Tom's Blog

Monday, January 16, 2012

Variable precipitation data; southern Wisconsin

The lack of snow this winter got me thinking about how variable our precipitation is in southern Wisconsin. I obtained from the State Climatology office data for our area that permitted me to study this variability in some detail.

The data I used came from weather stations nearby. For the years 1995 through 2001 the data were from Blue Mounds, and (after that station was closed) from Mazomanie from 2002 to 2011. These are both close enough to Pleasant Valley Conservancy to be reasonably useful, although some of those heavy summer rains are often spotty. (This year we had one 1.5 inch rain whereas neighbors less than a mile away had less than an inch.)

The data were provided as an Excel file with monthly averages. Note that the snow is converted into liquid water equivalents. Thus, I have a table with data for each month of the year for the years 1995 through 2011. Even this is too much data to present on this blog, so I have reduced it further. (Daily data are also available but these are much too detailed to handle easily.)

I'd be happy to send the big Excel file to anyone who is interested. For this post, I am using the month-by-month annual averages, and the maximums and minimums for each month.

As the graph shows, there is marked year to year variability. The 17-year average of 35 inches rarely happens, and the range is from a low of 24 inches (in 2005) to a high of 44 inches (in 1998). The years 2001 through 2005 constitute a dry period, and the years 2006-2010 a wet period.

However, if you look at the monthly data, you find that some years there may be a single month with a very high or very low total. For instance, even though everyone remembers the huge rainstorms of June of 2008, this was not the extreme high, which was May 2004, even though the rest of 2004 was fairly dry.

The table below gives average data for each month, with both the maximum and minimum for that month, and the year that extreme occurred.

If you look at how the annual precipitation is spread out monthly, Wisconsin can be divided into three separate periods. The period of lowest precipitation is winter, Dec/Jan/Feb/and part of Mar, with about 18% of the total. The months of Apr through Aug constitute the bulk of the precipitation, 61%, and Sep Oct and Nov about 21%. When you consider this breakdown in relation to temperature, it turns out that the months with the highest temperatures are also the months with the most precipitation. This, of course, is our growing season. However, other parts of the United States are quite different. Most of California get almost no rain during the hot summer, but lots during the winter, when temperatures are lowest, which explains why California agriculture is based primarily on irrigation.

Although these monthly figures are useful, it should be remembered that there have been months when the data have been much different. Take a look at the max and min values below.

Month 17-year average Maximum Year Minimum Year
Jan 1.22 inches 2.7 (1996) 0.23 (2004)
Feb 1.483 2.58 (2008) 0.01 (1995)
Mar 2.041 5.54 (1998) 0.34 (2001)
Apr 3.96 7.05 (2006) 1.49 (2003)
May 4.43 11.5 (2004) 1.36 (2007)
Jun 5.26 9.8 (2010) 1 (1995)
Jul 3.79 6.71 (2010) 1.19 (2001)
Aug 4.15 14.9 (2007) 1.36 (2008)
Sep 2.88 6.7 (2001) 0.9 (2004)
Oct 2.55 5.47 (1995) 0.82 (2000)
Nov 2.167 8.1 (2003) 0.22 (2007)
Dec 1.55 3.28 (2007) 0.28 (1998)
Annual 35.483 44.35 (1998) 24.35 (2005)

As the table show, although June of 2010 was very wet, in 1995 June was very dry. In fact, there have been summer periods as long as 4 weeks when it did not rain at all.

In a discussion of Indian grass and little bluestem in an earlier post, I discussed our observations that the former grass is favored by wet summers and the latter by dry summers. During that dry period centered on 2005, the large Indian grass stand in the prairie remnant called Unit 4 was virtually replaced by little bluestem. In the wet period that we have had since 2007 the reverse is the case, and Indian grass is now dominant, as it is on major parts of the south-facing slope.

These precipitation data obviously have phenological implications.


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