Archive July 2011

Crowd-sourcing for snow depth data Waiology Jul 28


By Ross Woods

On Monday morning I posted an album of photos of the snow near my house on my Facebook page, including shots like this one:


My son, who likes to tease me about spending too much time working, then asked me ‘what are your predictions for gutter flow rates due to snow melt in the coming days …’

This got me thinking about how to find out quickly how much snow there was around Christchurch, and then I had the idea of using networks on Facebook and at NIWA to get some crowd-sourced data on snow depth. At noon on Monday I created an open Facebook group called ‘How Deep is the Snow at Your Place? (July 2011, NZ)’, and offered some guidance on how many measurements to take (ten) and how to choose a good spot (away from fences, buildings, valleys). I circulated the same message at NIWA in Christchurch. Most of my replies came in within the first 3 hours.

I got 44 responses from Facebook and NIWA, from as far north as Wellington (‘no snow’) and as far south as Dunedin. I also got a nice report from an experienced hydrologist who had driven from Picton back to Christchurch, and taken note of how deep the snow was at quite a few locations. The most common snow depths were in the range 10-15 cm, and the statistical distribution of snow depths looks like this:


Where was the deepest snow? I couldn’t see any particular pattern in the snow depth data: there’s a map of the data points on the Google Maps page I quickly put together. The page is still under development — the colour of the placemarkers has been giving me trouble but I’ve nearly got it.

In the meantime, the Terra satellite was capturing some nice imagery on where the snow was, and NIWA staff were out doing a scientific survey of snow depth and density — more of that to come later…


New Zealand’s next top model Waiology Jul 20


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.

Nature, at your service Waiology Jul 13

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By Daniel Collins

Humans have depended on nature since we were knee-high to Australopithecus afarensis. Our ancestors drank river water, harvested berries from bushes and took refuge beneath trees. Today, we drink tap water, buy groceries from supermarkets and build houses out of trees. Over the two million intervening years, humans have increasingly co-opted and engineered natural ecosystems to meet their evolving needs (e.g., agriculture and hydropower), but we are still very much dependent on ecosystem services.

Ecosystem services are the direct and indirect benefits that we receive from ecosystems. They include the provision of food, water and minerals, regulation of natural hazards or pollination, support of nutrient cycling and seed dispersal, and the cultural services behind recreation and spiritual values.

Identifying an area’s ecosystem services is becoming a useful means of recognising and meeting competing environmental needs, and is the focus of the Royal Society of New Zealand’s recent issue paper (see the SMC press briefing here). The paper brings together scientists from around NZ to lay out what ecosystem services are all about and how they’re being incorporated into environmental management and research. As a concrete example, they draw on a Lincoln University study of the effects of the Opuha Dam in Canterbury (PDF).

An important point of the Opuha Dam study is that, over the 10 years since the dam was built, the ecosystem services provided by the Opihi River have changed. Water supply and flow regulation have tangibly improved, as intended. And while other ecosystem services can be identified — provision of salmon, erosion control, perceived beauty, mauri[1], etc — they are much harder to quantify. In fact, for the Opihi River, it is unclear whether many of the ecosystem services have improved or degraded. What is clear, though, is that by simply listing an area’s ecosystem services we can build a richer and collective picture of the various values that different parts of our society place on that area.

As Waiology is a hydrology blog, I want to highlight those ecosystem goods and services that relate specifically to the amount and movement of water. The RSNZ paper and the original report list several associated with the Opihi:

  • water supply for irrigation, drinking, industry and livestock;
  • hydroelectricity generation;
  • flow regulation (minimum flows, flushing flows);
  • flood and drought protection;
  • habitat for salmon, trout, mahika kai[2], flax and invasive plants; and
  • flow characteristics that underpin river bank stabilisation, boating, swimming, beauty and natural character.

Not surprisingly, there are few ecosystem services affected by reservoirs that are fully outside the domain of hydrology. I’ll elaborate on nature’s hydrological services in the near future.

In the end, ecosystem services offer a framework for examining resource use and environmental change. While the framework is not fully operational yet, it does help us to better understand how we use and modify natural resources.


[1] Mauri = in Maori custom, the life force unique to a resource that includes both physical and spiritual attributes.
[2] Mahika kai or mahinga kai = food and resources gathering area (in te reo Maori).

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