I took that historical chart, converted it to a CSV file, then plotted the 5 years surrounding the peak. The graph below is what I got. It shows that things were very noisy then and that we had a situation very much like the last 24 months, some months up, some down, but no real clear trend til a few years had passed.




Click on the image to see it full sized. I didn't want the graph to blow up the entire page formatting. I've saved the image at flickr. Hopefully I've got the URL right for others to see it.

At a guess, it's going to be a lot sharper for fields that have been doing water flooding for a while.
GZ, thanks. This is exactly what I was looking for.  Interesting.