Answer key: Water Cycle Journey Activity

For teachers. This page is hidden from search engines and not linked from student pages, but it is not password-protected, so please don’t share the link with students. Student worksheet: Water Cycle Journey Activity.

Goal: Follow one water droplet through Earth’s reservoirs, identify the processes that move it and what drives them, and compare how long water stays in each place.

Connects to NGSS MS-ESS2-4 (Develop a model to describe how water cycles through Earth’s systems, driven by energy from the sun and the force of gravity.); 5-ESS2-2 (Describe and graph the amounts and percentages of salt water and fresh water in Earth’s reservoirs as evidence of how water is distributed on Earth.)

Completed results table

Chance code 10 always makes this journey from the ocean in today’s climate. If a table differs, check that the code is 10, the climate is Today and the droplet started in the ocean.

A river. Total time: about 3,260 years (3,200 years in the ocean, 60 years in a mountain glacier and only about 2.3 months at all the other stops).

Answers

1. In which place did your droplet spend the most time? Why does water stay there so long?

   The ocean: about 3,200 years. The ocean is so big (about 96.5% of Earth’s water) that, even though a lot evaporates every year, an average droplet waits thousands of years for its turn.

2. Sort the processes on your journey: which were powered by the sun’s energy, and which by gravity?

   Sun’s energy: evaporation, uptake by roots, transpiration and melting. Gravity: precipitation and snow accumulation (rain and snow fall because of gravity). Condensation happens when vapor cools, before the rain or snow falls.

3. About what percent of Earth’s water is in the ocean? What percent is fresh? Where is most of the fresh water?

   About 96.5% is in the ocean, and only about 2.5% is fresh. About 68.7% of the fresh water is ice and snow and 30.1% is groundwater, so lakes, rivers, the air and living things share only about 1.2% of it.

4. Try Colder and Warmer. When a droplet leaves the air, how does the chance of snow on a glacier change? What would happen to sea level in a much colder climate?

   Today 1%, colder 2.9%, warmer 0.5%. In a colder climate more water falls as snow and stays frozen for a long time, so less flows back to the ocean and sea level falls, as it did in the last ice age.

5. Thought experiment: switch off the sun’s energy, start the droplet in the soil and let chance decide until it stops. Where does it end up, and why?

   It always ends in the ocean and stays there. With no energy from the sun nothing evaporates, transpires or melts, so gravity can only move water downhill until it reaches the sea.

6. In this model, which place holds a droplet the longest on average, and which the shortest?

   Longest: an ice sheet, about 20,000 years (Antarctica). Shortest: living things, about 1 week, then the atmosphere, about 9 days.