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Regional climate variability and more play role in Missouri River Basin's elevated streamflow

Stretching from Montana to Missouri, the Missouri River basin is one of the country's largest river systems. Over the past century, its average streamflow has increased by about 40%, with multiple years since 1990 experiencing extremely high flow. Streamflow is affected by factors including precipitation, temperature, humidity, and atmospheric carbon dioxide (CO₂), as well as changes in the way land is used, such as replacing forest with cropland. Understanding how these factors interact is critical in helping the public prepare for hydrologic extremes such as floods, which cost the United States billions of dollars per year.

in AGU Advances, Dannenberg and colleagues used land surface models and water budget simulations to examine the role these factors played in streamflow changes, seeking to assess the relative roles of natural climate variability and anthropogenic influence.

They estimated past flow using daily discharge estimates near the mouth of the basin, as well as water budget models based on past precipitation and evapotranspiration within the basin. They also divided the basin into six regions on the basis of seasonal precipitation to better understand streamflow changes across different climates.

From 1931–1960, the mean annual water flow in the Missouri River basin increased by more than 900 cubic meters per second. The researchers found that about 765 cubic meters per second of this increase was attributable to natural climate variability. Changes in land use and land cover accounted for another 100 cubic meters per second.

Land cover-related changes were more pronounced during wet years, suggesting that cropland is less effective than the forests it replaced at buffering the basin against extreme precipitation. This effect was particularly noticeable in the lower basin. The remaining 65 cubic meters per second increase was caused by elevated levels of atmospheric CO₂.

The researchers suggest that this period of increased flow is transient and could be replaced by a drier mean state. This drying, they say, could be further exacerbated by human-caused warming.