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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.
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