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surface run-off  Water runs off a clay surface. Clay is an impermeable rock. This means that water flows over it. (A little water can enter the clay, but this only causes the clay particles to expand and become even more impermeable.) The more water that falls, the greater the flow over the surface; this causes small gullies to form on the clay. In addition to the amount of rain, a steeper gradient will also increase the quantity of overland runoff. | Overland transfer of water after a rainfall. It is the most rapid way in which water reaches a river. The amount of surface run-off increases given (1) heavy and prolonged rainfall, (2) steep gradients, (3) lack of vegetation cover, (4) saturated or frozen soil. A hydrograph can indicate the time the run-off takes to reach the river. Throughflow is another way water reaches a river. |
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