Monday, 20 June 2005: 5:00 PM
North & Center Ballroom (Hilton DeSoto)
Land use change can modify root zone moisture distribution, energy partitioning and subsequently near surface energy balance. Various modeling studies also provided evidence of these changes. For example, land use change from natural grass land to irrigated land use would significantly increase and decrease latent and sensible energy flux, respectively. This type of long-term modification of energy balance would also change near surface temperatures. The Great Plains of North America experienced significant overturning of land from natural grass land to irrigated land use during 20th century. It is expected that this change would also modify long-term temperature records. This study investigated the changes in the historical near surface temperature records in the Ogallala aquifer region of USA. This includes eastern Wyoming, eastern Colorado, eastern New Mexico, Nebraska, western Kansas, western Oklahoma, and western Texas. Long-term mean monthly maximum, minimum, and monthly mean air temperature data from 33 irrigated and 33 non-irrigated sites (total 66) has been analyzed. Length and homogeneity of time series and stability of stations were primary determinants in selection of these stations. The time series include both Cooperative Weather Observation Network (COOP) and Historical Climate Network (HCN) data sets. Hence for 50 stations, data analyses were completed for both COOP and HCN time series. Pairwise comparison of temperatures between irrigated and dry land locations for pre- and post-1945, -1950, and -1955 periods were completed for both data sets. These breakdowns of time series helped to identify periods of widespread land use change. Results show notably cooler temperatures over irrigated areas. For example, mean maximum growing season temperature at irrigated Alliance, NE was 0.69 °C and 1.46 °C cooler compared to non-irrigated Halsey, NE during pre-and post-1945 period, respectively. Hence, there was a 0.77 °C cooling during post-1945 years. Moreover, there has been greater cooling during the second half of 20th century. Bootstrap re-sampling method was applied and trend analyses were completed. These assessments largely show decreasing trend in mean maximum growing season temperatures over irrigated areas. To further verify the results and to determine the impacts of extreme values (including extremely cool temperatures), 20% trimmed mean approach was applied. It is found that impacts of extreme values have been minimal. Based on the results it is concluded that the land use change in the Great Plains has modified near surface temperature records.
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