Texas State Climatologist: “It may well be the worst drought on record for agriculture” and “probably … the most unbearable”

John Nielsen-Gammon is the Texas State Climatologist and a Professor of Atmospheric Sciences at Texas A&M University. What follows is analysis first posted on his Houston Chronicle blog.

Texas Drought: A Fingerprint

Drought has a variety of impacts. Some droughts can hit agriculture hard. Others affect water supplies. Others affect fire danger. Most affect all three, to varying extents. The length and timing of the drought relative to such things as the growing season determines the impact profile.

I like to compare drought intensities through a type of diagram I created. (Maybe someone else created it before me, but at least I created it independently.) I call it the Drought Fingerprint Diagram. It’s drawn relative to a particular month of the year. Here, we’ll talk about the July diagram.

The x-axis is the number of months of accumulated precipitation. Suppose the precipitation in July 1993 at a particular location was 2.00 inches. Then the one-month data point for 1993 would be at 2.00″. If June 1993 brought 1.50″ of precipitation, the two-month accumulation would be a total of 3.50″, so that’s the value plotted at two months. Another 3.00″ of rain back in May means that the three-month total is 6.50″, and that’s what’s plotted at three months. And so on, as far back as you’d like to go.

All the lines slope upward to the right, because each point adds more precipitation from an older month to the current total.

Here’s such a diagram for Texas for the period 1898-2010:

Texas Drought Fingerprints, 1898-2010

There are 114 lines, one for each July in the period. Some of the more extreme lines are labeled. For example, the driest 4-month period ending in July took place in 1998. Other prominent drought years are 1917, 1956, 1925, and 1918. Conversely, it was very wet leading up to and including July 2007, 1941, and 1992.

Okay, it’s time to play a game we like to call “predict the likelihood of an extreme event”. Based on historical data such as this, civil engineers all the time have to come up with the probability of an extremely rare event occurring.

In practice, they do this by fitting a probability distribution to the data, meaning they figure out which bell curve (for example) best fits the distribution of the data. With the curve in hand, they can directly calculate the probability of any event occurring. In principle, they can do this even for events that lie outside the envelope of past events, though the farther you get from the envelope the less accurately you can estimate the probability.

So we’ll do it the simple way: by eye. Ready?

Okay, look at the graph and estimate for yourself the accumulation over 7 months that would correspond to a 1 in 100 dry event, that is, an event with a probability of 1%.

Since we’ve got about a hundred lines plotted, the simplest approach would be to take the lowest value among the data already plotted. Since 113 out of 114 events were wetter than that in the past, it’s reasonable to guess that about 99 out of 100 future events will also be wetter.

Maybe you’re already beyond that point. Maybe you’re looking at the graph and seeing that the number of dry outliers (graphs that lie well below the general cluster) is quite small, especially for the period 5-10 months. Meanwhile, at the high precipitation end, the spacing between the lines tends to spread out more gradually. Maybe you’re thinking that we’re overdue for a dry outlier.

Maybe you’d like to take climate change into account. Precipitation in Texas has been increasing over the long-term, and you can see that the highest graphs tend to be more recent than the lowest graphs. That would make a dry event less likely. Conversely, climate models project a slow decrease in precipitation: is a dry event more likely?

Okay, that’s fine for a 1 in 100 event. What about a 1 in 200 year event? 1 in 500?

Here’s the same graph again, except I’ve added in the 2011 data.

Drought Fingerprints, 1898-2011


The year-to-date precipitation (7-month accumulation period) for 2011 is about 6 inches. The next lowest value is about 9 inches, and most curves seem to be concentrated around the 15-20 inch mark.

2011 is not the low mark just at 7 months. It also takes the record for the following durations ending in July: 2 months, 3 months, 5 months, 6 months, 8 months, 9 months, 10 months, 11 months, and 12 months. Everything from 5 to 10 months breaks the record by a large margin. In fact, everything from 8 to 10 months breaks the record for consecutive months, no matter the ending month.

So we now have a dry outlier that’s even more an outlier than the wet outlier of 2007. What are the odds of such an event occurring? I had them at about 1 in 250. What about you?

Whatever the exact value, it’s obvious that Texas is living through a very unusual weather event. I don’t consider it to be the worst drought on record, because the 1950s drought lasted for seven years, and 1956 alone gives 2011 a run for its money. But, combine it with July being the warmest month on record for Texas, and it probably becomes the most unbearable. It may well be the worst drought on record for agriculture.

John Nielsen-Gammon is the Texas State Climatologist and a Professor of Atmospheric Sciences at Texas A&M University (but the opinions he expresses here are solely his own and are not intended to represent those of Texas A&M

Below are old comments from the earlier Facebook commenting system:

The Daily Impact has something of a scoop this morning on another effect of the spreading drought and heat — a national epidemic of toxic algae blooms.

4 · Like · Reply · Subscribe · August 8 at 10:51am

Mike twotwo · Top Commenter (signed in using Hotmail)

Texas in 2011 is a great opportunity to study tree mortality thresholds. There is a lot of forest here in the US that could see these conditions toward the end of this century. Without water, the trees cannot cool themselves, and at somewhere around 115 deg F the tissue just dies–but there is very little literature on the topic. The number of reports of tree death from Texas are growing.

3 · Like · Reply · Subscribe · August 8 at 10:31am

Colorado Bob · Top Commenter

Mike …. apricots , magnolia , sycamores are dying all around me.

1 · Like · Reply · August 8 at 2:01pm

  • Mike twotwo · Top Commenter (signed in using Hotmail)

Reports are that the cedars (scrub juniper?) are dying also. I do not envy you.

