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Sun, Snow and the City's Dam

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Sun, Snow and the City's Dam

Model how sunlight and gravity move the water that fills a city's taps and spins its turbines.

From the ocean to the tap

Read the passage. Then answer questions 1–3.

Planning for Larkspur's Water and Power

1The city of Larkspur is growing, and its planning board must decide how to supply more water and electricity over the next 30 years. Each year the board invites students to a youth advisory panel, and this year sixth grader Wren Adeyemi was chosen. City water engineer Dariusz Kowal walked the panel through the system. The city and its numbers are made up, but they are based on real water systems.

2Most of Larkspur's water starts in the ocean, about 300 km to the west. Sunlight warms the ocean surface, and some of the water evaporates into water vapor. Winds carry the moist air inland until it reaches the Granite Range. There the air is pushed up the slopes, and as it rises it cools, so much of its water vapor condenses into cloud droplets and ice crystals. In winter, snow piles up on the high slopes.

3In spring and early summer, sunlight melts the snow. The meltwater runs downhill in streams, and some soaks into the ground and seeps slowly into the streams later. The streams fill Clearfork Reservoir, a lake held back by a dam high in the hills above the city. From the reservoir, water drops 150 m through large pipes to turbines in a power plant at the base of the dam, and the spinning turbines make electricity. The water then flows down the Clearfork River, which reaches the ocean. A separate pipe carries drinking water downhill to Larkspur without any pumps.

4In summer, the reservoir loses some water to evaporation. The board is considering covering part of the reservoir with floating solar panels. To test the idea, Mr. Kowal filled two identical test basins at the water plant with reservoir water. Each basin is 10 m long, 10 m wide and 2 m deep, and both sit side by side in full sun. Floating panels covered 70% of one basin; the other was left open. No rain fell, and no water was added, so the drop in water level shows how much water evaporated. A 1 mm drop over 1 m² of water is 1 L.

1.

Sort each step in Larkspur's water system by what mainly drives it.

2.

Which order shows a path that some water could take to reach a Larkspur tap?

A. Ocean → water vapor → reservoir → clouds over the Granite Range → snow → streams → city pipeB. Ocean → Clearfork River → reservoir → water vapor → clouds over the Granite Range → snow → city pipeC. Ocean → water vapor → clouds over the Granite Range → streams → snow → reservoir → city pipeD. Ocean → water vapor → clouds over the Granite Range → snow → streams → reservoir → city pipe

3.

Use paragraph 2. Why does much of the snow fall on the Granite Range rather than on the land between the ocean and the mountains?

A. The rocks of the range give off water vapor, which freezes into snow in winter.B. The air rises and cools over the range, so more of its vapor condenses there.C. The high peaks sit closer to the Sun, so they heat the air until it snows.D. The high peaks pull clouds toward them with gravity, so the clouds drop snow there.

Power from falling water

Use paragraph 3 and Table 1 to answer questions 4–7.

Table 1. Clearfork Dam Power Plant, Last Year
Month	Water through turbines	Electricity made (MWh)
February	6 million m³	2,100
April	20 million m³	7,000
June	30 million m³	10,500
August	10 million m³	3,500

4.

In every month in Table 1, each million m³ of water that dropped through the turbines made MWh of electricity.

5.

Suppose 16 million m³ of water drops through the turbines in May. Based on Table 1, the plant would make about MWh.

6.

Gravity pulls the water down through the turbines. Where did the energy that let the water fall 150 m most likely come from in the first place?

A. Cold winter nights store energy in the snow, and the meltwater carries it to the plant.B. Sunlight evaporated ocean water, and rising air lifted the vapor high over the mountains.C. Heat from deep inside Earth warms the reservoir, which pushes the water into the pipes.D. The turbines make new energy as the water spins them, so no earlier source is needed.

7.

Suppose next winter brings about half the usual snow to the Granite Range. Which prediction for next spring and summer fits the model best?

A. The plant would make more electricity, since less snow would block the streams.B. The plant would make about the same, since the ocean is still the same size.C. Less water would likely reach the reservoir, so the plant would make less electricity.D. The reservoir would overflow in spring, since the snow would melt faster than usual.

Shading the water

Use paragraph 4 and Table 2 to answer questions 8–12.

Table 2. Drop in Water Level in the Test Basins (mm)
Month	Open basin	Basin 70% covered
June	150	60
July	190	75
August	170	70

8.

From June through August, the water level in the covered basin dropped mm less than in the open basin.

9.

Which explanation of Table 2 fits the passage best?

A. The panels shaded the water, so less energy from sunlight reached it to evaporate it.B. The panels caught summer rain and dripped it back, so the covered basin was refilled.C. The panels pressed down on the water, so gravity pushed less of it into the air.D. The panels pulled water vapor out of the air, so it condensed back into the basin.

10.

Why did Mr. Kowal use two identical basins filled with the same water and set side by side in full sun?

A. So the panel cover would be the main difference between the two basinsB. So the open basin would lose the same amount of water as the covered oneC. So the basins would fill with rain at the same rate during the summerD. So the panels could make enough electricity to pump water between them

11.

Use paragraph 4 and your answer to question 8.

From June through August, the panels saved about L of water in the covered test basin.

12.

A board member says, "Our dam's electricity and our tap water come from gravity alone. The Sun has nothing to do with them." Do you agree? Write a claim, give evidence from the passage and both tables, and explain the roles of sunlight and gravity in Larkspur's water system.