Pan Evaporation Trends in Dry and Humid Regions of the U.S

This is an extension of research initiated at the National Climatic Data Center to study the long-term trend of pan evaporation in the United States. Previous research found a downward trend in pan evaporation from 1948 to 1993 using a network of 746 homogenous stations. These findings along with similar results from the former Soviet Union suggest that, for large regions of the globe, the terrestrial evaporation component of the hydrological cycle has been decreasing. However, some researchers suggest that, because pan evaporation measurements are influenced by humidity or the amount of moisture in the surrounding landscape, pan evaporation trends do not accurately reflect the true change in evaporation that has occurred over the past half century. Temperate areas that receive increasing amounts of precipitation would be expected to show decreasing pan evaporation trends as wetter surroundings give up more moisture to the atmosphere which, in turn, inhibits evaporation of water from the water-filled pans. To address these issues, we extend the study of pan evaporation trends through 1998 within nine geographic regions of the United States. Within each climatological region, we will access the relationships between long-term trends in pan evaporation and long-term trends in other climatic variables such as precipitation, cloud cover, and diurnal temperature range.