The water cycle is at the center of many scientific topics, from watersheds (like this unit) to weather patterns to ice ages. The main idea is that almost all the water that exists on Earth today was there since the planet formed 4.6 billion years ago. However, water molecules do not stay in one place for long, at least not on a geologic time scale. The sun drives a continual process of evaporation, condensation, freezing, and melting that allows any given water molecule to travel from location to location on Earth. Thus the water cycle is the journey that water takes through its various phases (or states) – solid ice, liquid water, and gaseous water vapor – as it travels through Earth’s systems.
Water can exist in 4 phases or states – solid, liquid, gas and plasma – although plasma has little relevance to most everyday events including the water cycle. The molecules of a solid are tightly packed and bonded together so that the substance retains its shape. The molecules of a liquid are closely packed but can move relative to each other so that the substance flows. The molecules of a gas are independent of each other and move about freely in 3 dimensions. The transition from one phase to another is governed by temperature and pressure. As temperature increases and pressure decreases, a solid substance will generally transition to a liquid (it melts) and then from a liquid to a gas (it evaporates). As temperature decreases and pressure increases, a gas will generally transition to a liquid (it condenses) and then from a liquid to a solid (it freezes). It is possible for substances to transition straight from a solid to a gas in a process called sublimation. For instance, snow sometimes sublimates without turning to liquid water first. Similarly, dry ice sublimates straight to carbon dioxide gas.
With respect to the water cycle, water as a gas may travel huge distances across oceans and continents before it condenses and turns to rain or snow. Water as a liquid will flow across the Earth’s surface and percolate into the ground. Thus, water travels a lot as it undergoes phase changes. Most of this is governed by temperature as the result of the sun’s energy and less by pressure. One can therefore think of the water cycle as powered by the sun.
The water cycle is illustrated below. There are 6 major storage locations for water:
- Surface water (lakes, rivers, reservoirs, oceans, etc.) - The vast majority of Earth’s water, 97% of it, is stored in oceans.
- Atmosphere (clouds, fog, humidity, etc.)
- Precipitation (rain, snow, sleet, hail, and ice) – Water stored as precipitation in the form of snow is critical to the state of California since most Californians depend on snowmelt to provide them with fresh water throughout the dry summer months.
- Glaciers – These giant, slowly moving ice sheets form from snow that compacts and recrystalizes over time to form large ice crystals. Glaciers are more important than many realize. Approximately 75% of the Earth’s fresh water is stored as glaciers, primarily around the polar ice caps.
- Living organisms – Our bodies are 50-70% water!
From each of these storage locations, water has ways to travel to other locations. Water may:
- Evaporate –from surface water into the atmosphere
- Condense – from the atmosphere into precipitation
- Melt – from precipitation as snow to surface water or from glaciers to surface water
- Freeze – from precipitation as snow to glaciers
- Percolate – from surface water to groundwater and back again
- Transpire – from living organisms to the atmosphere
- Drink – from surface water (or sometimes groundwater) to living organisms
- Excrete – from living organisms to surface or groundwater
Previous exposure to the phases of matter and the water cycle is helpful but not necessary. If students have not learned about these topics in the past, then on day 1, do the phases of matter mini-investigation and discuss the locations where water is stored. On day 2, analyze the results of the mini-investigation and discuss the transitions between locations in the water cycle.