THE WET DECADES OF THE 80'S AND 90'S IN THE GREAT PLAINS: HOW WET?

Authors: Garbrecht, Jurgen; Rossel, Frederic - Fred

Submitted to: American Society of Civil Engineers Water Resources Conference Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 6/1/2000
Publication Date: 6/1/2000
Citation: N/A

Interpretive Summary: Decade-long precipitation variations affect agricultural production, urban and industrial water supply, hydro-electric power generation, flood and drought planning, and other industries and government functions that are related to weather and water resources. This study investigated the extent and magnitude of the precipitation increase in the closing decades of the 20th century in the Great Plains of the U.S. Results showed that the increase in precipitation that began in the 1980s affected a large portion of the Great Plains, with the greatest amount in the Oklahoma-Kansas region. The magnitude of the increase corresponded to about 10% of the long-term mean annual precipitation and to about 40 to 50% of the inter- annual variability in annual precipitation. The magnitude and the duration of this precipitation increase were the largest observed in this century. Recognition of the exceptional magnitude and duration of the current decade-long precipitation increase is important for agriculture, as well a government, urban, industrial and recreational water resources planning and management institutions. Agricultural planning and management strategies can be developed to diversify and increase production during such periods of increased precipitation, and water planning and management institutions can assess the impact of long-lasting variations in precipitation on the ever growing demand for water.

Technical Abstract: Annual precipitation for climate divisions in the Great Plains east of the Rocky Mountains and west of 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. Spatial gradients in annual precipitation are removed by standardizing the annual precipitation data, and year-to-year variations in annual precipitation ar filtered out by applying an 11-year moving average to the time series of the standardized annual precipitation. Visual interpretation was used to group climate divisions into broad regions with similar patterns in the time series of filtered standardized annual precipitation. A total of nine regions were defined in this way. Four regions taht represent 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 eduration and amplitude of the increase is greater than for any other increase during earlier times in the 20th century. The peak amplitude of the increase corresponds to roughly 10% of the annual precipitation and up to 50% of the inter-annual variability of annual precipitation. Cumulative distributions of the annual precipitation further show 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 were less dry than those for the 1895-1978 period. Thus, water deficits due to dry years have been less severe and less frequent over the last two decades.