September 13 – How dry I am

Today’s factismal: The rainiest part of the year in Arizona are the months of July and August; the driest is the month of June.

We’ve all seen the Western movie where the evil land baron dams up the river so that the poor hardworking farmers can’t water their crops (don’t ask me which movie; it has been remade under a thousand different titles). But what few of us have asked (because it was, after all, only a movie) is “How realistic is that?” Could you actually hold up enough water to dry out a farm? Or would Mother nature rain all over your evil plans?

Average monthly rainfall for two Arizona cities (Data from NOAA)

Average monthly rainfall for two Arizona cities
(Data from NOAA)

For a change, it turns out that the movies aren’t lying to us; the pattern of rainfall in Arizona and other parts of the West are such that you pretty much have to build a dam to catch the water when it rains and hold it to keep you going through the dry part of the year. That’s because most of the West is a desert or near-desert that only gets about a foot of water each year. And that foot is far from evenly distributed. From March through June (that’s planting season to you city slickers), Arizona is at its driest, with only about 1/4 inch of rain a month. Then July and August get more than two inches of rain in each month before things slide down to about 3/4 inch of rain for the rest of the year. Because the rain arrives only after your crops have turned to dust, a farmer in that region would need to have some way of storing the water from the late summer to use the next spring (or live right next to a river).

A map of Arizona's annual rainfall averages (Image courtesy NOAA)

A map of Arizona’s annual rainfall averages
(Image courtesy NOAA)

Of course, though we know a lot about the rainfall in Arizona in gross, there is still a lot to learn about the fine details. And that’s why the Arizona Department of Water Resources, The US Bureau of the Interior, and several other groups have come together to create Rainlog. They are looking for locals to host a rain gauge and report their data so that the folks in government can do a better job of using what rain does come Arizona’s way. If you’d like to help, drop in on them at:

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