Lesson 2 of 6 · Atmosphere, hydrosphere & cryosphere
Water connects the air and ground
8 minute lesson · 3 checks along the way
Your goal for this lessonTrace water through several possible pathways and balance the inputs, outputs, and storage of a simple watershed.
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One substance, several places and states
Earth’s includes water in oceans, lakes, rivers, soil, underground spaces, and the atmosphere. Frozen water is often studied separately as the , including snow, glaciers, ice sheets, and sea ice. These categories overlap deliberately. They let scientists focus on particular processes without forgetting that ice, liquid water, and water vapor are different states of the same substance.
The atmosphere is a mixture of gases surrounding the planet. Water vapor is the invisible gaseous form of water; a visible cloud contains tiny liquid droplets, ice crystals, or both. A photograph of white cloud does not show a tank of visible vapor. Separating the gas from the condensed droplets helps explain why moisture can be present even where the sky looks clear.
Check 1 of 3
10 pointsUse the reading above. You can try again without losing points.
Follow pathways instead of one circle
Energy from the Sun supports evaporation from water and land. Plants also release water vapor through . When vapor condenses, water transfers energy to its surroundings; when liquid evaporates, it takes in energy. Air transports moisture, and precipitation returns water toward the surface. Gravity helps move water downhill and through the ground. The water cycle therefore transports energy as well as material.
Rain does not have to complete the same sequence each time. It may run into a stream, infiltrate soil, remain as snow, or reach an aquifer. Groundwater occupies pores and fractures; it is not always an underground river. Water may return quickly to the air or stay stored for a long time. Pathways and residence times depend on the setting, including soils, rocks, vegetation, and human use.
DSCOVR’s EPIC camera observed this sunlit Earth on July 6, 2015. The color view combines red, green, and blue measurements. Clouds, ocean, and land occupy one connected scene; groundwater and invisible atmospheric water vapor require other observations.
Look closely
White clouds are visible above the ocean. Does a clear patch of sky necessarily contain no water, and does this picture measure all of Earth’s water stores?
See what to notice
Clear air can contain invisible water vapor. A visible-light view also does not directly measure groundwater or the water held inside soil. The image shows useful patterns, while a complete water account combines several kinds of observation with defined boundaries and units.
Work through a decision
Balance a small watershed
A is the area draining toward a chosen outlet. For a hypothetical month, suppose 100 units of water enter a watershed, 60 leave through the outlet, and 30 return to the atmosphere. If all other crossings of the boundary are already included or negligible, how does storage change? It increases by 10 units: inputs minus outputs. The units must represent the same amount of water throughout.
A positive storage change might be divided among soil moisture, groundwater, snow, and surface water. The calculation alone does not tell us which store gained it. If a later survey reports no change anywhere, investigate missing flows or measurement uncertainty before claiming water vanished. The example is an accounting exercise with invented numbers, not a forecast or measurement of a real basin.
Check 2 of 3
10 pointsUse the reading above. You can try again without losing points.
Connect an orbital view with measurements below
Satellites can observe cloud patterns, surface water, snow, and other clues over large regions. Ground instruments add rainfall totals, stream discharge, and water levels in wells. Neither viewpoint replaces the other. A bright patch in a photograph needs context before being labeled cloud or snow, while a single rain gauge cannot represent every mountain slope and valley across a large watershed.
Changing land cover can alter the pathways. Paving may reduce and send more water rapidly into drains, while pumping can remove groundwater faster than it is replenished. These effects depend on local conditions; they do not mean water leaves Earth when it leaves a town. Explain one pathway from rain to a river and a second that passes through the ground. Identify the boundary and timescale for each explanation.
Check 3 of 3
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Put it together
Your lesson progress
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