LAUSR

This study uses the Standardized Precipitation Evapotranspiration Index (SPEI) at a 6‐month timescale to document the spatial and temporal patterns of drought in Oklahoma from 1901 to 2014. A principal component analysis (PCA) with a VARIMAX rotation was used to divide Oklahoma into four spatially contiguous regions with similar drought patterns that are located in northwestern, northeastern, southwestern and southeastern regions of the state. Droughts tend to occur more frequently in northwestern Oklahoma, but all four regions show that droughts in Oklahoma have substantial variability at decadal timescales. A Mann–Kendall trend test demonstrates that there has been a statistically significant increase in the SPEI in Oklahoma since 1901. This indicates that conditions are generally getting wetter in Oklahoma. However, there is substantial variability in moisture conditions that is evident at shorter timescales. Prolonged periods of drier than normal conditions occurred in the 1910s, 1930s, 1950s, 1970s and 1980s, while conditions from the 1980s to the 2000s have been wetter than normal. Recently, the SPEI has decreased, which suggests that Oklahoma may be at the beginning of another dry cycle. In addition, while most droughts during the study period were caused by decreases in precipitation, recent droughts, such as the severe drought in 2011, were caused by both decreases in precipitation and increases in potential evapotranspiration. This may be indicative of future drought conditions in Oklahoma.