Submitted to: Journal Hydrologic Engineering
Publication Type: Peer reviewed journal
Publication Acceptance Date: 1/1/2001
Publication Date: 1/1/2001
Citation: Interpretive Summary: Annual precipitation at a location, such as a city or county, generally varies from year to year. Sometimes a sequence of ten or more years with mostly above or below average precipitation can occur. It is important to identify such sequences of mostly wet or dry years, because they impact productivity of dry-land agriculture, urban and industrial surface water supply, irrigation water needs, and water conservation strategies. This study identified the size and geographic extent of an increase in precipitation within the Great Plains at the end of the 20th century. It was found that the majority of the Central and Southern Great Plains experienced above average precipitation conditions over the last two decades of the 20th century (1980-1999). The observed precipitation increase was primarily the result of a reduction in dry years, as opposed to an increase in precipitation during wet years. The findings have immediate implications for agriculture in the Great Plains. Above average precipitation in sub-tropical dry climates may provide opportunities for diversification that directly benefit farmers and ranchers. However, further north, in temperate climates, the additional precipitation may lead to excessive soil moisture that can be detrimental to agriculture. On the other hand, should the wet period of the last two decades come to an end, adaptive measures and water conservation strategies may be needed to deal with potential water shortfalls that could affect an agricultural economy that has come to rely on the additional water supplied by the the recent wet period.
Technical Abstract: An increase in annual precipitation in the Great Plains at the end of the 20th century has been investigated. The object of this study was to quantify the magnitude and geographic extent of this increase with the purpose to expose subtle, yet consequential changes in annual precipitation in the closing decades of the 20th century. Many regions in the Central and dSouthern Great Plains experienced prevailing above average precipitation conditions during the 1980-1999 period. The size of the increase ranged from about 6% to 12% of the mean annual precipitation, and from about 25% to 60% of the inter-annual variability of the annual precipitation. A cumulative frequency distribution analysis showed that very dry years have been less frequent and/or less severe in many regions, whereas very wet years have not increased in frequency and/or severity in many regions. Thus, the decade-scale precipitation increase has been attributed primarily yto a reduction in dry years, as opposed to an increase in the intensity of wet years. The seasonal distribution of the increase in precipitation showed that, for most regions, several months during the year captured a disproportionate amount of the annual precipitation increase. These months were generally in early summer and in autumn. Also, a slight decrease in precipitation during one or two summer months was observed in many regions. For all regions in the Central and Southern Great Plains this two decade- long wet period was the longest and most intense for the entire 1895-1999 period of analysis.