The Water Cycle


ANNE THOMPSON: All the water on Earth today, every drop, is all

the water there has ever been on the planet. Freshwater is

actually millions of years old. The same water, flowing in a

continuous loop- falling as rain and snow from clouds to the

Earth's surface, running in rivers, pooling in ponds,

flowing from faucets, irrigating crops, traveling through plants,

generating power, eventually evaporating into the air and

condensing into clouds again.

ANNA MICHALAK: Why is there life on Earth? The reason

there is life on Earth is because Earth has this

perfect water cycle.

THOMPSON: The Water Cycle, so simple even small children

understand the basics, yet so complex, the most advanced earth

scientists, hydrologists, geologists, and biogeochemists

are studying every part and process.

MARTHA CONKLIN: The Water Cycle is fascinating. It's something

that's around us all the time, and yet we don't

really understand it.

THOMPSON: How to summarize what is known about the Water Cycle?

With two words- flows and stores. The Water Cycle is a

series of flows of water between various water stores or

storages. Clouds in the atmosphere...

TOM HARMON: There's always a little bit of water in the

atmosphere. We talk about relative humidity- it's a humid

day, it's a dry day- either way, there's water, sometimes a

little, sometimes a lot.

THOMPSON: There's a lot of water in the oceans – 70% of all

the water on Earth, in the ice sheets and glaciers - two-thirds

of all the fresh water on Earth, in the snow packs atop mountains

like the Sierra Nevada, in the Great Lakes, in rivers and

streams, in reservoirs and watersheds, in wetlands, in the

soil, in and on plants and trees rooted in the soil, and beneath

the soil, in water tables and underground aquifers like the

Ogallala-High Plains, which runs underneath parts of eight

states, from South Dakota to Texas. All this storage is

temporary. Water, in all its forms, is always in flux and

always moving. And there's a name for every kind of movement

in the Water Cycle starting with precipitation.

MICHALAK: Precipitation is the process of water falling onto

the surface of the Earth. You can have precipitation in many

forms- rain, snow, hail.

THOMPSON: Rain is falling water in liquid form. Snow,

ice, hail and sleet are falling water in solid, or frozen form.

Fog and mist? Falling water in gas or vapor form. Precipitation

that falls directly into the oceans becomes part of surface

ocean and can be churned by wave and wind action into ocean

currents. Rain and snow that falls directly on rivers and

streams becomes one part of stream flow. Rain that falls

onto land takes a different path to the river as does the

snow and ice that falls and collects on mountaintops

when temperatures warm.

CONKLIN: When snow melts, some of it runs through the snowpack

and goes into small streams, tributaries that feed

into large rivers.

THOMPSON: What about the precipitation that falls on and

over land? Some is intercepted by vegetation -

plants and trees.

HARMON: Like you might imagine, someone in a game of football

intercepting a pass, these are raindrops trying to come to the

ground, and leaves on the tree intercept them before they

hit the ground.

THOMPSON: And the precipitation that does hit the ground? It can

run off if the ground is hardscaped - covered with

asphalt or concrete- or if the soil is too wet, or saturated to

absorb more water, like an over soaked sponge. Otherwise,

precipitation infiltrates the soil surface, percolates

into the ground.

HARMON: Think of it as the water percolating through your

coffee grounds in the morning. Gravity continues to pull it

downwards so it will move through.

THOMPSON: Through the topsoil, into spaces between soil and

rock particles, down to bedrock, and further, into fractures,

into deep underground aquifers. Even groundwater here

is moving sideways, or laterally, discharging toward a

river, lake or the sea, generally the deeper the flow,

the slower the flow.

CONKLIN: Some of that fractured water might take a very long

time, thousands to millions of years, to get out.

THOMPSON: And how does water get back out into

the atmosphere? It evaporates, is turned from a liquid

into a gas or vapor, by the heat of the sun.

MICHALAK: If you put a bit of water into a bowl and you set it

outside on a sunny day, it's going to disappear. It's still

water, it's just in the form of a gas rather than in the form

of a liquid.

THOMPSON: Water evaporates from every wet surface – even from

wet air. Some rain and snow evaporates into the air while

falling. Water evaporates through our respiration and

perspiration and from plants, through transpiration. Trans

means through or across. Plant roots draw up groundwater.

MICHALAK: And plants pull that water up through their stems

into their leaves and then release it back out

through evapotranspiration.

THOMSPON: Evaporanspiration, a spelling bee worthy term for

evaporation from soil and water surfaces, plus transpiration

from plants. Evaporated water molecules are tiny enough to

flow into the air. Mix with smoke and dirt particles in the

atmosphere. Cool, condense, into visible masses of water vapor –

clouds. Winds move clouds into colder air, water droplets

collide and merge, grow bigger and heavier, until they

are so heavy, they fall again as rain or snow, sleet or hail.

Precipitation. Collection. Runoff. Interception.

Infiltration. Percolation. Discharge. Transpiration.

Evaporation. Condensation. The Water Cycle.