This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: The same amount of rain falling on the same landscape at different times can send very different amounts of water into its rivers, according to a new study in Nature Water. The research offers a global picture of how catchments behave—not where rivers are or how much water they carry, but how consistently landscapes turn rainfall into river flow.
"We often imagine a catchment as a pipe—put in a certain amount of rain and get a predictable amount of runoff," said Dr. Ali Ameli, a hydrologist at the University of British Columbia (UBC) and lead author of the study. "But most catchments behave more like dynamic systems with a memory.
How much of today's rainfall becomes runoff and reaches the stream depends on both the conditions created by earlier weather and which parts of the landscape supply water to the river during the storm." Drawing on more than 2 million rainfall–runoff events, the researchers assessed the behavior of more than 80,000 catchments across 97 countries. They found that complex behavior, in which similar amounts of rainfall can generate very different runoff responses, covered 87% of the land area evaluated in the study, some 121 million square kilometers (46.7 million square miles). Simple catchments, in which a given amount of rain delivers roughly the same amount of water to the river every time, covered just 1.5%.
A third group, which the researchers call intermediate, sits between the two. These catchments have more than one stable way of responding to rain—perhaps two or three—and switch between them. Complex catchments dominate across Africa and much of Asia, across most of France, Spain, eastern Germany and Denmark, across the central United States and across most of South America.
Simple catchments are far rarer, concentrated along the Coast Mountains of British Columbia, Canada, and the U.S. Pacific Northwest, with smaller pockets in the northern and western United Kingdom, parts of Ireland and northern Spain, and in Tasmania and New Zealand. Intermediate catchments are common across the eastern United States.
The team has released an interactive global map of the results, along with a free web application that returns a classification for any catchment boundary. Both are publicly available. The strongest factor separating simple and complex catchment behavior was the frequency with which meaningful rainfall occurs over time.
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