Reading passage Grade 12 Science
Earth's Water Budget and Residence Time
A grade 12 science reading passage on how Earth's water is distributed and how residence time explains why slow reservoirs like deep groundwater are hard to protect.
Objectives
- Students will describe how Earth's water is distributed among oceans, ice, and fresh water.
- Students will define residence time and estimate it from storage and flux.
- Students will explain how residence time affects pollution and water supply decisions.
Vocabulary
- Reservoir
- A human-made lake, usually created by a dam, that stores water for later. Reservoirs supply cities and farms and help control floods.
- Residence time
- Residence time is how long a drop of water typically stays in one part of the water cycle before moving on. Water passes through the air quickly but can stay underground for a very long time.
- Aquifer
- An aquifer is an underground layer of rock, sand, or gravel that holds water in the tiny spaces between its grains. Wells reach down into aquifers to bring water up to the surface.
- Recharge
- Recharge is water soaking down through the ground to refill an aquifer, the way rain refills an underground sponge. Aquifers stay healthy only if recharge keeps up with pumping.
Also: flux
Teacher overview
This passage gives students a systems view of the water cycle built around one quantitative idea: residence time. It explains how Earth's water is distributed, defines residence time as storage divided by flux, and shows why slow reservoirs such as deep groundwater are so hard to clean or refill. The reading pairs well with a unit on Earth systems or water resources, and the questions ask students to combine the prose with the residence-time table.
Passage
Earth looks like a water planet from space, yet the water people can actually use is a thin slice of the total. About 97 percent of Earth's water sits in the oceans as salt water. Roughly 2 percent is locked in ice caps and glaciers. That leaves less than 1 percent as fresh water that people can readily reach. This small share has to supply every farm, city, and factory on the planet.
Water is never still. It evaporates from oceans and lakes, forms clouds, falls as rain or snow, and flows back toward the sea. Scientists describe this motion using two ideas: reservoirs and fluxes. A reservoir is a place where water is stored, such as the atmosphere, a river, or an aquifer. A flux is a flow of water into or out of a reservoir. Evaporation is a flux out of the ocean, and precipitation is a flux into the land. The same water moves through many reservoirs over its lifetime.
One number ties reservoirs and fluxes together. It is called residence time, and it is the average time a water molecule spends in a reservoir before it leaves. You can estimate residence time by dividing the amount of water stored in a reservoir by the rate of flow through it. A large reservoir drained by a slow flux has a long residence time. A small reservoir with a fast flux has a short one. The same simple ratio works for the atmosphere, a lake, or a deep aquifer.
The differences among reservoirs are striking. The atmosphere holds only a little water at any moment, and that water cycles quickly. On average, a molecule spends about 9 days in the atmosphere before it falls as precipitation. Rivers hold even less water than the atmosphere, but far less water flows through them each year, so river water has a residence time measured in weeks. Deep groundwater is the opposite. Water can sit in a deep aquifer for centuries, and in some places for thousands of years, because the flow through it is very slow.
| Reservoir | Typical residence time |
|---|---|
| Atmosphere | about 9 days |
| Rivers | weeks |
| Deep groundwater | centuries to millennia |
Residence time matters a great deal for pollution. A pollutant that reaches a fast reservoir, such as a river, is flushed downstream in a matter of weeks. The same pollutant in a deep aquifer may remain for generations, because the water around it barely moves. This is one reason groundwater contamination is so hard to reverse. Cleaning a slow reservoir can take far longer than cleaning a fast one, and sometimes the practical choice is to protect the source rather than try to restore it later.
Residence time also shapes how communities use water. A town that draws from a river relies on a supply that refills quickly but can run low during a drought. A town that pumps ancient groundwater is spending a reserve that refills slowly, sometimes over thousands of years. When pumping outpaces recharge, the water table drops, the remaining water sits deeper, and it costs more energy to lift. A reservoir with a long residence time is easy to overdraw because it does not refill on a human timescale.
Demand keeps pressure on this thin supply. Agriculture accounts for about 70 percent of the fresh water that people withdraw worldwide, and cities and industry take much of the rest. Because usable fresh water is less than 1 percent of Earth's total, and because part of it refills only over centuries, planners treat fresh water as a limited resource rather than an endless one. Understanding residence time helps them decide which sources can be used freely and which must be guarded for the long term.
Questions
According to the passage, about what share of Earth's water is in the oceans, and about how much is fresh water that people can readily reach?
What is the average residence time of water in the atmosphere?
Using the reservoir-and-flux idea from the passage, explain what residence time means and how you would estimate it.
Explain why a pollutant that reaches deep groundwater tends to last far longer than one that reaches a river.
The passage states that agriculture withdraws about 70 percent of the fresh water people use. Connect this fact to why the passage treats fresh water as a limited resource.
Using the table together with the passage, rank the atmosphere, rivers, and deep groundwater from fastest to slowest by how quickly water moves through them, and explain your ranking.
Define "flux" as the passage uses it, and give one input flux and one output flux for the atmosphere.
From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.
Answer Key
Answer: About 97 percent is in the oceans, and less than 1 percent is reachable fresh water.
The first paragraph states that about 97 percent of Earth's water is ocean salt water and that less than 1 percent is fresh water people can readily reach.Answer: About 9 days.
The fourth paragraph and the table both give the atmosphere a residence time of about 9 days.Answer: It is the average time water stays in a reservoir, found by dividing storage by flux.
The passage defines residence time as the average time a molecule spends in a reservoir, estimated by dividing the water stored in the reservoir by the rate of flow through it.Answer: Deep groundwater has a very long residence time because it moves so slowly.
The passage explains that water in a deep aquifer barely moves, so a pollutant can stay for generations, while a river flushes a pollutant out in weeks.Answer: High demand meets a small, slowly refilling supply.
Because usable fresh water is less than 1 percent of Earth's total and part of it refills only over centuries, a single use like agriculture taking about 70 percent of withdrawals shows how heavily a limited supply is drawn upon, which is why planners treat it as finite.Answer: Atmosphere fastest, rivers next, deep groundwater slowest.
The table gives the atmosphere about 9 days, rivers weeks, and deep groundwater centuries to millennia, so water moves fastest through the atmosphere and slowest through deep groundwater.Answer: A flux is a flow into or out of a reservoir; for the atmosphere, evaporation is an input and precipitation is an output.
The passage defines a flux as a flow of water into or out of a reservoir and names evaporation and precipitation as examples, with evaporation adding water to the atmosphere and precipitation removing it.
Differentiation
Support
Provide a labeled diagram of the water cycle with the reservoirs and fluxes named, and let students answer the literal questions by pointing to the matching part of the diagram before writing.
Extension
Ask students to estimate a rough residence time for a reservoir not in the table, such as a lake, given a stated storage volume and inflow rate, and explain how their estimate compares with the atmosphere and deep groundwater.
ELL supports
Pre-teach "reservoir," "flux," "residence time," and "recharge" with simple definitions and a picture for each. Allow students to answer questions 1 and 2 with short phrases and to use a sentence frame for question 3.
Extensions
Have students find a reported residence time for a specific aquifer or lake, compare it with the values in the table, and write a short explanation of what that residence time means for protecting the source from pollution.