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Why we changed the rainfall data source |
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For two years we used the rainfall from the Weetangera weather station to compare monthly rainfall on The Pinnacle from month to month and year to year. However, following some large discrepancies between the Weetangera data and other observations in 2013, we carried out a search for alternative sources of rainfall that would be representative of The Pinnacle Nature Reserve. An analysis of these alternative data sources against the Weetangera data was then performed, as summarised below, confirming that (a) the Weetangera weather station had a consistent low measurement bias, and (b) that bias has been increasing with time. The weather stations chosen as alternative sources were the three Bureau of Meteorology rainfall stations closest the The Pinnacle, namely Aranda, Huntly and Melba (see details below). When monthly rainfall data from these three sites for the last 4 years were regressed against each other, they showed virtually a 1:1 relationship, as seen in the following graph. It was therefore concluded that the average of these three provided a reasonable representation of rainfall in the area between them.
When the monthly rainfall from the Weetangera weather station was regressed against the composite average (average of Aranda, Huntly and Melba), the slope was considerably less than 1, namely 0.85, as shown in the graph below. In other words, on average the Weetangera weather station was underestimating rainfall by 15% compared with the composite average.
In addition, by graphing the ratio of the monthly rainfall at Weetangera to the composite average monthly rainfall against time, a regression showed a steady decline in this ratio of 0.47% per month, or more than 2% over a 4 year period.
Source of Composite Rainfall Data The composite rainfall data summarised in the chart and tables above are the averages of data from the three Bureau of Meteorology Rainfall stations nearest to The Pinnacle (shown on the map below).
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