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Our transcription: The earth's external heat engine is essentially solar powered. It sets in motion a cycle known as the hydrologic or water cycle that has important ramifications for the landscape of this planet. When rain or snow falls on the land's surface, more than half the water returns to the atmosphere by evaporation or by transpiration from plants. The remainder percolates down into the Earth to become ground water or flows over the land's surface as runoff in streams and rivers. Over long periods of time, this moisture at the Earth's surface helps rocks decompose, forming soil. Water washing down hillsides and flowing in streams loosens and carries away the rock and soil particles.
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