Unit 1 · Topic 1.7 Beta

The Hydrologic (Water) Cycle

The water cycle moves water among the ocean, air, land, ice and groundwater.

Practice 1: Concept ExplanationPractice 2: Visual Representations

Part 1 · Hook

Why this matters

The water in your glass may have fallen on a mountain last month, been pumped from rock where it sat for a thousand years, or risen from a tree leaf yesterday. Water never runs out globally, but where it is, how fast it moves, and how much of it is fresh and reachable decide whether a town has enough.

Part 2 · Before you start

What this builds on

Part 3 · Prerequisite check

Quick check before you start

1. What is a reservoir in a cycle?

A place where something is stored for a time
A movement between two stores
A living thing
A kind of rock

Show the answer

A reservoir stores; a flux moves.

Correct: A place where something is stored for a time:
A movement between two stores:
A living thing:
A kind of rock:

2. What does leaching do to nitrate?

Washes it down through the soil with water
Turns it into N2 gas
Fixes it from the air
Locks it in rock

Show the answer

Leaching carries dissolved substances down through soil.

Correct: Washes it down through the soil with water:
Turns it into N2 gas:
Fixes it from the air:
Locks it in rock:

Part 4 · See it

See it first

The water cycle. The sun drives evaporation from the ocean and transpiration from plants. Water vapor cools and condenses into clouds. Precipitation falls on land. Some water flows over the surface as runoff to the ocean. Some soaks into the ground by infiltration and becomes groundwater, stored in an aquifer, a layer of rock and sediment whose pores hold water; groundwater flows slowly toward the ocean. — The water cycle: evaporation, transpiration, condensation, precipitation, runoff and infiltration into an aquifer. LevlPrep original diagram.

Part 5 · Step by step

How it works, step by step

The sun heats water and plants transpirewater vapor rises into the air
Rising vapor cools and condensesclouds form and precipitation falls
On land, water either flows over the surface or soaks inrunoff feeds rivers; infiltration refills groundwater in aquifers

Part 6 · Key ideas

Key ideas

The hydrologic cycle is driven by the sun (evaporation, transpiration) and gravity (precipitation, runoff, infiltration).
About 96.5% of Earth’s water is in the ocean; most freshwater is ice or groundwater, and lakes and rivers hold a tiny share.
An aquifer is porous rock or sediment that holds groundwater; deep aquifers refill very slowly.
Pavement and cleared land mean less infiltration and more runoff: more floods and less groundwater recharge.

Part 7 · Misconception

A common mistake

The wrong idea: Groundwater is an underground lake or river.

What actually happens: Most groundwater fills the tiny spaces between grains of sand, gravel and rock in an aquifer, like water in a sponge. It moves slowly, often centimeters to meters a day.

Part 8 · Check yourself

Check yourself

Exam-style questions. Anything you miss goes into your review queue.

1. What is an aquifer?

Porous rock that holds groundwater
The area of land that drains into one river
A large lake formed behind a dam on a river
A low cloud that is full of water droplets

Show the answer

An aquifer is porous rock or sediment that stores and transmits groundwater, which can be pumped from wells.

Correct: Porous rock that holds groundwater: Right: an underground layer that holds and moves groundwater.
The area of land that drains into one river: That describes the land that drains to one river, which is a different idea from an aquifer.
A large lake formed behind a dam on a river: A lake behind a dam is a surface reservoir.
A low cloud that is full of water droplets: Clouds are in the air, not underground.

Model or diagram

Where Earth’s water is stored

2. About what share of Earth’s water is in the oceans?

About 96.5%
About 50.0%
About 3.5%
About 1.7%

Show the answer

The ocean bar is at about 96.5%.

Correct: About 96.5%: Right: almost all water is salty ocean water.
About 50.0%: The ocean bar is far above half.
About 3.5%: 3.5% is roughly everything that is not ocean.
About 1.7%: 1.7% is the groundwater (or ice) bar.

