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.