Words to know
reservoir (in the water cycle)
A place where water is stored for a while, such as the ocean, the atmosphere, a glacier, a lake or an aquifer.
flow (flux)
A movement of water from one reservoir to another, such as evaporation, precipitation or runoff.
residence time
The average length of time a water molecule stays in one reservoir before moving on.
infiltration
Water soaking into the ground from the surface.
percolation
Water trickling down through soil and rock after it infiltrates.
sublimation
A solid changing directly into a gas, such as snow turning into water vapour.
deposition
A gas changing directly into a solid, such as water vapour forming frost.
transpiration
Water vapour given off through tiny openings in plant leaves.
Remember from Grade 7
In Grade 7 you described the water cycle with five steps: evaporation, transpiration, condensation, precipitation, collection and runoff. In Grade 8 we look at it like a system: water is stored in reservoirs and moves between them, at very different speeds.
How old is your water?
A drop of water might stay in the air for about a week and a half, then fall as snow. It could then stay frozen in a glacier for hundreds of years, or soak into the ground and stay in an aquifer for thousands of years. The water cycle never stops, but some parts are fast and some are very, very slow.
Reservoirs and flows
Think of the hydrosphere like a set of bank accounts (reservoirs) with money (water) moving between them (flows).
Reservoirs: oceans, the atmosphere, ice sheets and glaciers, lakes, rivers, wetlands, soil, groundwater, and living things.
Flows that go up (into the air):
- Evaporation from oceans, lakes and wet ground.
- Transpiration from plant leaves.
- Sublimation from snow and ice.
Flows in the air: winds carry water vapour, often hundreds or thousands of kilometres. When the air cools, vapour condenses into cloud droplets (or turns straight into ice crystals by deposition).
Flows that come down: precipitation as rain, snow, sleet or hail.
Flows on and under the land: runoff over the surface into streams; snowmelt in spring; infiltration into the soil; percolation down to the groundwater; slow groundwater flow to springs, rivers, lakes and the sea; glacier flow downhill; and rivers flowing to the ocean.
Ocean circulation: ocean currents move huge amounts of water (and heat) around the planet.
What powers the cycle?
- The Sun gives the energy to evaporate water and melt ice. (Remember from Grade 7: changes of state need thermal energy.)
- Gravity pulls precipitation down and makes rivers, glaciers and groundwater flow downhill.
Residence time
The residence time is how long, on average, water stays in one reservoir. A simple way to estimate it:
residence time = amount stored ÷ rate of flow out
Hands-on activityFair test: what speeds up evaporation? (A1.2, A1.5)
You need: 3 identical small plates or lids, a measuring spoon, water, a marker, a sunny window or a fan, paper and a pencil.
- Question: Does wind (moving air) make water evaporate faster?
- Hypothesis: write an "If… then… because…" statement.
- Put exactly 2 tablespoons (30 mL) of water on each plate. Mark the edge of the water with the marker.
- Put plate A in front of a fan on low (moving air). Put plate B right beside it but out of the fan's air (still air). Keep plate C as a spare to repeat the test later.
- Controlled variables: same amount of water, same plates, same room, same temperature, same start time. Only the air movement changes.
- Check every 30 minutes. Record how much water is left (draw the outline or measure the width of the wet area).
- Repeat with a new variable: surface area (30 mL in a narrow glass vs. on a plate) or temperature (sunny window vs. shade).
- Communicate: make a table and a bar graph. Did your results support your hypothesis?
What to look for: moving air, more surface area and warmth all make water evaporate faster. A good answer names the independent variable (what you changed), the dependent variable (how fast water evaporated) and the controlled variables.
Safety: keep water away from the fan's plug and motor; dry any spills right away.
For parents and teachersBig idea (E2.1): water circulates among reservoirs (ocean, air, ice, land, groundwater, living things) by flows powered by the Sun and gravity, and it stays in each reservoir for very different lengths of time. The evaporation fair test practises the scientific experimentation process (A1.2): one independent variable, a measured dependent variable, controlled variables, and communicating results (A1.5).
How to help: run the evaporation test together and help your child name the variables. On a frosty morning, point out deposition; after a storm, watch where runoff goes on your street.
Common difficulties:
- Thinking the water cycle is a neat circle. Water can take many paths and skip steps.
- Mixing up condensation and precipitation. Condensation forms the cloud; precipitation is what falls.
- Residence time calculations: remind your child to divide amount stored by flow rate and check the units.
Sources: Great Lakes retention times (Erie about 2.6 years, Superior about 191 years) and Lake Erie volume (about 484 km³) are from the U.S. EPA Great Lakes Facts and Figures; other residence times are rounded textbook estimates.