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The hydrological cycle of the earth is difficult to study. Why is this? Largely because the water cycle has no starting or ending point. The sun, which drives the water cycle, heats water in the oceans. Some of it evaporates as vapor into the air. Ice and snow can sublimate directly into water vapor. Rising air currents take the vapor up into the atmosphere, along with water from evapotranspiration, which is water transpired from plants and evaporated from the soil. The vapor rises into the air where cooler temperatures cause it to condense into clouds. Air currents move clouds around the globe, cloud particles collide, grow, and fall out of the sky as precipitation.

However, this precipitation is highly variable in time and location throughout a year and will be less than an inch in desert regions and more than 400 inches per year in some tropical rain forests. The total precipitation reaching land each year is about 160% of what left the land surface as evaporation and transpiration through vegetation. This value is greater than 100% due to a portion of the evaporation from oceans first returning to land before running back into the oceans.

Some precipitation falls as snow and can accumulate as ice caps and glaciers, which can store frozen water for thousands of years. Snowpacks in warmer climates often thaw and melt when spring arrives, and the melted water flows overland as snowmelt. A portion of runoff enters rivers in valleys in the landscape, with streamflow moving water towards the oceans. Runoff, and ground-water seepage, accumulate and are stored as freshwater in lakes. Not all runoff flows into rivers. Much of it soaks into the ground as infiltration.

Some water infiltrates deep into the ground and replenishes aquifers (saturated subsurface rock), which store huge amounts of freshwater for long periods of time. Some infiltration stays close to the land surface and can seep back into surface-water bodies (and the ocean) as ground-water discharge, and some ground water finds openings in the land surface and emerges as freshwater springs. Over time, the water continues flowing, some to reenter the ocean, where the water cycle renews itself.

Once all these factors are taken into consideration there is a final number most hydrologists can agree on. According to hydrologists and climatologists, every year about 24,000 cubic miles of water actually reaches the land as precipitation. This is equivalent to about 26 inches for every acre of land. Most precipitation falls back into the oceans or onto land, where, due to gravity, the precipitation flows over the ground as surface runoff.

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