A University of Oklahoma research study, led by professor Xiangming Xiao, revealed the divergent trends of open surface water bodies in the contiguous U.S. (CONUS) from 1984 to 2016 -- specifically a decreasing trend in water-poor states and an increasing trend in water-rich states.

Surface water resources are critical for public water supply, industry, agriculture, biodiversity and ecosystem services, the university said.

"The data, information and knowledge on long-term trends of surface water bodies across the CONUS at high spatial resolution are valuable for planning and management of surface water resources; however, they are not widely available yet," said Xiao, professor of ecology and remote sensing with the department of microbiology and plant biology in the University of Oklahoma College of Arts & Sciences.

Xiao and his team analyzed approximately 370,000 Landsat images and generated annual surface water body frequency maps for 1984 through 2016 at a 30 m spatial resolution.

Zhenhua Zhou, first author on the study and one of Xiao's graduate students, said, "Google's Earth Engine cloud computing platform, freely available Landsat image data, together with the novel and robust mapping algorithms derived from years of remote sensing studies in the past, allowed (Oklahoma) researchers to carry out the continental-scale image data analysis."

The spatial-temporal dynamics and trends of year-long water bodies in the CONUS during 1984-2016 were quantified by states and watersheds. During that period, 10 water-poor states in the Southwest and Northwest U.S. had statistically significant decreasing trends in surface water area, but 20 water-rich states in the Southeast U.S. and the northern Great Plains had statistically significant increasing trends.

Climate was the primary driving factor for such diverse trends in year-long surface water area since 1984, the researchers said.

"According to climate predictions, annual precipitation is likely to decrease in the Southwest but increase in the eastern U.S. during the 21st century; therefore, the observed diverse trends of surface water body areas since 1984 could continue to occur in the future," Xiao said.

University of Oklahoma researchers combined the surface water area data with the land water storage data from the National Aeronautics & Space Administration's Gravity Recovery & Climate Experiment for 2002-16 to investigate the data dynamics in severe drought and pluvial years.

The detailed analyses in California and the southern Great Plains (Kansas, Oklahoma and Texas) clearly highlight a large withdrawal of groundwater in those drought years and slow post-drought recovery of surface water area, groundwater and land water storage over years.

A paper on this study by Xiao and his team, "Divergent Trends of Open Surface Water Body Area in the Contiguous United States During 1984 to 2016," has been published in Proceedings of the National Academy of Sciences of the United States of America.