1. Introduction
In Unit 1, we discussed water’s chemistry and properties. Now we’re going to zoom out and look at water on a planetary scale.
The Earth has about 1.386 billion cubic kilometers of water. That water makes up the hydrosphere: that’s all the water on, under, and above the Earth’s surface. It includes oceans, rivers, lakes, groundwater, glaciers, and water vapor in the atmosphere.
Here’s how that water is distributed.
| Source | % of Total Water | Description |
|---|---|---|
| Oceans | ~96.5% | Salt water in seas and oceans |
| Glaciers & Ice Caps | ~1.7% | Mostly in Greenland and Antarctica |
| Groundwater | ~1.7% | Includes both fresh and saline groundwater |
| Lakes & Rivers | ~0.013% | Surface freshwater |
| Atmosphere | ~0.001% | Water vapor and clouds |
| Biological Water | Negligible | Water in plants, animals, and microbes |
The sources above are the reservoirs where water is found. The water cycle describes the movement or fluxes of water between these reservoirs through evaporation, condensation, precipitation, and transpiration. Let’s see how this cycle works.
2. The water cycle
The water cycle is distinct from the carbon or nitrogen cycles in that water never changes its chemical form. It’s always H2O. That means that for the purposes of understanding the water cycle, we don’t track biological processes in which water is broken down (such as occurs during the light reactions of photosynthesis) or created (during dehydration synthesis reactions).
There are two types of transformations that we do track.
- Changes in water’s physical phase — from liquid water to ice (a solid) or to water vapor (a gas).
- Changes in salt content — from saline water (in the oceans) to freshwater (in lakes, rivers, and streams). Note that there’s also brackish water (water with less salt than ocean water) found in marshes and estuaries, but for the purposes of AP Bio, we’re not going to include it in the discussion below).
The water cycle is driven by the sun’s heat. On the oceans (A), rivers (B) lakes (C), and land (not lettered), solar heat drives evaporation (1). Gaseous water vapor rises into the atmosphere. As water-carrying air rises, it cools, and its ability to hold water vapor decreases. As a result, the vapor condenses (2) onto tiny particles in the air, forming clouds (E). By the way, now you know something that most people misunderstand. Clouds aren’t made of water vapor. They’re made of tiny droplets of liquid water floating in the air.
About 60-70% of the moisture in the atmosphere comes from evaporation, most of which is from the oceans. Another flux of water into the air (about 30 – 40%) is from transpiration (3). Transpiration is the process by which water moves from the soil, into a plant’s roots, and then up through stems and out through leaves. Click here for a tutorial that covers the details of transpiration and water movement within plants.
When the water droplets in clouds becomes too heavy to stay suspended in the air, gravity pulls them to Earth as precipitation (4). The water can fall as rain, snow or hail.
Snow on a mountaintop is shown at letter “F.” An additional (and enormous) reservoir of frozen water can be found in the polar ice caps at the Earth’s north and south poles (not shown above). When snow and ice melts, this causes runoff (5). Rain falling on the land also becomes runoff, which flows into streams and rivers (B), which feed lakes (C), or which can flow directly into the oceans (A).
Some of this water percolates down below the surface to become groundwater (D). Groundwater flow (6) will bring groundwater to lakes, rivers, and oceans. Together, water in the ground, in rivers, in lakes, and frozen on the surface makes up the planet’s reservoir of freshwater, which is only about 2.5% of the Earth’s water.
3. Water cycle flashcards
[qdeck qrecord_id=”sciencemusicvideosMeister1961-water cycle flashcards”]
[h]Water Cycle Flashcards
[i]
[q]In the diagram below, identify the fluxes.
[a]1 – evaporation; 2 – condensation; 3- transpiration; 4-precipitation; 5-runoff; 6-groundwater flow
[q]In the diagram below, identify the reservoirs
[a]A – oceans; B – rivers; C- lakes; D-groundwater; E-clouds; F-ice
[q]What is the hydrosphere?
[a]All the water on, under, and above the Earth’s surface — including oceans, rivers, lakes, groundwater, glaciers, and water vapor in the atmosphere.
[q]What are water cycle reservoirs? List 5
[a]Places where water is stored: oceans (A), glaciers and ice caps (F), groundwater (D), lakes (C) rivers (B), the atmosphere (E). Biological organisms (not shown) also store a relatively minute quantity of water.
