With the help of a local historian and two inveterate weather observers, rainfall on the Sonoita Plains has been recorded since 1857.

The collecting of rainfall data, as well as other climate recordings, was initiated in 1826 by the U.S. Army Surgeon General, who established the Army Surgeon General Climate Network “to question whether in a series of years there be any material change in the climate of a given district of country; and if so, how far it depends upon cultivation of the soil, density of population, etc.” Each Army Fort surgeon was provided with a Smithsonian brass cylinder, rain and stick thermometers, and forms to be sent back to headquarters.

Weather reporting in the Sonoita area began in 1857 with Bernard Irwin, the Fort Buchanan Surgeon. He filled out the meteorological register each day recording dry and wet bulb thermometer, rainfall and miscellaneous observations (“Northern Lights! Oak leaves falling in April!”) Using Irwin’s records, I compiled rainfall data from Fort Buchanan from August 1857 to April 1861 and from Camp Crittenden from April 1868 to Jan 1873. The NOAA “Fort Project” also provided rainfall data from Fort Huachuca from 1886 to 1893, and then from 1900-1917, and 1955-1981. Three other observers have contributed data. The Ewing family at the Canelo Hill Ranch, near the Canelo Ranger Station, started keeping rainfall data in 1910 and continued through 1983. The Ewings were official Cooperative Observers for the U.S. Weather Service (USWS) and their monthly records are preserved in the USWS archives. Stan Lee Sims, who lived in Papago Springs, took daily precipitation records from 1963 to 2002. Paul Thornburg has maintained observations to the present.

Although we all know the monsoon rains can drop an inch in one area but leave the adjacent area dry, comparisons of observations from different points reveal that rainfall amounts ultimately even out. Rainfall at Canelo and Sonoita, about 25 miles apart, is remarkably similar. The long-term average for the Sonoita plains is 17.57” with no linear trend showing an increase or decrease over the years.

Rainfall comes both in the winter rains (October-March) and the monsoons (April-September). When the data are analyzed, it is clear that winter rains vary considerably from year to year, and the contribution of monsoon rains relative to total rainfall has increased.

I was able to find a total of 11 sources of rainfall data scattered over the Sonoita Plains. When I had more than one observation for a yearly total, I used the median values. Barbara Thomson, University of Toronto, provided the statistical analysis showing that a line fitted through all the data points has a very low chance of varying from dead level. Although the annual total has not varied over the past 150 years, there are significant cycles of wet and dry years.

The amount of summer and winter rains varies each year, as shown on this graph spanning the years 1858 – 2008.

It is very hard to predict the rainfall in any given year, but one can probably be about right if predicting the long-term average of about 17-18”. In the next decade, several models predict more rain in the monsoons, as sea temperatures are already higher than the long-term average, thus making more moisture available to the warm season tropical storms that can blow up from the Gulf of Mexico. We will see.