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Posts Tagged Climate change

Thirsty trees and water yields: Vegetation, water and a changing climate Waiology Oct 23

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By Cate Macinnis-Ng

2014IconFuture climate projections predict that some parts of New Zealand will become drier with droughts being more severe and frequent. This is particularly true for the north and eastern parts of the country. We know that soil moisture availability will decline due to reductions in rainfall and increased evaporative demand will lead to faster transfer of water back to the atmosphere. However, we do not yet fully understand the impact of climate change on water balances of vegetated catchments.

In forested areas, a large proportion of rainfall (up to 90% or more) is lost back to the atmosphere as evapotranspiration, the sum of loss of water through plants (transpiration) and evaporation of water from bare soil. Transpiration is the dominant component of evapotranspiration. The amount of water remaining after evapotranspiration (often known as the water yield) changes because plants respond to environmental conditions such as soil moisture and atmospheric evaporative demand. Climate change conditions often lead to reduced tree water use but during dry periods, the proportion of rainfall lost to the atmosphere often increases.

Simplified annual water budget

Q= P – ET

Where Q is the water yield available for human use, P is the precipitation and ET is evapotranspiration.

The interplay between rates of evapotranspiration, climatic conditions and plant functional responses to climate is summarised in the following diagram. In general, tree water use will decline in future climates as rainfall declines, CO2 increases, air temperature increases and humidity decreases.

Schematic diagram of relationships between tree water use and changing climatic conditions. Reproduced from Macinnis-Ng and Eamus 2009.

Schematic diagram of relationships between tree water use and changing climatic conditions. Reproduced from Macinnis-Ng and Eamus 2009.

What does this mean for catchment water budgets? In a semi-arid area of Australia, evapotranspiration accounted for around 90% of rainfall during a non-drought year, increasing to 98% of rainfall during a drought year. This leaves very little water for other water requirements of a catchment such as river flows or groundwater recharge because when rainfall declines, the proportion of rainfall used in evapotranspiration increases.

This type of scenario occurs in dry climates or during periods of low rainfall but the situation in New Zealand remains unquantified. Very few studies have investigated the seasonal and inter-annual variation of transpiration in our vegetation. Plants adapted to drought conserve water by closing their stomata (leaf pores) during dry periods but we do not know if native species have water saving strategies. Seedling trials suggest there is a wide range of physiological responses to drought ranging from water spenders to water ‘super-savers’. But field measurements on mature plants are required to determine the physiological responses to drought and the resulting effect on catchment water balances.

Our research is exploring the water relations of native trees and seedlings to identify the varying sensitivity of different species to drought and eventually determine the impact of climatic changes on forest water balances. Sap flow sensors are monitoring transpiration of kauri and associated trees 24 hours a day. Together with meteorological and soil moisture data, this information helps us understand the responses of trees to varying soil and atmospheric conditions. In addition to quantifying the forest hydrological cycle, our research will help us determine whether Northland forests are vulnerable to drought-induced forest dieback.


Dr Cate Macinnis-Ng is a Senior Research Fellow in Plant Ecophysiology in the School of Environment at the University of Auckland. You can follow her on Twitter @LoraxCate.

Impacts of climate change on water quality Waiology Dec 16

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By Daniel Collins

Un-muddying the Waters : Waiology : Oct-Dec 2013Climate change goes beyond warmer weather and more extreme floods and droughts. Effects are expected to include changes to the water quality of our rivers and lakes as well. This has implications for the vulnerability and sensitivity of freshwater ecosystems, as well as water quality limit-setting and catchment nutrient management. These issues, and more, were discussed at a Department of Conservation workshop held last week on the implications of climate change for freshwater conservation.

Middle-of-the-road projections of climate change indicate about a rise of about 2 degC in average temperatures across New Zealand by 2080-2099 compared with 1980-1999.

Middle-of-the-road projections of climate change indicate a rise of about 2°C in average temperatures across New Zealand by 2080-2099 compared with 1980-1999.

