Thursday, July 26, 2012

Drought vs Heat

When you read the news these days about US agriculture, the headlines generally start with "US Drought."  My guess is that when a reporter sees a wilted plant, the first reaction is that precipitation must have been below normal.

Below is a graph that plots cumulative precipitation for the areas where corn is grown in the US. Similar to the previous post about extreme heat, the red line is for 2012, while the grey lines give the historic data for 1960-2011.

Precipitation has been below normal this year, although, it isn't at a new record low.  So what is more important, extreme heat or lack of precipitation? Well, both matter, and they usually occur in sync, as it only gets very hot when it is dry. When it is wet, evaporation results in cooling (if you have ever been to Arizona, you might have seen the mist sprays that keep outdoor seating at restaurants cooler).

In the debate about extreme heat versus low precipitation, I'd put my money on extreme heat, as it has been a much better predictor in statistical studies. Why does it matter? Well, pretty much all climate models predict an increase in temperature, but the effect on precipitation is more debated.

Drought is a relative concept. Here's a (bad) example what I mean: on some days I force my old body to go for a jog. If you ever run, you might have recently seen a warning that you need to drink more because it is hot. Well, crops aren't that different: if it gets hot, their water requirement is going up. So even if precipitation is at the historic average, hotter temperatures might give you wilted plants as the historic average precipitation is not enough for the increased water requirement of the plant.  To stick with the previous example: it is like running in really hot weather and drinking as much as you usually do, which will get you dehydrated.

So what is going on this year? Well, corn is getting the double-whammy. Remember the 2008 Chicago Marathon. It was very hot that day, and some water stations closed because they ran out of water - it wasn't a good day to run: in the end one person died and 250 were hospitalized.

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