[q]What is the largest water reservoir on Earth?
[a]The oceans (~96.5% of all Earth’s water).
[q]What percent of Earth’s water is freshwater?
[a]About 2.5%.
[q]Which reservoirs store Earth’s freshwater?
[a]Glaciers & polar ice caps, groundwater, lakes, and rivers.
[q]What are the four main processes that move water between reservoirs in the water cycle?
[a]Evaporation (1), condensation (2), transpiration (3), precipitation (4), runoff (5) and groundwater flow (6).
[q]What drives the water cycle?
[a]The sun’s heat.
[q]What happens during evaporation?
[a]Liquid water becomes water vapor (gas), usually from oceans, lakes, rivers, and land surfaces.
[q]What happens during condensation?
[a]Water vapor cools and changes back into liquid water, forming clouds.
[q]What are clouds made of?
[a]Tiny droplets of liquid water (not water vapor).
[q]What is transpiration?
[a]The process by which water moves from soil, through plants, and out through their leaves into the atmosphere.
[q]Where does atmospheric moisture come from?
[a]About 60–70% from evaporation (mostly oceans), and 30–40% from transpiration.
[q]What is precipitation?
[a]Water falling from clouds as rain, snow, or hail, when droplets become too heavy to stay suspended.
[q]What is runoff?
[a]Water from rain or melting snow that flows over land into streams, rivers, lakes, and oceans.
[q]What is groundwater?
[a]Water that infiltrates the soil and percolates down into underground reservoirs.
[q]What is unique about the water cycle compared to the carbon or nitrogen cycles?
[a]Water doesn’t change its chemical form — it’s always H₂O.
[q]What two types of transformations occur in the water cycle?
[a]Changes in physical phase (between liquid, solid, and gas) and changes in salt content (between freshwater and saltwater).
[q]Identify 1 – 5 in the diagram below
[a]1-evaporation; 2-condensation into clouds; 3-transpiration; 4- precipitation; 5-runoff
[/qdeck]
4. Water cycle quiz
[qwiz random=”true” qrecord_id=”sciencemusicvideosMeister1961-water cycle quiz”]
[h] Water cycle quiz
[q] In the diagram below, evaporation is represented by which letter or number?
[textentry single_char=”true”]
[c]ID E=[Qq]
[f]IEV4Y2VsbGVudCEgTnVtYmVyIDEgc2hvd3MgZXZhcG9yYXRpb24u
[c]ICo=[Qq]
[f]IE5vLiBFdmFwb3JhdGlvbiBpcyB3aGVuIGxpcXVpZCB3YXRlciBlbnRlcnMgdGhlIGF0bW9zcGhlcmUgYXMgd2F0ZXIgdmFwb3Iu
[q] In the diagram below, transpiration is represented by which letter or number?
[textentry single_char=”true”]
[c]ID M=[Qq]
[f]IEV4YWN0bHkuIFRyYW5zcGlyYXRpb24gaXMgc2hvd24gYnkgbnVtYmVyIDMg4oCUIHdhdGVyIHZhcG9yIGV4aXRpbmcgcGxhbnQgbGVhdmVzLg==
[c]ICo=[Qq]
[f]IE5vLiBUcmFuc3BpcmF0aW9uIGlzIHRoZSBtb3ZlbWVudCBvZiB3YXRlciB2YXBvciBmcm9tIHBsYW50cyBpbnRvIHRoZSBhdG1vc3BoZXJlLg==
[q] In the diagram below, precipitation is represented by which letter or number?
[textentry single_char=”true”]
[c]ID Q=[Qq]
[f]IFllcyEgTnVtYmVyIDQgc2hvd3MgcHJlY2lwaXRhdGlvbiDigJQgd2F0ZXIgZmFsbGluZyBmcm9tIGNsb3VkcyB0byBFYXJ0aC4=
[c]ICo=[Qq]
[f]IE5vLiBQcmVjaXBpdGF0aW9uIGlzIHdoZW4gd2F0ZXIgZmFsbHMgdG8gdGhlIHN1cmZhY2UgYXMgcmFpbiBvciBzbm93Lg==
[q] In the diagram below, runoff is represented by which letter or number?
