US runoff trends and projections
The US National Climate Assessment observes that large increases in heavy precipitation have occurred in the Northeast, Midwest, and Great Plains, where heavy downpours have frequently led to runoff that exceeded the capacity of storm drains and levees, and caused flooding events and accelerated erosion.
As for annual runoff and streamflow, these have increased in the Mississippi Basin and Northeast. In the largest rivers of the US, including the Ohio, Missouri, and Mississippi, the number of days with high stream flow (the top 25 percent of readings) has risen over the past 30 years (1987-2016).
There are no clear trends in annual runoff in much of the rest of the continental US, although a declining trend is emerging in annual runoff in the Colorado River and Rio Grande Basins. A May 2017 paleoclimate study of runoff ratio—the portion of precipitation that ends up in the river each year, rather than evaporating—for the Upper Rio Grande basin finds a declining trend in runoff ratio from the 1980s to present day that is unprecedented in the last 445 years.
Runoff and streamflow have also declined in some locations in the last half-century, notably in the Northwest.
Models project that annual runoff and related river-flow will decline in the Southwest and Southeast and increase in the Northeast, Alaska, Northwest, and upper Midwest regions broadly mirroring projected precipitation patterns.
Global and regional runoff trends and projections
Heavy precipitation is contributing to increased runoff and flooding around the world. Flooding is worsened by regional climate trends such as increases in heavy rain and snow, early snowmelt, and increased seasonal precipitation. Flooding is also affected by non-climatic factors such as land development, deforestation, levee placement and local topography, making it challenging to determine an overarching climate signal in flood trends.
A 2009 study by Dai et al. looked at the 925 most downstream stations on the largest rivers—accounting for 80 percent of the global land area that drains into the ocean and 73 percent of the continental runoff. One-third of the 200 largest rivers (including the Congo, Mississippi, Yenisey, Paraná, Ganges, Colombia, Uruguay and Niger) showed statistically significant trends during 1948–2004, with 45 rivers showing downward trends and 19 showing upward trends.
The IPCC finds that decreases in runoff are likely in southern Europe and the Middle East, while increased runoff is likely in high northern latitudes, consistent with projected precipitation increases.
Scientists have not observed a consistent long-term trend in discharge for the world’s major rivers on a global scale.