Graphs of the Day: More on Texas Drought
August 31, 2011
Figure 1 shows the year of the worst 6-12 month drought for various areas in Texas. For 55.8 percent of the state, the current drought is the worst on record. No other drought was as bad in so many places. The previous standard for a one year drought, 1925, can now only be considered the worst ever in 14.6 percent of the state.
For July, the statewide Palmer Drought Severity Index (PDSI), which is a measure of dryness that takes both temperature and moisture into account, recorded its lowest ever reading. This surpassed the worst July readings for 1918, 1925 and 1956, the droughts of record in Texas.
Figure 2 shows whether the twelve-month precipitation amounts for the period 1896-2011 were above or below normal, and by how much. With less than six inches of rain since January and a 13 inch rainfall deficit since last August, this period of dryness is unprecedented in recorded Texas history, significantly below the previous minimum readings in the 115-year record.
However, the drought is not unprecedented in every way, and much longer droughts have occurred in the past. The worst extended drought remains the massive 1950’s event when Texas suffered under drought conditions for 10 years from the late 1940’s until the late 1950’s. In the 1918 case, severe dryness began in 1917 and peaked in 1918 before rebounding to wetter than normal conditions. As the current drought has only been ongoing for the past 6-12 months, it can only be described as the most acute in Texas history; it is nowhere near the longest—yet. With the possibility of another La Nina developing in the Pacific—an event historically correlated with Texas drought—there is no sign that the current drought will break anytime soon, but it is impossible to predict whether its duration will ultimately match its intensity.
Even though we can’t predict what will happen with individual droughts, Figure 2 does communicate useful information about drought risk in Texas going forward. The record shows that 10-year droughts are possible. Going back even further in time, climate data from tree rings shows that in the past, Texas has suffered through droughts that are measured in multiple decades.