Saturday, 12 January 2008

Correlation between Rainfall and Elevation

Six months or so ago I analysed the data from the three climate monitoring sites closest to and surrounding Ochre Arch. These locations are Forbes, Quandialla and Grenfell. One observation was that the mean annual rainfall at Grenfell (elevation of 410 metres above sea level) was 602 mm whilst Forbes (elevation 240 metres) was 503 mm and Quandialla (elevation 250 metres) was 521 mm. Given that the elevation at various points on Ochre Arch increases by approximately 100 metres (from 315 metres near the house to 415 metres at Lookout Rock) it struck me that there was a reasonable prospect that the rainfall on our place is greater at the highest point and that the order of magnitude would be around 20 %. We now have rain gauges near both the house and Lookout Rock and we are finding that we consistently measure 20 % more rain near Lookout Rock – especially in instances where the rainfall in the area is general rather than narrow storms and such like.

Following on from my previous post looking at temperature and elevation I decided to use the climate data from the locations in and around the landholder sites to see what the correlation was between annual rainfall and elevation.

In the accompanying graph you can see the results. The vertical axis represents both elevation in metres above sea level and mean annual rainfall in millimetres. The horizontal axis shows each of the locations – sorted by ascending elevation from left to right.

Whilst it would not be appropriate to conclude that what occurs in these locations could be expected to be experienced in locations in all regions the graph does show rainfall continuing to increase with elevation increase. At lower elevations the annual rainfall is a much higher multiple of elevation (roughly 3 times) whilst at the higher elevations the rainfall is a much lower multiple (roughly 1 to 1 at Crookwell). This suggests that the trend is likely to reverse at a certain point and a quick look at the data Guyra (elevation 1332 metres, annual rainfall 926 metres) tends to confirm this – although Guyra is in a totally different region.

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