Archive October 2011

On the philosophy of modelling Waiology Oct 27


By Daniel Collins

Rodin's 'TheThinker'Back in June I introduced you to a model we use, called TopNet, and I talked about modelling on the ‘Ever Wondered?’ episode back in September. I’d now like to give you a richer picture of what models are in general.

Models feature widely in hydrology. Not the fashion models usually, but the mathematical or computer variety. In fact, to be pedantic, everything in hydrology is based on a model in one way or another, right down to the measurement of river flow. So to understand hydrology properly, you need to understand modelling. But be warned: modelling cannot be explained in one blog post. Whole books are written on the subject. Whole careers are built on building models. Instead, what I will try to do here is share some of the fundamentals of the philosophy of modelling as I see them.

The first rule to remember is that all models are to some extent wrong, but some models are useful.

Models are simplifications or idealisations of reality with the purpose of serving a particular function. This goes for fashion models, who are idealisations of the human form and whose job it is to make clothing look good. Or model or toy aeroplanes, which are easy-to-build miniatures that you can play with. Or mathematical models, which attempt to explain some phenomenon as a calculable function of defined variables.

The same goes for hydrological models. Mostly they are of the mathematical variety, but sometimes they can take on aspects of the fashion or toy models.

A mathematical model is basically an algebraic relationship with one or more variables, like Einstein’s e = mc2. You can add randomness, make it self-reflexive, or combine a suite of individual equations together if you want, but it’s still a mathematical model. One of the strengths of mathematical models is that they typically allow for more reproducibility, one of the hallmarks of science. And as soon as you write it into computer code, it becomes a computer model too. We take this step whenever it’s too burdensome to calculate the equation(s) by hand.

The model could be based on empirical observations, like the Rational Method for peak flood flow:

Qpeak = C I A

Or on physical principles, like Richards equation for water movement in unsaturated porous media:


But no matter what the model looks like, it would have been developed over the course of much time and research. For it to become an accepted model of reality, it will have been seeded by observation, shaped by imagination, potentially tweaked by calibration, and will have successfully jumped through the hoops of validation and either verification or corroboration.

And as modellers build their models, they should remember (though we don’t always) that models should be as simple as possible, but no simpler.

This basically means that models shouldn’t include features that don’t help them serve their purpose, but that they must still be useful somehow (in contrast to a spherical cow). For example, water temperature won’t improve Manning’s equation of river flow noticeably, so it’s left out, even though a liquid’s viscosity is temperature-dependent. While too few parameters in a model can lead to huge errors, too many parameters can be overly burdensome in terms of data collection or cause the model to lose all generality.

Now remember that hydrological models can sometimes resemble fashion or toy models?

If you’re trying to explain an idea, such as the water cycle, you need a conceptual model that makes sense – a model that makes people want to buy an idea. This is much like a fashion model who is used to sell clothing. And if you’re trying to explore the implications of existing scientific theories, such as the role of carbon fertilisation on catchment water yield, you need a toy mathematical model whose variables you can experiment or play with.

In the end, of course, we must remember that while good models serve a purpose, the results will be no better than the veracity of the model. It takes skill to use a model properly, and to interpret its results. And no matter how sophisticated our modelling skills, we should never lose sight of the observational data that feed our models and the questions that drive them.

Measuring snow and rain with a crashed spaceship Waiology Oct 25


Guest post by Dr Tim Kerr (post-doctoral fellow funded by the Ministry of Science and Innovation, and hosted by NIWA)

Back in June, Daniel described New Zealand’s rain as coming mainly from the plains mountains. In the South Island, that generally means the Southern Alps. One of science’s tasks is to refine our understanding of where, why and how much of this precipitation is falling. In preparation for doing this I’ve installed ten new rain gauges in some fairly remote regions of Westland Tai Poutini National Park.


These are no ordinary rain gauges.


A standard rain gauge is made up of a funnel that guides rainfall onto one of two small buckets on either end of a see-saw.

When a known amount of water falls into one of the buckets, the see-saw drops and empties the bucket at the same time as raising the other bucket up into line with the funnel.

Every time the see-saw tips a small switch closes, which enables an electronic recorder to keep track and timing of the tips. This works well for rain, but If you put this type of gauge somewhere where it may snow, then the funnel clogs and nothing is measured, or worse still, the entire gauge gets buried.


My gauges consist of two-metre high pipe filled with mono-propylene-glycol (an agricultural food supplement that also works as an antifreeze!). An overflow tube runs from the top of the pipe down to the base where it feeds into a normal rain gauge.

