Just as a quick look at the data provided from CSIRO for the Drought Exceptional Circumstances Report, I made density plots (frequency histograms) for the rainfall data over two periods, 1900-2010 and 2010-2040 for the South-west of Western Australia, the area with the highest drought predictions. The plot below is the result, with the past in blue and the future in red. You can see the extremely skewed distribution of the data, with a large number of zeros (no drought areas). The future data (red) has a bump at the right hand end of the plot indicating a higher frequency of 100% drought declared areas than the past data.

The question in: What is the appropriate test of the difference between these two populations?

Here is the R code. You will have to delete the summary rows at the bottom and save a file as csv, until I develop a script for processing all the data files in native format.

d< -read.table("RSWWA.csv",header=T,sep=",")
pr<-as.matrix(d[1:110,2:14])
dim(pr)<-NULL
fr<-as.matrix(d[111:141,2:14])
dim(fr)red")
lines(density(pr),col="blue")