Lesson 1 of 6 · the water cycle
Follow water through connected reservoirs
10 minute lesson · 3 checks along the way
Your goal for this lessonTrack water across a defined boundary without confusing a store, a transfer and a phase change.
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Words with a dotted underline open a short definition right here in the lesson.
A cycle has branches, not one compulsory route
Earth’s water occupies oceans, ice, lakes, rivers, soil, groundwater, the atmosphere, and living organisms. A is the amount stored in one of these places; a describes a transfer per unit time. A molecule can take many routes between reservoirs. Water evaporating from the ocean may rain back into the ocean, while groundwater can remain underground for much longer than water passing through a storm.
The familiar cycle drawing compresses those different scales into one picture. Its arrows do not imply that every reservoir empties and refills once a year. Sunlight supplies much of the energy supporting , while gravity influences falling precipitation and flow through landscapes. Human withdrawals, dams, irrigation, and changes to vegetation also alter the routes and timing. Repeated circulation does not guarantee clean freshwater wherever or whenever people need it.
DSCOVR’s EPIC camera observed Earth’s sunlit side on July 6, 2015. This color composite combines red, green and blue measurements. Clouds and surface water appear together, but much of the water beneath the ground remains out of view.
Look closely
Find a visible water store and one important store this view cannot inventory. Could one image reveal the annual amount exchanged between them?
See what to notice
The ocean is visible, while groundwater and much subsurface storage are hidden. A snapshot records appearance at one time; exchange rates need repeated observations, measurements and a defined boundary.
Check 1 of 3
10 pointsUse the reading above. You can try again without losing points.
Keep phase changes distinct from transport
Evaporation turns liquid into vapor; turns vapor into liquid. Neither process splits water molecules into hydrogen and oxygen. transfers water through plants into the air, and evapotranspiration combines that pathway with evaporation. Infiltration moves water into the ground. Some remains in soil, some reaches groundwater, and some returns to streams. Groundwater usually occupies pores and fractures, rather than one continuous underground lake.
A view of Earth from DSCOVR shows ocean, land, and clouds in one connected system. It cannot reveal every underground store or show the rate of each exchange. Rain gauges, river measurements, soil sensors, and satellite observations add different constraints. A satellite can observe radiation related to moisture or precipitation, but retrieving a quantity still requires calibration, assumptions, and attention to the part of the system actually sampled.
Work through a decision
Worked budget: choose what lies inside
Consider an invented one-square-kilometer region whose boundary includes its lake, soil, and groundwater. During one month, precipitation totals 40 millimeters and totals 15 millimeters over that area. Their volume equivalents are 40,000 and 15,000 cubic meters. Surface and groundwater inflow together add 3,000 cubic meters, while all outflow and withdrawals together remove 18,000. Treat these terms as complete and measured over the same interval.
The storage change is 40,000+3,000−15,000−18,000 = 10,000 cubic meters. Water infiltrating from the lake into soil stays inside this chosen boundary, so it is not another external loss. If the boundary instead enclosed only the lake, that transfer would need its own term. The positive result describes a change, not the total stored volume, which also requires the initial inventory.
Count transfers across the chosen boundary
Original schematic of the lesson’s fictional one-square-kilometer region. Arrows label monthly transferred volumes, not instantaneous rates; infiltration remains internal because both stores are included. Arrow widths and reservoir sizes are not to scale.
Open full-size diagram in a new tab: Count transfers across the chosen boundaryCheck 2 of 3
10 pointsUse the reading above. You can try again without losing points.
Test a changing water supply
Suppose a river’s flow decreases while rainfall appears unchanged. Possible explanations include altered evaporation, storage, withdrawals, and delayed groundwater response. A rainfall total alone cannot select among them. Compare the same drainage area and time interval, include uncertainty, and look for a missing budget term. Snow temporarily storing winter precipitation can also shift streamflow into a different season without creating new water.
Water availability combines amount, timing, accessibility, and quality. A large ocean inventory does not mean that a nearby community has usable drinking water, and a full reservoir at one date does not establish its long-term balance. An informative map identifies both where water is observed and which exchanges were inferred. Following the boundary consistently makes local measurements connect with the global view from space.
Check 3 of 3
10 pointsUse the reading above. You can try again without losing points.
Put it together
Your lesson progress
Each check connects to a part of the story. Complete all 3 to earn your 25-point lesson bonus.
Go to the source
Explore the evidence behind this lesson. These optional readings and videos open in a new tab.