One of the most important aspects of water quality is temperature, and as the air warms so too will the water. New Zealand has already seen an increase in air temperature of about 1°C over the last century; over the next century, projected changes are closer to 2°C, depending on how global greenhouse gas emissions develop. Warmer waters would shift the range of freshwater fish south and higher in elevation, depending on the species. The ranges of some species may increase, while others may decline. For natives, this would be influenced further by shifts in the ranges of predator species such as trout. Warmer waters also make algal blooms more common.

Water temperature is also susceptible to changes in the flow regimes of rivers and streams. With more frequent or longer droughts over much of the country, particularly the east, we can expect more pronounced periods of low flow, at least the smaller catchments. These lower flows would mean that water temperatures climb even higher.

At the other extreme, the more intense rainfalls would lead to more intense erosion, delivering sediment to waterbodies – another important aspect of water quality. But we must be careful about extrapolating here. While the most intense storms are likely to become more intense, the less intense storms may not and there may be fewer of them. Studies of long-term erosion under climate change have thus been equivocal.

For nutrient such as nitrate (N) and phosphorus (P), there is little we can say at present, as a lot will depend on land use change. One study out of the University of Waikato warns that by the end of the century the warming of lakes could have a similar effect on trophic status as a 25-50% increase in catchment nutrient loads.

With rising sea levels of about 50-100 cm by 2100, lakes, rivers, wetlands and aquifers along the coast would experience an increase in salinity. Intermittently closed and open lakes would be open to the sea more often, and salt wedges and tidal influence in streams would reach further inland, both influencing aquatic habitat and biodiversity.

Waterborne diseases are also on researchers’ radars with an interactive map of projected impacts recently released by ESR in collaboration with other organisations. Changes in temperature and precipitation, floods and droughts, can influence the potential to contract these diseases for better or for worse.

But on top of climate change we also have land use change, and the two will act in concert. Some land use changes are themselves liable to result from climate change, whether for the purposes of mitigation (e.g., carbon-farming forests or smaller herds of methane-emitting ruminants) or adaptation (e.g., shifts in crop choice and management in response to water resource stress).

So what can we do about all of this? In some respects water quality is almost bound to change (temperature, salinity near the coast), in others it’s unclear (sediment). The first suite of options to consider, then, would be those with no regrets: riparian planting along streams, erosion control, no more than optimal fertiliser use, and so on. These are actions with benefits no matter how the climate changes. Another matter to consider, at some point in the future, is the setting of water quality and catchment load limits that account for the “likely effects of climate change”, as recommended by the Land and Water Forum‘s third report. And of particular relevance to DoC would be the development of conservation strategies that account for the biodiversity threats posed by degrading or shifting water quality in a warming world.


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

How to drought-proof New Zealand as droughts get worse Waiology May 03

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By Daniel Collins

For the most part, droughts are natural events. Rainfall and river flows wax and wane, and there will be times when there just isn’t enough water to fully meet our needs, whether to grow crops or to quench a city’s thirst.

Wairarapa drought, February 2013. (Credit: D. Allen, NIWA)

Wairarapa drought, February 2013. (Credit: D. Allen, NIWA)

And when it comes down to it, that’s really the best definition of a drought: when water supply is insufficient to meet demand. If no rain falls on the land, and there is no-one there to go thirsty, is it a problem?

But there is a growing part of drought that isn’t natural. Increases in water use, beyond the capacity of the environment to supply the water, have led to what are called “demand-driven droughts”. Changing climate has been implicated in changing patterns of drought around the world (e.g., Dai, 2013), and this is expected for New Zealand in the future.

So how can we adapt to more frequent and more severe droughts?

One option being discussed in New Zealand lately has been to increase the drought relief from the Government, but the Government has indicated that farmers will not be able to rely on this.

Another option is to build more water storage reservoirs and siphon off some winter river flows for use in spring and summer. So long as this does not increase dependency during times of plentiful rain and is reserved as a form of insurance as a drought approaches, this is a possible option, depending on economics and environmental impacts.

But we shouldn’t become fixated on just one or two strategies. There are many to choose from and it is likely that the best approach will be a balanced portfolio of options, tailored to specific needs and adaptive capacities.