Using this system, if it snows or rains, the level of the fluid in the main pipe rises and pours through the overflow tube to the tipping-bucket mechanism and a measurement is made. The height of the gauge helps prevent it from being buried by snow but has the draw back that it is susceptible to stronger wind, which is known to reduce the amount of snow or rain that falls into the gauge (under-catch). To get around this, the top of the gauge is surrounded with a circle of metal slats called an Alter Shield. Even an Alter Shield is not perfect, so a temperature and wind speed sensor have been installed near each gauge. Using the measurements from these devices a correction for any extra under-catch can be made. The whole thing looks a bit like a crash landed space ship! The gauge shown here is in the upper Boyd Creek. The equipment on the pole to the right of the precipitation gauge are the wind speed and temperature sensors.


The gauges were installed at the end of March 2011. When last checked (at the end of May 2011) the gauges were measuring from between 1.4 times (at the Lower Spencer Valley site) to 2.2 times (at the Upper Callery site) the amount that was measured at the Franz Josef Village airport (the nearest long-term NIWA rain gauge site). The Franz Josef airport has an estimated average annual precipitation of 4 m, so the Upper Callery is certainly a bit damp. After two years of measurements at these sites, some much-needed extra detail will be known about the distribution and magnitude of rainfall in the area. It will then be time to move on to the next blind spot on the rainfall map.

How much water do we use? Waiology Oct 21


By Daniel Collins

One of the arguments being used at the moment to promote water storage and irrigation schemes is that much of the water that falls on New Zealand flows to the sea, not to the farm. Conor English, CEO of Federated Farmers, wrote in an opinion piece earlier this year:

“It’s not that New Zealand is running out of water, it’s that water is running out of New Zealand.”

As it turns out, about 80% of the water that falls on New Zealand flows out to sea, the rest evaporates back into the atmosphere.

As for what we use, Lachlan McKenzie, previously from Federated Farmers, is cited as saying that 97% of New Zealand’s water flows wastefully to the sea (audio). Similarly, in a Q&A file released in May, the Government states that 2% of our freshwater resource is used.

McKenzie’s 3% used and the Government’s 2% are basically the same. They ultimately come from two sources: the 2006 Statistics NZ report mentioned previously, and a report on water allocation by Aqualinc Research Ltd. The numbers refer to the fraction of total annual water supply that can be abstracted legally, as an average for New Zealand as a whole. (It’s important to remember that water doesn’t need to be abstracted to be useful or beneficial, even for commercial purposes, but that’s a discussion for another time.)

According to the Aqualinc report, nearly 27,000 x 106 m3 of water may be abstracted from rivers and aquifers each year. That’s almost half the volume of Lake Taupo*. Of this, 16,000 x 106 m3 is for the Manapouri hydropower scheme that takes water from the Waiau River and discharges it directly to the sea. If we’re just thinking about water that can be consumed for irrigation, drinking and so on, then we’re left with 11,000 x 106 m3. Using the Statistics NZ data (either 2006 or 2011 reports), this is 2% of New Zealand’s annual freshwater supply.

But this is a national and annual average, which is adequate for a big-picture view but irrelevant for practical purposes. It implicitly assumes that the water is equally available everywhere, all the time. It is not. The water in Southland is of no use to Canterbury or Hawke’s Bay, and unless we build pipes or tunnels across the Southern Alps, nor is the West Coast’s. The water in winter is of no use in summer unless we build reservoirs, hence the drive for more water storage. (Expect more on the hydrology of water storage in the future.)

For a more useful description of the annual water used in each region, check out the figures below. Omitting Southland’s hydropower, most of the water allocated for consumptive purposes is in Canterbury, followed by Otago. Canterbury’s allocation is 5,000 x 106 m3/yr — nearly half of New Zealand’s total. As a fraction of each region’s annual water supply, Canterbury and Otago are also on top, at 8.3% and 7.7% respectively. Cool and wet Southland and West Coast come in at the bottom with 0.2% each, as does less developed Gisborne.

It’s easy to see how the regional picture is a lot different from the national picture, but in terms of how we use our water, this is really only part of the jigsaw puzzle. Stay tuned for more pieces.

Water Allocation

* The volume of Lake Taupo is about 59 km3 or 59,000 x 106 m3. 27,000 x 106 m3/yr is about 21 Olympic swimming pools worth of water per minute.

Public perceptions of NZ’s freshwater management Waiology Oct 19

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By Daniel Collins

In my previous post I wrote about how Kiwis perceived the state of our freshwaters and how certain they were. Drawing from a report by Lincoln University, I’ll now cover what Kiwis think about freshwater management and what they want management to achieve.

Most people believed that all aspects of the environment, freshwaters included, are managed at least adequately, and that this management has improved over the last 10 years. However, people were most negative about management of rivers, lakes and groundwater. And again, just as people were least certain about the state of wetlands and groundwater, people are also less certain about the management of these freshwaters.

In terms of activities, more than half of the respondents thought that management of farm effluent and runoff was bad or very bad, while they were most satisfied with management of sewage disposal. Indeed, most people thought that the degradation of freshwaters was due primarily to farming, though a substantial portion also implicated sewage and storm water, as well as industrial activities. The primary drivers of wetland degradation were considered to be pests and weeds, farming, and sewerage and storm water. Looking down ethnic lines, Maori implicated sewage and storm water as the main culprit for degraded freshwaters, followed by farming; for NZ Europeans, it was the other way round.