3. Earth has about $1.39 \times 10^{9}$ cubic kilometers of water. Using the chart, about how many cubic kilometers are groundwater? Use 1.7%.

Type a number, then its unit (km^3). Scale words (2.5 million) and powers of ten (2.5 × 10^6) are read.

Show the answer

$\frac{1.7}{100} \times 1.39 \times 10^{9}$ $= 0.017 \times 1.39 \times 10^{9}$ $= 2.4 \times 10^{7}$ km³ (about 23.6 million cubic kilometers; 2.4 × 10⁷ and 2.36 × 10⁷ are both accepted).

Answer: 23,630,000 km^3

4. Which conclusion about freshwater does the chart support?

Most freshwater is ice or groundwater.
Lakes and rivers hold most of the freshwater on Earth.
Freshwater makes up about half of the water on Earth.
Groundwater is the largest of the water reservoirs.

Show the answer

Ice and groundwater are about 1.7% each, while lakes, rivers, soil and air together are well under 0.1%.

Correct: Most freshwater is ice or groundwater.: Right: ice and groundwater dwarf surface freshwater.
Lakes and rivers hold most of the freshwater on Earth.: Lakes and rivers are in the smallest bar.
Freshwater makes up about half of the water on Earth.: Freshwater is only a few percent of all water.
Groundwater is the largest of the water reservoirs.: The oceans hold far more water than groundwater.

Data

Where rain goes on two kinds of land

What happens to 100 cm of yearly rain on a forested hillside and on a paved parking lot nearby (invented data for practice)
Path	Forest (cm)	Parking lot (cm)
Evaporation and transpiration	50	25
Runoff over the surface	10	70
Infiltration into the ground	40	5

5. How does paving change where rain goes, according to the table?

Runoff rises and infiltration falls.
Infiltration rises from 40 to 70 cm, and runoff falls.
Evaporation and transpiration double, from 25 to 50 cm.
Evaporation changes, but runoff and infiltration stay put.

Show the answer

On the parking lot, runoff is 70 cm instead of 10, and infiltration is 5 instead of 40.

Correct: Runoff rises and infiltration falls.: Right: much more runoff, much less infiltration.
Infiltration rises from 40 to 70 cm, and runoff falls.: The table shows the opposite.
Evaporation and transpiration double, from 25 to 50 cm.: They fall from 50 to 25 cm.
Evaporation changes, but runoff and infiltration stay put.: Runoff and infiltration change the most of the three paths.

6. What percent of the forest’s yearly rain infiltrates the ground?

Type a number, then its unit (%). Scale words (2.5 million) and powers of ten (2.5 × 10^6) are read.

Show the answer

$\frac{40}{100} \times 100 = 40\%$ .

Answer: 40 %

7. Based on the table, what happens to the groundwater below a town as more land is paved?

It refills more slowly.
It is refilled faster, because the runoff soaks in.
It does not change, because the rain is the same.
It turns salty from chemicals on the pavement.

Show the answer

Infiltration falls from 40 to 5 cm a year, so far less water reaches the groundwater.

Correct: It refills more slowly.: Right: less infiltration means less recharge.
It is refilled faster, because the runoff soaks in.: Runoff flows away over the surface rather than soaking in.
It does not change, because the rain is the same.: The same rain is split differently, so recharge changes.
It turns salty from chemicals on the pavement.: Paving does not make groundwater salty by itself.

Part 9 · Summary

Summary

The water cycle moves water among the ocean, air, land, ice and groundwater. The sun drives evaporation and transpiration; vapor condenses into clouds and falls as precipitation; on land it runs off to rivers or infiltrates to recharge aquifers. Almost all water is salty ocean water, and most freshwater is ice or groundwater. Paving increases runoff and reduces recharge.

Part 10 · Up next

What comes next

Part 11 · Connections

Connections

Question set for this topic