http://blog.chron.com/climateabyss/2011/11/texas-drought-the-executive-summary/

The 2011 drought in Texas has been unprecedented in its intensity.
The year 2010 had been relatively wet across most of the state, except for
extreme eastern Texas. Beginning in October 2010, most of Texas experienced a
relatively dry fall and winter, but the record dry March 2011 brought widespread
extreme drought conditions to the state. A record dry March through May was
followed by a record dry June through August, and the 12-month rainfall total
for October 2010 through September 2011 was far below the previous record set in
1956. Average temperatures for June through August were over 2 °F above the
previous Texas record and were close to the warmest statewide summer
temperatures ever recorded in the United States.

As the drought intensified, the previous year’s relatively lush growth dried
out, setting the stage for spring wildfires. Conditions were so dry during the
spring planting season across much of the state that many crops never emerged
from the ground. Continued dry weather through the summer led to increasing
hardship for ranchers, who generally saw very little warm-season grass growth
while stock tanks dried up. The record warm weather during the summer in Texas
was primarily a consequence of the lack of rainfall, but the heat and resulting
evaporation further depleted streamflow and reservoir levels. By early fall,
trees in central and eastern Texas were showing widespread mortality and dry and
windy conditions allowed forest fires to burn intensely and spread rapidly in
Bastrop and elsewhere.

Twelve-month rainfall was driest on record across much of western, central, and
southern Texas, and many stations received less than 25% of their normal
12-month precipitation. The area near, north, and east of Dallas was
comparatively well off compared to the rest of the state, but still endured
serious drought conditions and record heat.

This drought has been the most intense one-year drought in Texas since at least
1895 when statewide weather records begin, and though it is difficult to compare
droughts of different durations, it probably already ranks among the five worst
droughts overall. The statewide drought index value has surpassed all previous
values, and it has been at least forty years since anything close to the
severity of the present drought has been experienced across Texas.

Because of the return of La Niña conditions in the tropical Pacific, a second
year of drought in Texas is likely, which will result in continued drawdown of
water supplies. Whether the drought will end after two years or last three years
or beyond is impossible to predict with any certainty, but what is known is that
Texas is in a period of enhanced drought susceptibility due to global ocean
temperature patterns and has been since at least the year 2000. The good news is
that these global patterns tend to reverse themselves over time, probably
leading to an extended period of wetter weather for Texas, though this may not
happen for another three to fifteen years. Looking into the distant future, the
safest bet is that global temperatures will continue to increase, causing Texas
droughts to be warmer and more strongly affected by evaporation.

http://thinkprogress.org/romm/2011/11/30/378412/texas-drought-historic-off-the-charts-says-state039s-climatologist/#jump

After examining tree-ring data going back to 1550, researchers at
Columbia University found that this year’s drought was only rivaled once in the
last 461 years. According to the Palmer Drought Severity Index, a system for
measuring wet and dry conditions, the last time Texas experienced a drought this
bad was in 1789.

The state’s climatologist, John Nielsen-Gammon, explained the historical
significance of the ongoing drought in an interview with CBS:

“This is basically off the charts. Based on past history, you wouldn’t expect to
see this happening in maybe 500 or 1,000 years. One more year and we’re already
talking about a drought more severe than anything we’ve ever had. And this will
become for them, the drought of record.”

The drought, which Nielsen-Gammon says could stretch over a number of years, has
devastated cotton crops, livestock, pumpkin crops, and, as the below CBS story
points out, Christmas trees. The dry conditions have been exacerbated by a
combination of human-caused global warming and La Niña, which pushes unusually
cold air from the Pacific Ocean and causes drier-than-average conditions in the
Southern U.S.