Many people did not know how effective different planning or policy mechanisms were in regards to freshwater sustainability. They did, however, have more faith in regulatory approaches and least faith in voluntary programmes, while economic approaches like water trading or pollution fees were intermediate. Combined approaches, that use all of these methods, were the most popular. People generally approved of water-use meters, and of leaving water in rivers for environmental and recreational needs. And while more than half of the respondents thought no further abstraction should occur from lowland streams, almost a third did not know about the quality or management of these streams.

As for the desired outcomes of freshwater management, people generally wanted safe water to swim in, safe water to drink, no further significant pollution, and protection of the most important fishing rivers. Opinions on other outcomes, such as biodiversity and hydropower, were more divided. Over 20% of respondents considered customary Maori values as being irrelevant in terms of freshwater resources. And looking down ethnic lines again, 70% of Maori believed that Maori values should be considered a lot more in freshwater issues; only 17% of NZ Europeans thought the same. Also, more NZ Europeans and other ethnicities found aquatic biodiversity loss to be acceptable than did Maori.

So, overall, we want a clean freshwater environment, but we don’t see eye-to-eye on all the issues or how to get there. This is quite apparent in the two recent developments related to Southland’s Waituna Lagoon and the Manawatu River. Developing a better template for solving these types of problems is in fact the job of the Government-appointed Land and Water Forum, and I’m sure Waiology will cover that in more detail in posts to come.

Public perceptions of NZ’s freshwater environment Waiology Oct 14


By Daniel Collins

As much as Waiology is about conveying science to the public and fellow water professionals, it’s also valuable for us all to understand how New Zealanders perceive our freshwaters and freshwater management. Excellent insight into this is provided by a biennial report from Ken Hughey and colleagues at Lincoln University. The 2010 report covers the environment in general, but has given special treatment to freshwater. I’ll cover the results in two parts: the first on the state and knowledge of freshwaters; the second on freshwater management. The main conclusion is that Kiwis are more concerned about water than any other environmental issue.

Most of the respondents believed the NZ’s environment and freshwaters were in a good or adequate state. However, the most negative responses of 11 environmental aspects considered was rivers and lakes (grouped together); wetlands and then groundwater followed not far behind. Most respondents believed that NZ has at least a moderate availability of freshwater resources, though 20% of people did not know much about the wetlands we have (Answer: 89,000 hectares, down from 670,000 before human settlement). Considering differences in responses between ethnic groups, Maori typically thought that water quality was worse than did NZ Europeans.

The confidence of the respondents was also interesting. A clear majority of respondents believed they have an adequate or good knowledge of environmental issues. When freshwaters were broken down into rivers, lakes and groundwater, it was clear that people were much less certain about the state of groundwater and its management; however, Cantabrians tended to be more certain about their groundwater, perhaps as an indication of how significant groundwater is to the region. People were also less certain about wetlands and lowland streams than other surface waters. Comparing the postal results with the online part of the study, online respondents were almost always more certain of their answers.

It’s clear that we New Zealanders don’t know all that we could about our freshwaters, particularly about groundwater, wetlands and lowland streams. This is perhaps because a lot more is said in the media about larger rivers and lakes. But even among us scientists, we know more about NZ’s rivers than its aquifers, or even groundwater-fed lowland streams. Waiology will do its part in filling those knowledge gaps soon.

How much water does New Zealand have again? Waiology Oct 06


By Daniel Collins

Today, Statistics NZ released a new assessment on the water accounts of the country. Back in June I distilled the previous 2005 assessment for you, and now here’s the update.

The average amount of precipitation that falls on New Zealand each year, now based on data from 1995-2010, is 610,000 x 106 m3 — just over 10 times the volume of Lake Taupo. This is a bit more than the last estimate. The proportion of this water that in turn reaches the sea is 80% – little change there.

Of course, averages hide variability among data. 1996 was the wettest year with 700,000 x 106 m3, and 2001 the driest with 550,000 x 106 m3. No surprise, then, that the largest drop in lake and reservoir storage also occurred in 2001. I wasn’t in the country then, but you may remember the news: drought, power crisis.

There are also differences among regions. On an areal basis, the West Coast is the wettest, Otago the driest, and both come about because New Zealand straddles a subduction zone that has pushed up the Southern Alps. Compared with the last assessment, there are slight differences in the order of the regions in terms of the equivalent depth of precipitation and freshwater (see the figure below).

You may also notice differences in rank between the two figures. Tasman is #2 when it comes to the average depth of precipitation, but #6 in terms of available freshwater. The difference is evaporation: P = FW + E. The difference in rank is because not all regions evaporate the same proportion of water. This is largely due to temperature, but the landscape also plays a role: there’s more evaporation and less runoff from taller vegetation, all else being equal.


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