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Why is the amount of water entering the oceans from land each year similar to the amount stored in rivers and lakes?

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Diagram illustrating Earth's environmental spheres, with labeled sections showing the geosphere (land and crust), biosphere (living things), atmosphere (air), and hydrosphere (water), each extending vertically from the Earth's surface. — hydrosphere Earth's environment includes the atmosphere, the hydrosphere, the lithosphere, and the biosphere.

The observation that the amount of water entering the oceans from land each year is similar to the total volume stored in rivers and lakes at any given time is a reflection of the rapid rate of water circulation within the hydrologic cycle.1

While the total volume of water stored in rivers and lakes is relatively small compared to other reservoirs like the oceans or groundwater, water moves through these surface systems quite quickly. The residence time—the average time a water molecule spends in a reservoir—is determined by the ratio of the reservoir's volume to the rate at which water enters or leaves it. Because the turnover rate of water in rivers and lakes is high, the annual discharge from land to the sea effectively matches the instantaneous storage volume of these surface water bodies.123