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Mission Briefing: Earth's Yearly Water Budget

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Mission Briefing: Earth's Yearly Water Budget

Use a satellite team's water budget to model how sunlight and gravity move water.

The yearly water budget

Read the briefing and Table 1, then answer the questions.

Planning a Water-Watching Satellite

1Sixth grader Soren Adeyemi won a spot at a summer program run by a national space agency. The agency team, the people and the briefing here are made up, but the water amounts are rounded from real scientific estimates. On the last day, Soren sat in on a planning briefing for a new Earth-watching satellite. Its job will be to measure how fast water moves between the ocean, the air and the land.

2Mission scientist Dr. Ngozi Halvard began with the water budget the team wants to check. The numbers are yearly totals for the whole planet, in thousands of cubic kilometers. One thousand cubic kilometers of water would fill a cube 10 km on each side. Over many years, the ocean, the air and the land each gain about as much water as they lose.

3Dr. Halvard explained that sunlight warms the surface of the ocean and the land. Water at the surface evaporates and rises into the air as water vapor. Plants also release water vapor from their leaves, and sunshine melts snow and ice. When moist air rises and cools, the vapor condenses into the tiny drops of clouds. Drops and ice crystals that grow large enough fall as rain or snow.

4On land, some rain and melted snow flow downhill in streams and rivers. Some soak down through soil and rock into groundwater, which slowly seeps toward the sea. In high mountains, snow packs into glaciers that creep downhill. The satellite will also measure how much sunlight the ocean surface absorbs, because the team wants to compare it with evaporation in different parts of the world.

Table 1. Earth's Yearly Water Budget (rounded)
Water flow	Thousand km³ per year
Evaporation from the ocean	413
Rain and snow on the ocean	373
Vapor blown from ocean to land	?
Evaporation and plants on land	73
Rain and snow on land	113
Rivers and groundwater to ocean	?

1.

Use Table 1. The air over the ocean gains and loses about the same amount of water each year. Vapor that does not fall back on the ocean is blown over land.

How much water vapor is blown from the ocean to the land each year? thousand km³

2.

Use Table 1. The land gains and loses about the same amount of water each year. Water that falls on land and does not evaporate flows to the ocean.

How much water flows to the ocean in rivers and groundwater each year? thousand km³

3.

Your answers to questions 1 and 2 are the same number. What does this show about the ocean over many years?

A. The rivers carry water vapor back to the ocean, so rain is not part of the cycle.B. The water it loses to the land as vapor comes back in rivers and groundwater.C. The ocean gains 40 thousand km³ each year from rivers, so it is slowly rising.D. The ocean loses 40 thousand km³ each year, so it is slowly drying up.

4.

Which flow in Table 1 moves the most water each year, and what mainly drives it?

A. Rain and snow on the ocean, driven by gravity pulling drops from cloudsB. Rivers and groundwater, driven by sunlight pushing water toward the seaC. Evaporation from the ocean, driven by sunlight warming the water's surfaceD. Rain and snow on land, driven by sunlight lifting the drops out of clouds

5.

Rain and snow on land (113) are larger than evaporation and plant release on land (73). Which explanation fits the budget best?

A. Winds carry extra water vapor in from the ocean, and some of it falls on land.B. Groundwater rises into the clouds over land and falls again as extra rain.C. Rivers flow uphill into the mountains, where their water falls again as snow.D. Plants turn sunlight into new water, which then rises and falls as rain.

Sunlight, evaporation and gravity

Use Table 2 and the briefing to answer the questions.

Table 2. Ocean Sunlight and Evaporation (rounded yearly averages)
Latitude band	Sunlight absorbed (W/m²)	Evaporation (cm/year)
Tropics (0° to 30°)	220	150
Middle (30° to 60°)	140	90
Polar (60° to 90°)	60	20

6.

Evaporation from the tropical ocean is how many times evaporation from the polar ocean? Do not round. times

7.

Which pattern does Table 2 show?

A. Evaporation is about 90 cm/year in each band, whatever sunlight it absorbs.B. The middle band loses more water to evaporation than the tropics do each year.C. The polar ocean absorbs the least sunlight but loses the most water to evaporation.D. Bands where the ocean absorbs more sunlight tend to lose more water to evaporation.

8.

The satellite will also study a cold ocean region that absorbs about 100 W/m² of sunlight. Based on Table 2, about how much evaporation would you predict there?

A. About 55 cm/yearB. About 10 cm/yearC. About 160 cm/yearD. About 120 cm/year

9.

Use paragraphs 3 and 4. Sort each process by what mainly drives it.

10.

Which order best models one path a bit of water could take from the ocean to a mountain and back?

A. Falls as snow → evaporates from the ocean → melts in spring → condenses in a cloud → flows down a riverB. Evaporates from the ocean → flows down a river → condenses in a cloud → falls as snow → melts in springC. Evaporates from the ocean → condenses in a cloud → falls as snow → melts in spring → flows down a riverD. Condenses in a cloud → evaporates from the ocean → melts in spring → falls as snow → flows down a river

11.

Suppose a thick layer of volcanic ash high in the air blocked much of the sunlight over the tropical ocean for a year. Based on both tables, which prediction fits best?

A. Evaporation there would rise, since a darker sky keeps the ocean surface warmer.B. Less water would likely evaporate there, so less vapor would be blown to the land.C. Evaporation would stay near 150 cm/year, since evaporation depends on gravity.D. Rivers on land would flow faster, since gravity would get stronger without sunlight.

12.

A mission planner says, "The water cycle runs on sunlight. Gravity just holds the ocean in place." Do you agree? Write a claim, give evidence from both tables and the briefing, and explain how both sunlight and gravity keep water cycling.