Here is a longer list. For more information, read the recent Ministry for Primary Industries (MPI) report on adapting to climate change. No option is favoured over any other, and important economic considerations are beyond our scope.

Around the house. Install dual-flush toilets and don’t flush every time. Switch to a front-loading washing machine. Take marine-style showers: turn the water off when soaping up. Plant your garden with drought-resilient species. Collect rainwater from the roof and stop watering when shortages loom. Sweep the path with a broom, not a hose. Buy food with a smaller water footprint, relative to the growing region’s climate, and reduce food wastage.

Around the farm. Increase water use efficiency. Match crops and livestock (number and species) to the available water. Accumulate feed reserves if seasonal weather forecasts indicate, and start de-stocking before the feed runs out. Build on-farm water storage or collaborate in large-scale reservoirs. Schedule irrigation based on short-term weather forecasts and distribute it based on soil moisture and crop conditions. Buy drought insurance with profits from more productive years.

Around the business. Use materials and products with a smaller water footprint, relative to the producing region’s climate. Reduce the water footprint of the manufactured products, adding value in the process. Identify parts of the supply chain that are more or less drought-sensitive and build in contingency plans based on climate forecasts. Seek drought insurance and relief. Adopt relevant actions from around the house or farm.

Around the town. Encourage water conservation through education, incentives, penalties or user-charges. Reduce reticulation leakage. Landscape green spaces with drought-resilient plants and cease irrigation when shortages loom. Secure alternative sources of water and protect existing supplies. Monitor weather and climate forecasts, and phase in voluntary or compulsory restrictions in advance. Develop long-term plans for residential and industrial growth that can be balanced by future water supplies.

Around the region and country. Develop policies and plans that account for the foreseeable impacts of climate change. Adapt water quantity limits as climate change projections indicate. Encourage personal and industrial water conservation through education, incentives, penalties or user-charges. Provide financial and logistical support for costlier adaptation options. Encourage the use and development of weather forecasts, both short-term and seasonal, among water users. Identify and develop new sources of water (e.g., inter-catchment transfers; inter-seasonal storage). Quantify the available water and how this may change in the future. Foster land covers that have higher water yields, particularly during times of low flow.

In the end, the only sure-fire way to drought-proof New Zealand is to live within our climatic means. Being resilient to some drought, however, may not be so bad. Both would require us to tailor our water demand to the vagaries of the climate, develop land uses and societal practices attuned to the water cycle, and build in flexibility to ramp usage up or down as variable water supplies dictate.


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

Managing our freshwater resources in a changing climate Waiology Mar 22

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By Daniel Collins

WaterGovernanceWaiology2013While water management is challenging enough as it is, climate change makes it harder. No longer can we rely solely on experiences from the past to guide our actions, but we must also consider forecasts of the future. And with New Zealand’s water resources expected to change in the coming decades – well within resource management planning horizons – it would be prudent to start to adapt sooner than later. So how does climate change affect the ways water may be governed, and how are current governance systems placed to deal with climate change?

First up, let’s review the potential effects. As the climate changes, temperatures are expected to rise and rainfall patterns shift both regionally and seasonally. This would result in more flow in some rivers and in some months, and less in others. Similar effects can be expected for our groundwaters too. In much of the country, droughts are expected to become more severe (PDF). Paradoxically, so too are floods, in part as warmer air is capable of holding more water, leading to more intense storms. Water quality is expected to decline in some lakes, and erosion may increase, though there has been much less study on this. As for the implications of climate change for our aquatic ecosystems, the jury is basically out at this time due to limited research. And in general, gradual changes in freshwaters will be combined with increasing uncertainty about the future as we move further away from past conditions.

These potential impacts would have implications for the amount of water that can be abstracted, the amount of nutrients that can be discharged, the productivity of both agriculture and hydroelectricity, flood and drought risk, and the natural character of the landscape. This would affect the social, cultural, economic and environmental values of New Zealand’s freshwaters making the balancing act all the more difficult.