1 · Like · Reply · August 8 at 7:20pm

Prokaryotes – · Top Commenter (signed in using Hotmail)

As long Texas can keep up it’s electrical infrastructure (air conditioning) nothing will change. But this will change in coming blackouts..

2 · Like · Reply · Subscribe · August 8 at 2:18pm

Colorado Bob · Top Commenter

Texas drought will harm wildlife habitat for years.
CANADIAN — In a muddy pile of sand where a pond once flowed in the Texas Panhandle, dead fish, their flesh already decayed and feasted on by maggots, lie with their mouths open. Nearby, deer munch on the equivalent of vegetative junk food and wild turkeys nibble on red harvester ants — certainly not their first choice for lunch.

As the state struggles with the worst one-year drought in its history, entire ecosystems, from the smallest insects to the largest predators, are struggling for survival. The foundations of their habitats — rivers, springs, creeks, streams and lakes — have turned into dry sand, wet mud, trickling springs or, in the best case, large puddles.

1 · Like · Reply · Subscribe · August 8 at 2:04pm

Jean Mcmahon

I guess I am the only person who thinks the state climatologist should be the chief activist to try to warn and educate the public of Global warming..Gary Mcmanus , Oklahoma’s climatologist, put out a great report years ago, but no one really got it.Lately he spoke of el nino, la Nina as poss causes for the heat danger Oklahoma has been living thru.I think we should organize against these climatologists…I think Mcmanus may not have a climatology degree..How come Climate Progress lets them get away with Murder?

1 · Like · Reply · Subscribe · August 8 at 4:12pm

Derek Ryter · University of Oregon

Good idea. Have you thought of using growing-degree days or potential ET along with PPT? I was looking at building a map time series showing the water balance (potential ET – PPT) for counties in ag states to show the shifts of drought from Kansas/Nebraska to Texas/OK and back again, and relative intensity. Of course, lower PPT wouldn’t be such a problem if it weren’t so hot.

1 · Like · Reply · Subscribe · August 8 at 9:14am

Paul Magnus · Top Commenter

July was also the warmest month on record for DC.
Why isn’t anyone getting out there and shouting this fact down inhofe’s throat?

Someone should construct an igloo and leave it on the legislative lawn to melt for all to see….

1 · Like · Reply · Subscribe · August 8 at 10:42am

Joan Savage · Top Commenter · SUNY-ESF

Surely you thought of the phrase, snowball in hell?

Like · Reply · August 8 at 12:04pm

Paul Magnus · Top Commenter

A giant snowball would do….

Like · Reply · August 8 at 12:31pm

Mason Ice · Dallas, Texas

I live in TX and find this graph very helpful in understanding the severity of this. I think we are all feeling it here…agriculture the most.

Like · Reply · Subscribe · August 10 at 1:17am

Thomas Jamison · Top Commenter

It may be an outlier now, but the predictions are that this kind of drought will become ever more common as this century progresses. It might occur once in 10 years, then 1 in 5 years, then every two years, then 2 out of 5, then 8 out of 10.

Like · Reply · Subscribe · August 8 at 9:43am

Joan Savage · Top Commenter · SUNY-ESF

John Nielsen-Gammon’s graphic projection that this year’s drought could cease to be an outlier by November 2011 depends on getting autumn rain, but he didn’t comment on that in his text. Straight-lining the trend without that rain is deeply scary.

Like · Reply · August 8 at 12:09pm

Paul Magnus · Top Commenter

It could also be that that region has already ‘tipped’ in to drier regime.
When things heat up we get more chaos (does that term have any meaning… ‘more chaos’?)

Like · Reply · August 8 at 12:33pm

Dan Phillips · UC Davis

Joan, the x-axis is running backwards in time because he’s plotting accumulated rainfall into the past. So “11” corresponds to 11 months ago, rather than November, and he isn’t extrapolating anything. But of course your comment is still valid going the other direction, and climatologists are not predicting much relief soon. Hard to imagine this becoming closer to the norm in the future.

Like · Reply · August 8 at 1:03pm

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Ron Baker · Dallas, Texas

we are finding cat-fish with ticks…

Like · Reply · Subscribe · August 11 at 7:47am

R Elton Goddard · Austin, Texas

This drought is so bad this year, that the animals in this state are migrating north in search of more water. This is very scary!

Like · Reply · Subscribe · 3 hours ago

Barry Saxifrage · Top Commenter

Senator Inhofe has started a successful new line of climate-themed ice cream snacks. The most popular one is the “Big Hoax” with an alluring image of frozen delight on a stick. Amazingly very few of his customers seem to mind that when they open the “Big Hoax” wrapper they find but hot air inside. People seem to just love it. They are buying it as fast as he can make it up. His signature dish is the elaborate “Inhofe Dust Bowl” with fancy packaging promising heaping helpings of the most delicious, cool, feel good fantasy. Expensive but very, very popular in Oklahoma.

Like · Reply · Subscribe · August 9 at 1:41am

Juan Cruz · Bronx Community College

Not that do not like Texas, but republicans there deserve this! I hope it hits them hard and makes them THINK about this serious issue!

Like · Reply · Subscribe · August 8 at 10:17pm

Ingrid Trujillo


Like · Reply · August 9 at 10:45am

Barry Saxifrage · Top Commenter

Oklahoma is KO.

Like · Reply · Subscribe · August 9 at 1:29am

Shannon Marie Pyle- Scott · Splendora, Texas


Like · Reply · Subscribe · August 9 at 7:26pm

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