[textentry single_char=”true”]
[c]ID U=[Qq]
[f]IEdyZWF0ISBSdW5vZmYgaXMgc2hvd24gYnkgbnVtYmVyIDUg4oCUIHN1cmZhY2Ugd2F0ZXIgZmxvd2luZyB0b3dhcmQgbGFrZXMgb3Igb2NlYW5zLg==
[c]ICo=[Qq]
[f]IE5vLiBSdW5vZmYgaXMgd2F0ZXIgZmxvd2luZyBhY3Jvc3MgdGhlIGxhbmQu
[q] In the diagram below, condensation is represented by which letter or number?
[textentry single_char=”true”]
[c]ID I=[Qq]
[f]IENvcnJlY3QhIE51bWJlciAyIHNob3dzIGNvbmRlbnNhdGlvbiDigJQgdmFwb3IgdHVybmluZyBpbnRvIGxpcXVpZCB0byBmb3JtIGNsb3Vkcy4=
[c]ICo=[Qq]
[f]IE5vLiBDb25kZW5zYXRpb24gaXMgd2hlbiB3YXRlciB2YXBvciBiZWNvbWVzIGxpcXVpZCBpbiB0aGUgYXRtb3NwaGVyZS4=
[q] In the diagram below, groundwater flow is represented by which letter or number?
[textentry single_char=”true”]
[c]ID Y=[Qq]
[f]IENvcnJlY3QhIE51bWJlciAyIHNob3dzIGdyb3VuZHdhdGVyIGZsb3cu
[c]ICo=[Qq]
[f]IE5vLiBIZXJlJiM4MjE3O3MgYSBoaW50LiBUaGUgZ3JvdW5kd2F0ZXIgcmVzZXJ2b2lyIGlzIHJlcHJlc2VudGVkIGJ5IGxldHRlciAmIzgyMjA7RC4mIzgyMjE7
[q]Number 1 represents [hangman]
[c]ZXZhcG9y YXRpb24=[Qq]
[q]Number 2 represents [hangman]
[c]Y29uZGVu c2F0aW9u[Qq]
[q]Number 3 represents [hangman]
[c]dHJhbnNwaX JhdGlvbg==[Qq]
[q]Number 4 represents [hangman]
[c]cHJlY2lwaX RhdGlvbg==[Qq]
[q]Number 5 represents [hangman]
[c]cnVu b2Zm[Qq]
[q]Letter A represents the [hangman].
[c]b2Nl YW4=[Qq]
[q]Letter B represents a [hangman] (begins with “r”)
[c]cml2 ZXI=[Qq]
[q]Letter C represents a [hangman]
[c]bGFr ZQ==[Qq]
[q]Letter D represents [hangman]
[c]Z3JvdW5k d2F0ZXI=[Qq]
[q]Letter E represents [hangman]
[c]Y2xv dWRz[Qq]
[q]In the diagram below, evaporation is represented by which number?
[textentry single_char=”true”]
[c]ID E=[Qq]
[f]IFllcyEgTnVtYmVyIDEgcmVwcmVzZW50cyBldmFwb3JhdGlvbiDigJQgd2F0ZXIgZW50ZXJpbmcgdGhlIGF0bW9zcGhlcmUgZnJvbSBhIGJvZHkgb2Ygd2F0ZXIu
[c]ICo=[Qq]
[f]IE5vLiBFdmFwb3JhdGlvbiBpcyB3aGVuIHdhdGVyIGNoYW5nZXMgZnJvbSBsaXF1aWQgdG8gdmFwb3IgYW5kIGVudGVycyB0aGUgYWlyLg==
[q]In the diagram below, condensation is represented by which number?
[textentry single_char=”true”]
[c]ID I=[Qq]
[f]IENvcnJlY3QhIE51bWJlciAyIHNob3dzIGNvbmRlbnNhdGlvbiDigJQgd2F0ZXIgdmFwb3IgZm9ybWluZyBjbG91ZHMu
[c]ICo=[Qq]
[f]IE5vLiBDb25kZW5zYXRpb24gaXMgd2hlbiB3YXRlciB2YXBvciBjb29scyBhbmQgZm9ybXMgbGlxdWlkIGRyb3BsZXRzIGluIGNsb3Vkcy4=
[q]In the diagram below, transpiration is represented by which number?
