Rainforests help maintain the water cycle. The water cycle, also known as the hydrologic cycle, describes the continuous movement of water on, above, and below the surface of the Earth.
The role of world forests in the water cycle is to add water to the atmosphere through the process of transpiration (where they release water from their leaves during photosynthesis). This moisture contributes to the formation of rain clouds which release water backto the rainforest. In the Amazon, 50-80% of moisture remains in the ecosystem's water cycle.
When forests are cut down, less moisture goes into the atmosphere and rainfall declines, sometimes leading to drought.