About Rainfall Ranges
Purpose of the Rainfall Ranges
Throughout the year, it can be useful to compare the current total rainfall for the year, or season with historical totals of rainfall. It can also be useful, particularly at times such as autumn when seasonal forecast models show low predictability, to look at what history says about the typical range of rainfall values for the remainder of the year or season.
For example, it may be useful to know, in the context of making an informed farming decision, that there is a high chance that the total accumulation over an entire cropping season will be above average (median), even if the rainfall in the coming months is reasonably low (e.g., around the 10th percentile).
Similarly, if rainfall for the coming season is in the highest 10% of all past rainfall totals (i.e., 90th percentile, or very wet) is still not enough to lift the accumulated rain to above average then, again, management decisions can be made to allow for such an outcome.
Understanding the Graphs
The Rainfall Ranges graphs show accumulated rainfall to the end of the previous month as a blue line, starting at a specific month selected from the drop down menus.
To place this accumulation into an historical context, the previous year's rainfall accumulation is shown as a grey dashed line along with the historical range of accumulated values. This historical range is indicated by the 10th percentile (a one-in-ten-year low rainfall outcome), the 50th percentile or median, and the 90th percentile (a one-in-ten-year high rainfall outcome). The range between the 10th and 50th percentile is shaded brown. The range between the 50th and 90th percentile is shaded green.
The 90th percentile is wetter than nine tenths of all years, while for the 50th percentile it is one half of the years and for the 10th percentile it is only one tenth of the years. Another way of saying this is that the 10th percentile is drier than nine tenths of all years. Please see the accompanying example rainfall range graph.
The rainfall ranges that look ahead from the present are simply based upon the spread of past rainfall totals. These scenarios for future rainfall show what conditions may be if rainfall over the coming period is in the range from the 10th percentile (very dry past conditions: red dashed line) to the 90th percentile (very wet past conditions: green dashed line). Median / 50th percentile conditions are also shown (normal conditions: blue dashed line). These scenarios are added to the current accumulation to arrive at a range of rainfall possibilities.
Example Graph
The example graph for Tatura shows that rainfall during the previous year was in record low territory, with values easily within the lowest 10% of all records. The current season's rainfall accumulation is just below the median (normal) - far better than for the previous year. If very wet conditions occur it will still take Tatura a month to return to an above average accumulation for the year, however, if Tatura only receives average rainfall, it will remain below average until at least the end of the year.
The following points explain the details of the graph further, with numbers corresponding to those data points on the detailed example.
- The median line at November is simply the accumulation (in the example, rainfall from all past September to November periods) that sits in the middle of the full range of past rainfall accumulations for that set of months.
- The 90th percentile line at December is the accumulation (in the example, rainfall from all past September to December periods) that is wetter than 90% of past all rainfall accumulations for that set of months.
- The previous year's graph shows the accumulation throughout the same period for the previous year. In this example, the value at February is the accumulated total rainfall from September 2006 to February 2007.
- This year's rainfall line shows the accumulation of rainfall to the end of last month. In this case, it is the accumulated rainfall total for the period September 2007 to February 2008.
- The "very dry" scenario takes the current year's accumulation (in this case September 2007 to February 2008) and adds it to the 10th percentile accumulation total (i.e., the historical accumulation that is only wetter than 10% of all past accumulations) for the subsequent period (in this case, calculated from all past March to June periods).
- The "very wet" scenario takes the current year's accumulation (in this case September 2007 to February 2008) and adds it to the 90th percentile accumulation total (i.e., the historical accumulation that is wetter than 90% of all past accumulations) for the subsequent period (in this case, calculated from all past March to July periods).
Please Note
It is important to note that the scenarios are not a forecast or an outlook. They are simply a range of possible values based upon long term rainfall records, with no one scenario favoured over another. A seasonal outlook would aim to use current climatic conditions, such as whether or not there was an El Niño in progress, to narrow the range of possibilities by providing the likelihood of each scenario occurring.
Not all Bureau rainfall stations are included in the list of Rainfall Ranges locations. At present, only those stations with at least 20 years of continuous data (i.e., no periods of missing data) up to the previous month are considered for calculating the Rainfall Ranges, as any shorter length of data will poorly represent the range of long term rainfall values. As new data comes to hand, it will automatically be included in the Rainfall Ranges. Further work is planned to fill the gaps in the data so even more sites will have a 20 year continuous record.
