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What rain information should we believe?

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Prime Time Specialty Mini Grid WIDGET

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POSTED: March 3, 2013 2:19 p.m.

OK, we got some rain recently, so now you are out there thinking the drought is over and you can go back to doing what you want.
I only wish it was that easy. I checked the Palmer Drought Index for the Southeast ending Feb. 16, and it showed Georgia all in gray, which means normal moisture. Then, for the same region on the same website, I clicked on the Crop Moisture Index and it showed North Georgia as wet, Coastal Georgia as abnormally moist (one notch drier than wet) and the rest of Georgia as excessively wet for the same day. Then I clicked the link to the U.S. Drought Monitor for Feb. 19, and it showed Coastal and Central Georgia in extreme drought, just one notch down from exceptional.
So what gives? Who do we believe? Now the fun starts.
All three of these charts are correct. Each of these moisture maps are assembled for different purposes. Just like a half-inch wrench is the right wrench for a half-inch bolt head, each one of these maps is right when it is applied to the question it is designed to solve. Now try using that same half inch wrench on a 9/16 bolt. It just doesn’t work; it is not the appropriate tool for the job. There is nothing wrong with the half-inch wrench. It still is a perfectly good tool. We just have a mechanic selecting the wrong tool for the job. Unless one selects the right tool, one will not make beneficial progress and will wind up with skinned knuckles and high blood pressure.
The Crop Moisture Index looks at how much water is in the plow layer right now. It is a good reflection of the past week’s rainfall. Just the sort of thing you want to know if you are down to picking a date to put your tractors in the field planting, or later in the year to see what the yield prospects will be for, say, corn harvest. Check moisture levels at critical water-demand times for crops in the field.
The Palmer Drought Index is a longer-term measure of available moisture. It includes rainfall but also factors in temperature and soil moisture recharge to get at whether there is adequate moisture in the soil-surface plow layer as well as the deeper subsoil that can recharge topsoil somewhat and can be tapped by deep rooted crops like corn, alfalfa and forages.
The PDI is good for year-to-year planning by farmers on a county-by-county basis, but how does one advise elected officials and the money folks like Department of Agriculture and the Internal Revenue Service about the intensity of drought being suffered by farmers? Enter the U.S. Drought Monitor. Let’s compare the Feb. 19 USDM to PDI for Feb. 16 — the latest available maps at the time I wrote this. Both agree that the central plain states are in extreme, exceptional droughts. We will learn the difference between the two by looking at where they seem to disagree. The PDI shows Louisiana, Mississippi and southern Alabama to be extremely moist to very moist over much of the states. The USDM shows these areas to be normal in moisture. My interpretation is that the PDI shows these areas have recently gotten a lot of moisture. The USDM says that moisture will be enough to recharge subsoil moisture and break the drought or weaken it in the portions of the states affected.
The same maps show all of Georgia to be at normal moisture on the PDI but in the USDM, it shows everything south of Atlanta to be in moderate-to-extreme drought. My interpretation is that we recently have received good but not excessive rainfall across the state. Yes, a few isolated spots may have received a deluge, but the interstates still are open through Macon, so it cannot be all that bad. The USDM shows that the rain we have received has not been enough to recharge plow layer or subsoil enough to meet moisture needs for crops and cattle.
Like the five blind Indians touching a different part of an elephant, a single picture does not tell the story. But a number of them together can make an educational movie. Watch out. There are a lot of folk out there telling you one part of the truth but neither all the truth nor the whole truth. Do your homework.

Gardner lives in Keller and is the UGA extension agent for Glynn County, serving South Bryan.

 

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