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A Desert Lake's Water Budget

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A Desert Lake's Water Budget

Follow the water in and out of a lake with no outlet.

The lake's water budget

Read the passage and study the budget table. Then answer the questions.

The Ring on the Rocks

1Sagebrush Lake lies in a desert basin in Nevada, with snowy mountains rising to its west. The lake and its numbers are made up, but they are based on real desert lakes. Each July, a summer kayak camp paddles along its shore. On the first morning, camper Inés Galarza noticed a white crust on the rocks about half a meter above the water. Ranger Dmitri Volkov explained that it is a ring of salt, left behind where the water once stood higher.

2The Silver River carries water from the mountains down into the lake, but no river flows out. River water holds tiny amounts of dissolved salts that it picks up from soil and rock along the way. The water of Sagebrush Lake is many times saltier than the river water. The lake is about 8 m deep in the middle.

3Rangers keep a water budget for the lake, much like a budget for money. They measure the water flowing in from the river and the rain and snow that fall on the lake itself. They estimate how much water evaporates from the lake's surface using a pan of water on the shore and weather data. Very little lake water soaks into the ground, so the budget leaves it out. For this lake, a change of 2 million cubic meters (m³) of water raises or lowers the lake level by about 1 cm.

4Most of the river's water starts as snow that piles up in the mountains from November through March. Ranger Volkov told the campers that last year was drier than usual in the mountains.

Sagebrush Lake Water Budget, Last Year (million m³)
Season	River in	Rain and snow on lake	Evaporation	Change in lake water
Winter (Dec–Feb)	20	15	10	+25
Spring (Mar–May)	90	10	40	+60
Summer (Jun–Aug)	30	2	110	−78
Fall (Sep–Nov)	15	8	50	−27

1.

Last year, the river and the rain and snow added million m³ of water to the lake in all, and million m³ evaporated.

2.

Use paragraph 3 and the table.

During the summer, the lake level dropped by about cm. By the end of the whole year, the lake was about cm lower than at the start.

3.

Why does the river bring the most water into the lake in spring?

A. Cool spring air sinks into the river channel and turns straight into liquid water.B. Spring rain falls mostly on the lake, which pushes extra water up the river.C. Gravity is stronger in spring, so it pulls more water vapor out of the mountain air.D. Spring sunlight melts the mountain snow, and gravity pulls the meltwater downhill.

4.

Which explanation best fits the evaporation column of the table?

A. In summer, more energy from sunlight warms the lake, so more water turns to vapor.B. The lake is deepest in summer, so more of its water can rise into the air.C. Summer winds blow drops of lake water onto the shore, where they soak in.D. The river brings hot water in summer, and that water boils off the lake.

5.

The river water is nearly fresh, but the lake water is very salty. Which explanation fits the passage and the budget best?

A. Salty water flows up the river from the lake in fall and settles in the mountains.B. Water leaves the lake mainly by evaporating, and the salts stay behind in the lake.C. Plants and fish in the lake make new salt as they grow each summer.D. Salt evaporates along with the water and falls back on the lake as salty rain.

Model the lake's water cycle

Use the passage, the table and your answers above.

6.

Complete this model of one path some of the lake's water can take. Use each word from the word bank once or not at all.

Sunlight warms the lake, and water into the air as vapor. Winds carry the moist air over the mountains, where it cools and the vapor into tiny cloud droplets. Snow falls on the peaks as . In spring the snow melts, and pulls the water down the Silver River and back into the lake.

7.

Suppose next winter brings twice as much snow to the mountains, and the summer is as sunny as last year. What is the most likely change in next year's budget?

A. Summer evaporation drops close to zero, because the extra snow keeps the lake cold.B. Spring river flow stays near 90 million m³, because the river bed is already full.C. Spring river flow rises, so the lake is likely to end the year higher than it started.D. The lake falls faster in spring, because the extra snow covers the lake's surface.

8.

A camper says, "The water that evaporates from Sagebrush Lake is gone for good." Which response fits the model best?

A. The vapor moves with the air until it condenses and falls, perhaps on the mountains.B. The vapor turns into salt on the rocks, which forms the white ring around the lake.C. The vapor floats out into space, since water vapor is lighter than the rest of the air.D. The vapor sinks straight into the desert soil and becomes groundwater right away.

9.

Sort each event by how the water moves.

10.

Inés says, "The lake drops every summer, so it will dry up in a few years." Do you agree? Write a claim, give evidence from the budget table and the passage, and explain your reasoning. Name the roles of sunlight and gravity in your answer.