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Water cycle
from oceans. The water rises as vapor and then changes back into droplets or ice crystals high in the air. When raindrops or ice crystals reach a large enough size, they can fall out against the rising currents of air and produce rain and snow.
Some of the water that reaches the ground is used up in wetting the surfaces of leaves and to a smaller extent roads and buildings. This water never reaches the soil but
is evaporated back to the air. The remaining water seeps into the soil. Some water is held back by the soil, this time trapped in the tiny cavities between soil grains known
as pores. This water will later
be sucked up by plant roots and returned as vapor to the air as the plants transpire.
Any water not lost by falling
on plants or trapped in soils will then drain slowly through the
soil and any permeable rock as groundwater before finding its way eventually to the banks and bed of a river. In times of heavy rainfall some water may also run directly over the ground as surface runoff, simply because the ground cannot soak it up fast enough.
Rivers return both surface runoff and groundwater to the oceans,
thus completing the water cycle. The water cycle can be severely
altered by people. The demand
for water is now so great that the whole flow of a river is sometimes consumed by people living near
its banks. As more and more areas become built up, the amount of impermeable surfaces increases, and less water flows into the ground. This water is instead taken through drains directly to rivers. This makes river flows much more variable. Rivers are more likely to flood during heavy rainfall and have little water left in times of drought.
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Rain and snow fall and provide moisture for soil and rivers.
Boundary of drainage basin
Water flows through soil and rocks, then enters the river channels.
Water flows back to the ocean in rivers.
Water evaporates from the oceans, rises, and cools to make clouds.
 Water cycle—The water cycle is the continuous exchange of water between oceans, atmosphere, and land.