The Water Cycle


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. ♫MUSIC♫.