Wednesday, April 20, 2011

Drought History in Texas

Climate Progress had a post yesterday discussing the current drought situation in Texas, including quotes like this:
Texas is in the midst of one of the worst droughts, in terms of the depth and expanse of drought conditions, since the early 1900s.

Dan Byrd, meteorologist with the National Weather Service in Jackson, Miss., said, “This is an unprecedented drought situation [in terms of] how widespread it is and the depth of the drought. We haven’t seen anything like this for the state overall since the early 1900s.

Koenig commented, “It’s pretty phenomenal and historic. The entire state is involved in this. When you look at the size of Texas, from the panhandle to the coast, you have about 1,000 miles.”

According to the latest analysis by the U.S. Drought Monitor on April 12, 2011, the entire state of Texas was experiencing abnormally dry or drought conditions with most areas in a severe to exceptional drought.
I thought it would be interesting to pull the data to see how abnormal the current situation is versus the history of drought in Texas.  Accordingly, I went to the National Climatic Data Center and obtained the data for the Palmer Drought Severity Index for Texas in March.  That data goes back to 1895 and looks like this:

I've circled March 2011, so you can see how it compares to the history.

Recall the meaning of the PDSI, in which values of -2 to -3 are "moderate drought", -3 to -4 are "severe drought", and -4 and below are "extreme drought".  So the current value of about -3.2 is indeed a severe drought.  At least if the PDSI is to be believed, it's not unprecedented however: I count 15 different years which have had a lower PDSI in March than 2011.

I've also added a trendline, which you will note slopes upward (ie to wetter conditions).  However, a regression comes up with a slope of 0.0056 ± 0.0075 PDSI units/year - ie. the trend is smaller than the uncertainty and so it's not a statistically significant trend.  At any rate, there certainly is no evidence in the record that Texas is getting drier overall (at least in March, at least so far).

Climate Progress also links to a different indicator: data at the US Drought Monitor for the southern region, showing the fraction of the land area under different degrees of drought.  The current map looks like this:

The data for this indicator only go back to 2000, so there's no way to do any kind of useful trend analysis.  However, I plotted the fraction in severe, extreme, and exceptional drought as follows:

So again, while there is certainly a serious drought in Texas at present, even in the context of this extremely short record it does not appear to be unprecedented in scale or severity.

Our sympathies should of course go to the farmers affected by the drought, as well as to those affected by the wildfires.  Let's not lose our sense of proportion and start exaggerating the significance of current weather conditions, however.


Jennie said...

I've heard that the Ogallala is going dry in parts of TX, do you think that is having any effect on the fires?

My thought is that less municipalities have access to the aquifer and thus have fewer acres that are irrigated and more that are dry tinder. But, I'm just conjecturing.

John said...

Don't know if the shrinking Ogallala aquifier has any impact on the fires or drought. But according to National Geographic the Ogallala is definitely shrinking.

Robert said...

One of my earliest childhood memories is being stranded in a black duster near Amarillo. However I do not recall any fires while growing up. My parents had a tiny cabin in a canyon near Amarillo. It once took five feet of water during a100 year flood but was salvaged. The following third picture of that canyon shows one of Boone Pickens' sons - on the left side in the black striped shirt - largely hiding Boone's first wife, Lynn O'Brien. There is some controversy as to what part Lynn's father played in Boone's start. Boone has some water rights in that area. The Ogallala has been disappearing for decades.

John Fleck said...

Stuart -

Excellent job in bringing data to bear on this question. One of the most important characteristics of drought is duration, and your PDSI data really makes the drought of the 1950s, which was epic, stand out.

John said...

Texas wildfires: why this season is one of the worst in state history

This article points out a lot of other variables that impact the severity of the drought and/or resultant wild fires.

-Lots of dry growth as a result of earlier rains.
-Humidity extremes
-Encroaching civilization

I guess the take away I have from the article is that the length and severity of the drought doesn't necessarily determine how destructive the drought is.

Alexander Ac said...

And, also, let's not fool people into feeling that CO2 reduction might do anything to alleviate the future wildfires in Texas... ;-)


Craig said...

I'll leave it to other to decide how bad the drought is in Texas. But there is one major difference between the current era and the dust bowl of the 1930s. The water table has continued to drop during those years.

In California, the low water table has probably contributed to the severe firestorms of the last twenty years and may be a factor in Texas.

Hal said...

Craig: "In California, the low water table has probably contributed to the severe firestorms of the last twenty years and may be a factor in Texas."

I'm scratching my head about this one. Could you explain what you mean by "low water table" and "severe firestorms?"

Mike Aucott said...

Based on limited but nevertheless incontrovertible data the PDSI is a questionable measure of at least one measure of drought - tree death. The drought in central New Jersey last summer was, according the PDSI, only moderately severe. Yet it led to wilting foliage and premature loss of all leaves of two large tulip trees near my house. This spring has revealed that these trees are dead.

Ted Nation said...

There are lots of ways at looking at drought. Your method of comparing the drought indexes for the month of March is one, but what does it say about the duration of the drought or the extent? Does the analysis in this artice tell us more?

Stuart Staniford said...

Ted: The graph at your link is pretty equivalent to my second graph, except it includes the D1 moderate drought also. Again, it suggests a similar conclusion: the present drought is not worse than prior droughts pretty recently.

Also, I did play a little bit with doing things like looking at the last 12 month average PDSI, etc, but it didn't produce different conclusions. Also, if you delve into the details of how the PDSI is defined, you'll find that it tries to incorporate a concept of the cumulative drought stress over time. So the reading in March is not just about temperatures and rainfall in March, but instead is about how temperatures and rainfall in March add to the preexisting level of soil moisture stress from the weather in previous months.

It is an admittedly somewhat crude heuristic.

But at the moment, I'm not aware of any reasonable quantitative drought indicator that shows the present Texas drought as being historically off the charts. As far as I can see, the apocalyptic rhetoric at places like Climate Progress and in certain news stories is not backed up by any serious quantitative analysis.

John said...

How Bad Is the Ogallala Aquifer's Decline in Texas?

The Ogallala runs from Texas into Nebraska and South Dakota. In general the levels are falling rapidly.

In Texas, Questions of Drought and Climate Change

Kathy said...

Take another look at the drought monitor data, what looks unprecedented about this drought based only on that data from 2000, is how widespread the drought is. If you look at the archive tables, it is the percentage of area in drought conditions that really stands out. In some years the whole state was considered dry but now over 90% of the state is in the severe drought range.

Stuart Staniford said...

Kathy: "Severe" is D2 and above. In the week of 4/19, 67.61% of the southern region is listed in that condition (from here). In August 2006, this particular indicator reached 62.65%, and prior to that, in October 2000, it reached 86.32%.

Kathy said...

Here's a link to just the Texas drought conditions which is worse than the South's conditions: In 2006 drought levels were close to those now but the percent of the state in drought conditions was not as bad. Looking just at Texas as of 4/19/11, 91.52% of the state is at D2 (severe drought) or above, 67.96% of the state is at D3 (extreme drought) or above and 14.99% is at D4 ("exceptional" drought). In the other years you mention when there were extreme drought conditions in Texas, 21.42% of the state was in extreme drought and in 2006 it got up to 48.59%. So the current conditions are worse.

Mike said...

What would be an updated opinion of the drought be now?