Skip to 0 minutes and 7 secondsDoesn't really matter how water got to Earth. What's important is how water functions on planet Earth, and how much water there is in particular regions. This is what affects the health and well-being of human societies, and also the functioning and health of ecosystems around the planet. The water cycle is made up of different stores of water and the movement between those stores. We'll begin our discussion of the water cycle with precipitation. Precipitation is the delivery of water from the atmosphere to the Earth's surface. This happens through a process called condensation, where water molecules in the atmosphere-- in clouds-- collect into larger and larger water droplets, such that they have enough mass that they start to fall out of the sky.

Skip to 1 minute and 6 secondsPrecipitation can fall as snow, as hail, as sleet, or as rain. And rain can be very heavy or it can be a light drizzle. But ultimately, all of those phases of precipitation deliver water to the land surface. Once the water arrives at the land surface, that water has different possible pathways. Some of that water can infiltrate into soils. Some will evaporate back into the atmosphere. Some will go into the deeper soil surface and recharge the groundwater within a region. Some of the water that ends up in the soil will be taken up by plant roots and returned to the atmosphere through a process called evapotranspiration. Evaporation is the main process of returning water to the atmosphere from the Earth's surface.

Skip to 2 minutes and 13 secondsIt occurs in three different ways. The first process is the movement of water from open water bodies-- lakes, oceans, and rivers-- and returning that water to the atmosphere. That happens once the water surface is heated up enough that the molecules on the surface can change phases from liquid to gas. And then they are delivered back into the atmosphere. So essentially, there's a recycling of water that landed on Earth that gets returned back into the atmosphere. The second way that water can get back into the atmosphere is through evaporation of water within the soil. Some of the water, as I mentioned, can drain and infiltrate into deep layers within the soil.

Skip to 3 minutes and 10 secondsBut it represents a much smaller amount of the total water that's delivered to the Earth's surface. Plants take up water through their roots in the soil, and send that water up through their stems, and deliver it from their leaves back into the atmosphere. That process is called transpiration. So the sum total of all of these processes is called evapotranspiration.

Stages of the water cycle

In this video, Dr Michael Singer describes the first stages of the water cycle.

He introduces the concepts of

  • precipitation
  • evaporation
  • transpiration

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The Challenge of Global Water Security

Cardiff University

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