The continuous circulation of water - on the Earth, below its surface and above in the air - is called the hydrological cycle. Throughout this cycle, water continuously changes states between solid, liquid and vapour. Individual water molecules come and go, but the amount of water on Earth remains fairly constant.
The sun drives the cycle by heating water in the oceans, causing it to evaporate into the air and become vapour. Snow and ice can sublimate (change from solid directly to gas) into vapour. Water also transpires from plants, and evaporates from the soil through a process called evapotranspiration. Rising air currents transport vapour into the air where cooler temperatures create clouds. That’s why we see so many clouds forming above mountains. Air currents transport clouds around the globe, causing vapour molecules to collide, grow and fall as precipitation in the form of rain, freezing rain, drizzle, ice crystals, snow, sleet (ice pellets), graupel and hail. Some rainwater soaks into the ground through a process called infiltration, while a portion flows as surface runoff, coursing into water stores such as rivers, lakes and wetlands. Some rain is stored in aquifers, which contain and transmit groundwater.
Snow that falls at high elevations or extreme latitudes (the Arctic and Antarctica), and accumulates over long periods of time can become compacted causing the lower layers to recrystallize into glacial ice. Glaciers and icefields can store large quantities of frozen freshwater for thousands of years. Gravity slowly pushes glaciers downslope like frozen rivers that eventually melt to become part of the streamflow. When seasonal snowpacks thaw and melt, mainly in the spring, meltwater is distributed via creeks and rivers. Eventually, water returns to the ocean to re-enter the cycle again as vapour.
- Hydrological Cycle – Vapour rises up from the ocean and is transported on air currents. When the vapour hits colder air high above the Earth’s heat-conducting surface, it forms clouds that shed rain or snow onto land or right back onto the ocean.
- Groundwater – Some rain and snowmelt seeps into the ground and percolates into the saturated zone through a process called recharge. Groundwater is stored in cracks and spaces in the bedrock, or on top of bedrock. In some places the bedrock supports flowing streams. Eventually this water is discharged above ground, often in springs, merging with lakes and rivers. While on average it takes two weeks for a river’s water to replace itself, groundwater can be stored for thousands of years. There is more water stored under Canada than on her surface.
- Wetlands – Wetlands, which include wooded swamps, bogs, fresh and saltwater marshes, sloughs and seasonally flooded forests, are either permanently or temporarily submerged in water. Plants growing in wetlands are uniquely adapted to saturated soil conditions. Once viewed as wastelands, wetlands are now appreciated – and formally protected - for their ability to filter sediments and toxic substances, act as reservoirs in the cycle of production, release and store important greenhouse gases and absorb the devastating impacts of floods. Wetlands also provide foods such as rice and cranberries, energy such as peat and charcoal and essential habitat for waterfowl shorebirds and mammals.
- Biological diversity –Scientists are just beginning to understand how diverse ecosystems provide countless services that are essential to the production of fresh water, such as biological water filtration, detoxification and breakdown of wastes, nutrient conversions, maintenance of soil cover, pollination, suppression of pests and diseases and perpetuation of healthy wildlife populations and fish stocks. Forests alone, through their health, maturity and the amount of cover they provide, help moderate climate and influence natural patterns of evapotranspiration, soil moisture and rainwater transportation into groundwater and surface water. Ecosystems that are biologically diverse are more resilient in the face of challenges, such as extreme weather events or conditions resulting from the effects of climate change that are being experienced around the world.
This interactive piece lets users select to learn about the continuous movement of water on, above and below the Earth’s surface. Each selection features detailed diagrams that describe water volume, hydrological cycles, and groundwater.