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When precipitation falls, the water percolates into the rock layer of the ground and then through flow occurs, leading the water back to underground reservoirs. Humans have disturbed this through urbanisation and deforestation. When cities are built in place of natural areas, the concrete and other materials used are not permeable, so the water cannot get into the ground to reach the level needed for through flow to happen. Deforestation causes the precipitation to directly hit the ground, often causing the maximum capacity of absorption of the ground to be surpassed, causing saturated overland runoff so water runs straight into the rivers rather than being absorbed.