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 two 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 per cent of the land area evaluated in the study, some 121 million square kilometers. 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 per cent.
A third group, which the researchers call intermediate, sits between the two. They 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 Canada's British Columbia 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.
It is not just how much rain falls, but how often
The strongest factor separating simple and complex catchment behavior was the frequency with which meaningful rainfall occurs over time.
"It's not simply the amount of rain that matters; it's also the rhythm of rainfall," Dr. Ameli said. "Persistent rain keeps the landscape in a more consistent state. When wet and dry conditions alternate, the same catchment can behave very differently from one event to the next."
Where rain arrives regularly, soils, subsurface stores and pathways connecting the landscape to the stream remain in similar states from one storm to the next. In these catchments, a comparable amount of rain is more likely to generate a comparable amount of runoff. The rain-soaked coast of Canada's British Columbia is one of the clearest examples in the study.
Where rain is less persistent, a catchment repeatedly moves between wet and dry conditions. A storm arriving after a dry period may soak into soils and subsurface stores while sending relatively little water to the river. A similar storm arriving when the landscape is already wet can send a far larger share of that rain downstream.
Impact of overall water availability, landscape
The researchers also found that a region's overall water availability strongly distinguishes simple from complex catchments.
They examined this using measures including an aridity index, which compares how much water the air can draw off a landscape with how much rain actually falls on it, and the long-term balance between rainfall and the water that evaporates or is taken up by plants.
Catchments with simpler, more consistent behavior were generally found in wetter and more humid conditions. Complex catchments were more commonly associated with drier conditions, where less water remains after evaporation and plant use are accounted for.
These findings show that overall water availability establishes the broad climatic setting, and that rainfall rhythm indicates whether the landscape is likely to remain in a stable hydrologic state or alternate among different states.
Climate was the dominant influence, but the physical character of a catchment helped refine its behavior at local and regional scales. Simple catchments tended to occur in steeper terrain. Steeper slopes can move water rapidly and maintain more direct connections between the landscape and the stream, contributing to a more consistent runoff response.
A new way to understand the world's catchments
The study and map give hydrologists a common language for comparing catchments in different parts of the world. Two catchments on opposite sides of the planet may convert rain to runoff in the same way, and the classification makes that visible.
It also offers a practical guide to how a catchment should be represented in a computer model. A simple catchment can be captured by a straightforward model. A complex one needs a model that allows different parts of the landscape to take the lead at different times.
The research was conducted by Dr. Ameli and Hamed Sharif of UBC and Dr. Jeffrey McDonnell, affiliated with the University of Saskatchewan, North China University of Water Resources and Electric Power, and the University of Birmingham.
