Texas Drought History Reconstructed and Analyzed from 1698 to 1980

Abstract

A selected group of nine climate-sensitive tree-ring chronologies from old post oak trees are used to reconstruct the June Palmer Drought Severity Index (PDSI) from 1698 to I 980 for two large regions in northern and southern Texas. Analysis of tree growth and monthly climate variables indicate that the June PDSI is the most robust climate signal evident in these chronologies. and principal component analysis (PCA) reveals a northsouth geographic pattern in the relationship between the regional tree-ring time series~ Serially random amplitude series from the first two significant eigenvectors of tree growth, which explain 65% of the total variance in the tree-ring data were entered into stepwise multiple regression as predictors of regionally averaged June POSI in north and south Texas for the common interval I 931-80. The regression models explain 59% and 6O%, of the variance in north and south Texas June PDSI, respectively, and both reconstructions are well verified against independent June PDSI data available on a statewide basis from I 888 to 1930. The weak persistence pl~ent in the observed June PDSI series was added to the serially random tree-ring reconstructions prior to verification, using autoregressive modeling procedures. The mean and variance of June PDSI during the 50-yr period of meteorological observation ( I 93 1 -80) appear to be representative of the last 283 yr, but significant changes in average June PDSI for Texas appear to have occured~l over both 30 and -90-yr time intervals. Moderate or more severe June droughts (PDSI ( -2.0) have an estimated recurrence probability of over 90% each decade, and the risk of extreme June drought (PDSI ( -4.0) is estimated at over 50% every I 5 yr in north Texas and every 10 yr in south Texas. The reconstructions faithfully reproduce the frequency domain properties of the actual June PDSI, and marginally significant spectral peaks are present at 2.3 yr and between 14 and I 8.67 yr in both reconstructions. Significant interannual persistence of June moisture extremes apparent in the statewide June temperature, precipitation, and PDSI data from I 888 to I 982 is also present in both regional reconstructions from I 698 to I 980. The reconstructions indicate that the risk for below average June moisture conditions increases to at least 65% in north and south Texas in the summer following a June drought (PDSI ~ -2.0), Interannual persistence is also indicated for June wetness anomalies and may have some modest value in statistical forecasts of growing season moisture conditions in Texas.