Posts Tagged precipitation

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.

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.

Snow doesn’t stop hydrologists Waiology Aug 22

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By Daniel Collins

With the sun shining and last week’s snow all but gone in Christchurch, it’s hard to imagine that the city almost ground to a halt. But not the hydrologists. While Ross was collecting snow in a saucepan, two other NIWA colleagues – Chris Appleby and Jeremy Rutherford – were out in the field doing more rigorous measurements, and far more of them. TV1 News caught them in the act.

They were making hundreds of measurements, recording how snow depth and density varied in and around Christchurch. As Jeremy put it, “snow is notoriously spatially variable”, so to get a decent picture of how much snow fell, measurements had to be made in as many locations as possible.

The one density measurement mentioned on the news clip was of some snow comprising 23% liquid water, the rest being air. Ross’s measurement from his garden was 14%.

As it turned out, many readers of The Press were also making measurements last week, at the paper’s request. A map of snow depth was printed in the paper’s Wednesday edition (17 August; alas, it’s not online). A high of 30 cm was reported in Cashmere and Huntsbury along the Port Hills, a low of 13 cm in Shirley and Burnside, while most of Christchurch were in the upper teens and low twenties. A nice example of citizen science.

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…


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