Example 1 - How Rare is the Current Drought?

This question is raised for reasons varying from curiosity to applying for disaster assistance. It is also asked in conjunction with the question, "How likely is it that the drought will end in the next X months?"  The Atlas can provide information to help answer the question. For example, consider a drought which started in June and has been in progress for two months in central Alabama. During these two months, only 45% of the average June-July precipitation fell. How unusual is this?   The first step is to look at the cluster map to determine which Atlas cluster should be used in the analysis. In this case, it is pretty clear that the area is represented by Cluster 105. In some cases it will not be that clear, and you will have to make a professional judgment. To get those Atlas statistics, download the Excel file for Cluster 105. You'll see tabs at the bottom of the screen for the four worksheets included in this file: Interface, Quantiles, Stationmeans, and Graph. Every Atlas precipitation spreadsheet has the same structure.  On the "Interface" sheet, select "June" as the starting month and "2" months as the duration. You'll notice that the spreadsheet returns specific precipitation depths for the particular precipitation station chosen, but in this example, you are using regional data (the 45% calculation), so the individual station statistics are not relevant. Click on "Quantile" and you will see a table of ratios that apply throughout the cluster area (Central Alabama, West Central Georgia, Central Tennessee, and South Central Mississippi). Near the top of the page, notice two rows in the table reproduced below:

The top row is the "non-exceedance probability" and the second row is the ratio of the precipitation for the non-exceedance probability in that column to average precipitation. Remembering that you selected a 2 month duration starting in June on the "Interface" page, this table tells you that there is a 5% chance that June-July precipitation will not exceed 0.419 of average precipitation for June-July. Some people would invert the 5% and say that the region was experiencing a 20 year drought if it were to receive 0.419 of average precipitation in the June-July period. You can see there is a 10% chance of getting at least 0.526 of average precipitation for the period. Since the region received 45% in this example, you can answer the question "How rate is the current drought" by saying: the odds of having a June and July this dry in central Alabama are a little more than 5% (or) we can expect a drought like this at least every 10 years on average (or) (interpolating graphically) we can expect a drought like this about every 15 years on average If you find the "once every 10 years" expression makes more sense to people, remind them that this does not mean that the drought occurs on a 10 year cycle, just that if you examined precipitation records for a very long period of time, about one in 10 of those years would have less June-July precipitation. Another dimension of drought is streamflow, which is of special importance from the standpoint of reservoir inflow, wastewater effluent dilution, and fish survival. It is usually the case that the frequency of occurrence of streamflow is not the same as the frequency of occurrence of the precipitation that generated the streamflow. It could be more or less frequent. Streamflow frequency is approached in essentially the same manner as precipitation. Use the streamflow regions selection map, and download the appropriate regional spreadsheet. But note that there is no regional analysis of streamflow stations, and you must compare current data for a station listed in the Atlas spreadsheet to the statistics for that station. Considering that watersheds above gauging stations have different drainage areas and land use, the frequency of occurrence of flows at nearby stations might be quite different. Return to Main Page, National Drought Atlas Precipitation Streamflow.

National Drought Atlas

Precipitation

Streamflow

revised 1 Aug 2006

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Precipitation

Streamflow