The Water Cycle

All life on Earth is based on water.
“Thousands have lived without love, not one without water.” – W. H. Auden…

Water is so sacred that in India, when the rains finally arrive, the women and children run out to dance in the rain chanting “Bishti, Bishti, rain rain“.

Dancing in the Rain

Dance for the Season’s First Rain
(M. Khosla, Dedicated to her Mother)

Doors slam as monsoon winds
whistle music through the house—
drawing closer, closer, big drops flattening months of dust,
sheets of rain dancing at the garden’s edge,
and the earth smells of iron and dense relief.

Fastening the payals onto your bare feet
motioning me to take off my shoes,
singing bishti, bishti, rain, rain.
Your arms undulate in rhythm with the flourish:
wet leaves, rich, wet breeze—
…. only this music for new grass, monsoon urging us with its swirl.

The water cycle is the movement of water on, in, around, and above the Earth. Earth’s water is always in movement and is always changing states – water is present as a liquid, a vapor, or a solid. The conversion of water between those states is an endless cycle. Water converts from liquid (rain, dew) to vapor to solid (ice, snow) and back again – and not necessarily in that same order.

The Hydrologic (Water) Cycle on Earth

The water cycle has no starting or ending point so we’ll start with how water is transported into the air. The sun heats water in the oceans (where most of the Earth’s water exists) causing some of the water to evaporate as vapor into the air. A relatively smaller amount of moisture is added to the air when the sun heats ice and snow which convert directly from their solid state into vapor. Water transpires from plants and evaporates from the soil (evapotranspiration) adding to the amount of water in the air. Rising air currents take the vapor up into the atmosphere where cooler temperatures cause it to condense into clouds.

Air currents move clouds around the Earth, and cloud particles collide, grow, and fall out of the sky as precipitation – precipitation can be in the form of either rain, hail, or snow, and in some cases, actual ice. 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 typically thaw and melt during spring, and the melted water flows overland as snowmelt. Most precipitation falls into the oceans or onto land, where the precipitation flows over the ground as surface runoff driven by gravity. A portion of runoff enters rivers in valleys, with streamflow moving water towards the oceans. Runoff and groundwater 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 of the water infiltrates into the ground and replenishes aquifers (saturated subsurface rock), which store huge amounts of freshwater for decades. Some infiltration stays close to the land surface and can seep back into surface-water bodies as groundwater discharge, and some groundwater finds openings in the land surface and emerges as freshwater springs. Some groundwater is absorbed by plant roots and is ultimately evapotranspired from the leaves back into the air. Over time, all of this water keeps moving in an endless circle, some to reenter the ocean, where the water cycle “ends” … or “begins.”

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