But the need to take heed of climate change when managing resources is being recognised, with climate change becoming part of major national governance documents. The Resource Management Act, amended in 2004, directs councils to have particular regard to [sic] the effects of climate change. The National Policy Statement for Freshwater Management 2011 (NPSFM) requires councils to have regard to the reasonably foreseeable impacts of climate change. Consequently, more specific references to climate change have been appearing in regional documents too, referring to both flooding hazards and water shortages (e.g., Taranaki Regional Policy Statement, section 7.2). And as councils respond to the relatively recent NPSFM, we can expect to see more consideration of climate change in future policies and plans.

Along similar lines, the non-governmental Land and Water Forum acknowledged the significance of “changing weather patterns” in its first report, which could be interpreted as climate change or as climate variability (e.g., ENSO, IPO). In its third report, the LWF recommended that water quality management take climate change into account (where the science vastly lags the policy aspirations), while water allocation (where the science of climate change impacts is clearest) only considered climate variability.

More recently, while the Government’s freshwater management proposals, Freshwater reform 2013 and beyond, do not mention climate change explicitly, they do acknowledge that…

“…future freshwater supplies may not be reliable, especially in the context of climate uncertainties.” (p.16)

So how can the Government, Regional Councils and other governing bodies (e.g., water user groups) take into account the “foreseeable impacts of climate change”? In various ways:

  • be prepared for less water and more droughts, more sensitive ecosystems, and an increase in extreme floods (see the MfE guidance on flooding), depending on the location. But also be prepared to take advantage of any opportunities (e.g., more river flow in winter);
  • accept and accommodate greater uncertainty in environmental limits and economic productivity;
  • adapt allocation limits, minimum flows and nutrient loads as the changing freshwater system dictates (preemptively or retrospectively);
  • foster resilience and robustness among water users and management regimes.

Putting these strategies into practice is going to take time, and regional councils and other governing bodies and advisory groups are taking the first steps on this journey. Progress will depend upon the science providing the necessary answers, and existing science being better translated into policy solutions. In either case, the science plays a fundamental role in assessing impacts and alternative interventions, and thus informing the governance process.


Collins, D.B.G.; Woods, R.A.; Rouse, H.; Duncan, M.; Snelder, T.; Cowie, B. (2012). Chapter 8. Water Resources. Water resource impacts and adaptation under climate change. In: Impacts of Climate Change on Land-based Sectors and Adaptation Options. Clark, A.J.; Nottage, R.A.C. (eds). Technical Report to the Sustainable Land Management and Climate Change Adaptation Technical Working Group, Ministry for Primary Industries, Wellington, pp 347 – 386.

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.

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.

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!

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.

New Zealand’s next top model Waiology Jul 20

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By Daniel Collins

When I see ads for New Zealand’s Next Top Model, I sometimes share a quiet chuckle with myself. I’m notorious for puns, I’m afraid, and the show’s name is ripe for the picking.

To many hydrologists, TOPMODEL is the name of a hydrological model developed by Keith Beven and Mike Kirkby in 1974. It describes how the wetness of ground in rolling hill country relates to upslope area and local slope (the “TOP” refers to topography). Back in 1997, my colleague, Ross Woods, and another of his colleagues visiting NZ at the time, took the idea of TOPMODEL and applied it across a river network. They christened the new model TopNet. TopNet has since become our primary catchment hydrology model, used for all sorts of studies across New Zealand, and as such it already is New Zealand’s next TOPMODEL.

Back in May I was filmed for the new season of ‘Ever Wondered?’ and I talked about TopNet and hydrological modelling in general. (You’ll have to wait until at least August to see the programme.) In a nutshell, TopNet is a computer model, comprised of umpteen mathematical equations, that simulates the movement of water through a landscape, from rain to river discharge. If you’re interested, I’ll explain my philosophical approach to modelling in another post.

As for its applications, we’ve used TopNet to make projections of river flow under climate change, and of flooding after land use change. We’ve used it to estimate the water balance of the regions, as seen in the Statistics NZ Water Stock Accounts mentioned previously. I am hoping that we can also use it to infer what rivers were like about the time when Polynesians arrived, and Europeans. It’s a great tool to help us understand the hydrology we have today, and how it can change.

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