The Wet Decades of the 80s and 90s in the Great Plains: How Wet?

Annual precipitation for climate divisions in the Great Plains between the Rocky Mountains and the Mississippi River have been used to define the duration, amplitude and geographic extent of increased mean annual precipitation in the closing decades of the 20th century. Visual interpretation was used to group climate divisions into broad regions with similar patterns in the time series of filtered standardized annual precipitation. Four regions that included the majority of the Central and Southern Great Plains showed a significant increase in mean annual precipitation over the last two decades. For these regions the duration and amplitude of the increase is greater than for any other increase in the 20^th century. The peak amplitude of the increase corresponds to roughly 10% of the annual precipitation and about 50% of the inter-annual variability of annual precipitation. Cumulative distributions of the annual precipitation further showed that within the 10% and 90% cumulative range increased by a similar amount. However, the wettest years in both time periods remained comparable, while the driest years in the 1979–1998 period did show an increase over the 1895–1978 period. Thus, water deficits due to dry years have been less severe and less frequent over the last two decades. This study shows that the increase in annual precipitation over a large portion of the Central and Southern Great Plains and over the last two decades is significant in amplitude and duration compared to variations in annual precipitation during the early part of the 20^th century.