The Web of Life (19-5) Water Cycle

Water Cycle

As water is the medium for all cellular chemical reactions, clean water is essential to all life.

Surface water on Earth is held in a massive aquatic reservoir (the oceans), with a smaller reserve on land (glaciers, rivers, and lakes).

The upper mantle holds at least 3.5 times as much water as the oceans, subducted in deep-sea fault zones.

The subduction zones at which the tectonic plates beneath the sea thrust into the deep Earth act as gigantic conveyer belts, carrying water, fluids, and volatile compounds into our planet. Water in Earth’s interior is released back into the oceans and atmosphere by volcanoes. These inputs and outputs constitute a global deep-Earth water cycle. ~ Donna Shillington

Earth’s water is incessantly in motion, and constantly mixed. Water evaporates from the ocean, then precipitates over land as rain, sleet, or snow; returning to the ocean through runoff and river flows.

The hydrological cycle (water cycle) circulates water through the biosphere, most actively the atmosphere. Via atmospheric exchange of water vapor, the water cycle is primarily a gaseous cycle. Water lingers in the atmosphere as vapor for 9–10 days.

Atmospheric bands of water vapor flow like rivers in the sky. These atmospheric rivers develop because of the temperature differences between Earth’s poles and tropics.

At any given time, 3 to 5 atmospheric rivers ferry water in each hemisphere. These rivers are over 1,000 km long and often no wider than 400 km, carrying in water vapor the equivalent of what flows at the mouth of the Mississippi river.

Water cycles from the atmosphere by precipitation: rain, sleet, snow, and hail. Each form has its own dynamics.

A single drop of water from a raincloud multiplies as it descends. Air resistance on an accelerating drop increases until the cohesive forces holding the drop together are overcome. The drop bursts into a shower of droplets.

That same friction that tears raindrops apart is lessened by the rain. The friction of falling raindrops dissipates wind. Rainfall soothes the atmosphere.

The power of precipitation is considerable. Globally, raindrops rip friction at a rate of 1015 watts. The rate of energy dissipation from worldwide precipitation is 100 times that of human energy consumption.

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Water cycles into the atmosphere via evaporation and transpiration. Evaporation is a taking, transpiration a giving.

Evaporation is the kinetic energy excitement of surface water molecules busting a move into vapor. As with many energetic transactions on Earth, the inspiring culprit is the Sun.

Oceanic evaporation is the source of most rainfall. But all surfaces are sources of evaporation: soils, bodies of water, and the bodies of organisms.

Evaporation and precipitation over land and ocean differ. Excess ocean evaporation is carried by the wind onto land, where more precipitation occurs than evaporation. 20% of global rainfall is over the oceans; 80% lands on land, where it is taken up by vegetation, stored in terrestrial reservoirs, or runs off into the oceans.

Transpiration comes from plant release of water; especially leaves, but also flowers, stems, and roots. Plant leaves are dotted with stomata: pores whose opening is regulated by guard cells.

Plants move water from the soil to leaves for photosynthesis. 90% of the water that plant roots absorb transpires.

Plants flow mineral nutrients from roots to shoots. In the finale of transpiration, plants let go of water to cool themselves. Only 1% of the transpired water that passes through a plant is used in the growth process.

A mature oak tree, fully leafed, transpires 4 tonnes of water a day in warm, dry weather. Globally, 10% of all water evaporation comes from transpiration.

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Over 35% of the forests in the world are in the humid tropics. Forests, especially tropical forests, promote rain. Tiny aerosols put out by rainforest inhabitants – especially trees – seed raindrop formation. Further, 25–56% of the rainfall in a tropical rainforest is recycled within the ecosystem.

Tropical rainforest is a gyral truism. Deforestation reduces rainfall for thousands of kilometers beyond the forest.

From 1970 to 2012, 20% of the Amazon rainforest was cleared. By 2050, a further loss of at least 40% is likely. Under that scenario, rainfall across the Amazon basin will drop over 20%.

Such disruption by itself would shift global rain patterns and promote aridity on other continents. The effect is accelerated with global warming.

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On land, gravity pulls water down into the ground, until the soil is saturated: the spaces between soil particles full of water. The level of ground saturation is the water table; the water found there: groundwater.

Groundwater provides 95% of the world’s supply of fresh water. If the land lies below the water table, surface water is present as wetlands, lakes, bogs, swamps, and streams.

Water is a restless medium: constantly moving. Groundwater recharges lakes, streams, and rivers. Some groundwater goes deep, recharging geologic pools of water: aquifers. Water running off to the sea forms watersheds, also termed catchments or drainage basins. In flowing back to the sea, river runoff closes the water cycle.

Owing to water’s restless nature, groundwater pollution quickly diffuses. If a part of a watershed becomes polluted, the entire watershed is debased in short order.

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Changes to the global water cycle alter precipitation patterns and redistribute freshwater on a geographically large scale. Such global water cycle changes have been apparent since 1950. The upshot is more extreme weather in many areas.

Since the mid-1990s, melting ice near the poles has changed the shape of the Earth: putting a bulge of ocean near the equator. This is merely an acceleration of a trend that begun in the mid-19th century, as industrialization got underway.

The water cycle is intensifying quickly under global warming – twice as fast as climate models have been predicting. ~ American science writer Richard Kerr