three bottles, each with very different prices: Tai Tapu water from The Store at $4.60/L, carbonated water from Springfresh for $10/L, and a bottle of 420 brand water that we’re told once fetched a lofty $150/L at Gordon Ramsay’s restaurant in London, Claridge’s. Slight differences in the composition of the water (and its marketing) can yield huge differences in perceived value, and hence price.

Most of the water exported from NZ, however, doesn’t travel in bottles, but is embedded in the production of the food that we grow. This is the trade in virtual water, derived from rainfall and irrigation, and is of great value to the NZ economy. But because water has often been seen as plentiful it is given a low value, or sometimes none at all.

This is basic economics: the value of a product or resource increases with its scarcity. And as the value of the water increases, there is more money to be made in capturing and using it more efficiently.

On this note, the show gives us two vignettes. One is of an Aussie economist who sees an economic opportunity in climate change, as it encourages the development of (and profit from) new water-saving technologies. And a second of two Kiwi entrepreneurs who have helped develop (and profited from) such technologies.

And so whether we take water for granted or not depends on the value that we give it. This value appears to be rising, though as I have described before, different people assign different values to water. Indeed, water can be valued as an economic good or also as a cultural, environmental or recreational good. Recognising all these different values within the water resource allocation process is a significant challenge for NZ.

I’ll leave my summary there, then, but do see the episode in full. Te Radar puts a comedian’s touch on an important issue, and how could he go wrong with lines like this?

‘It’s not rocket science, it’s just hydrology. Which is a science all of its own; doesn’t generally involve rockets.’

As seen on TV Waiology Nov 01

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By Daniel Collins

EverWondered

‘Water. It turns the turbines that power our country, and it irrigates the fields that power our economy.’

With that, John Watt introduced an episode of ‘Ever Wondered?’ about water. Three NIWA scientists, myself included, were among those who featured in the half-hour show.

The thrust of the episode was to examine how much water we have in New Zealand, how this might change in the future, and how we can help balance demand and supply. Waiology has broadly answered the first question here and here.

Looking to the future, we have to turn to models. Models are a synthesis of how we think reality works, and if the models do a good job at describing the present, we can use them to describe plausible future conditions. With several such models it has become apparent that drought prone parts of NZ are likely to become more so, particularly Canterbury where farmers would become ever more reliant on irrigation.

But models are powerless without data, and so we must continue to measure how much water there is, and why it’s there. The combination of better data and better models in turn allows decision-makers — be they policy makers or farmers — to make more informed judgments. Resource managers would have a better idea on how much water can be allocated from a river or aquifer without compromising other water needs. Farmers would have a better idea on how much to irrigate.

One innovative approach to increase irrigation efficiency highlighted in the show is offered by a couple of agricultural engineers from Precision Irrigation. They devised a system where they make a map of the soil properties of a field using an electromagnetic sensor, and then configure the individual nozzles of a sprinkler to turn on or off so that just the right amount of water falls to the ground.

Of course, I’m only giving you a synopsis of the episode here. I do recommend you view it in full, though the show is itself a synopsis of what we know and how we’re trying to solve the freshwater management problem.

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