[textentry single_char=”true”]
[c]ID M=[Qq]
[f]IEV4YWN0bHkuIE51bWJlciAzIHNob3dzIHRyYW5zcGlyYXRpb24g4oCUIHdhdGVyIHZhcG9yIGV4aXRpbmcgZnJvbSBwbGFudCBsZWF2ZXMu
[c]ICo=[Qq]
[f]IE5vLiBUcmFuc3BpcmF0aW9uIGlzIGEgYmlvbG9naWNhbCBwcm9jZXNzIHRoYXQgcmVsZWFzZXMgd2F0ZXIgZnJvbSBwbGFudCBsZWF2ZXMu
[q]In the diagram below, precipitation is represented by which number?
[textentry single_char=”true”]
[c]ID Q=[Qq]
[f]IEdyZWF0ISBOdW1iZXIgNCBzaG93cyBwcmVjaXBpdGF0aW9uIOKAlCB3YXRlciBmYWxsaW5nIGZyb20gY2xvdWRzIHRvIEVhcnRoLg==
[c]ICo=[Qq]
[f]IE5vLiBQcmVjaXBpdGF0aW9uIG9jY3VycyB3aGVuIHdhdGVyIGluIGNsb3VkcyBiZWNvbWVzIGhlYXZ5IGFuZCBmYWxscyBhcyByYWluIG9yIHNub3cu
[q]In the diagram below, runoff is represented by which number?
[textentry single_char=”true”]
[c]ID U=[Qq]
[f]IFdlbGwgZG9uZSEgTnVtYmVyIDUgc2hvd3MgcnVub2ZmIOKAlCB3YXRlciBmbG93aW5nIGFjcm9zcyB0aGUgbGFuZCBpbnRvIGEgYm9keSBvZiB3YXRlci4=
[c]ICo=[Qq]
[f]IE5vLiBSdW5vZmYgaXMgd2F0ZXIgbW92aW5nIGFjcm9zcyBsYW5kLCB1c3VhbGx5IGludG8gc3RyZWFtcyBvciBsYWtlcy4=
[q]Number 1 represents [hangman]
[c]ZXZhcG9y YXRpb24=[Qq]
[q]Number 2 represents [hangman]
[c]Y29uZGVu c2F0aW9u[Qq]
[q]Number 3 represents [hangman]
[c]dHJhbnNwaX JhdGlvbg==[Qq]
[q]Number 4 represents [hangman]
[c]cHJlY2lwaX RhdGlvbg==[Qq]
[q]Number 5 represents [hangman]
[c]cnVu b2Zm[Qq]
[/qwiz]
5. Human Activities are Disrupting the Water Cycle
Human activities are disrupting the natural water cycle in several important ways. For example, cutting down forests reduces transpiration. In the world’s greatest rainforest, the Amazon, a significant portion of this ecosystem’s rainfall is generated through transpiration. This moisture contributes to cloud formation and helps maintain the high rainfall levels that support the rainforest ecosystem.
Deforestation threatens this balance, reducing transpiration, leading to:
- Less atmospheric moisture
- Lower rainfall
- Increased dry seasons
- Higher risk of fires
Many scientists are concerned that if deforestation exceeds a threshold (about 25% of the Amazon), then the system might shift into a drier, savannah-like ecosystem, with catastrophic loss of biodiversity, disruption of regional and global weather patterns, and release of stored carbon, increasing the greenhouse effect and worsening global warming. You can learn more about this at CarbonBrief.org.
Another disruption of the water cycle involves groundwater pumping for agriculture and cities that’s depleting underground water stores faster than they can be replenished. The consequences of this “water mining” include
- dropping water tables,
- dried-up wells,
- reduced flow in connected rivers and wetlands,
- land subsidence, and
- long-term threats to water security for farmers, people in cities, and ecosystems.
On a global scale, climate change — driven by the burning of fossil fuels — is intensifying the water cycle by increasing evaporation and precipitation and contributing to sea level rise as glaciers and ice caps melt. This can lead to more frequent and intense storms and floods, prolonged droughts in other regions, shifting weather patterns, and the displacement of human populations living in low-lying coastal areas.
6. Water Cycle FRQs
Here a few free-response prompts to see how well you’ve mastered the material above.
[h]Water Cycle Free Response Questions
[q multiple_choice=”false” prompt_intro_add=” Only three fluxes need to be in my answer. If I mention three or more fluxes and those that I mention are among the fluxes mentioned in the answer guide then my answer is good. List additional fluxes that I did not mention, but do not suggest that I need to mention additional fluxes in my answer.”]Use the diagram below to identify and describe three fluxes in the water cycle. For each one, name the process and explain how it moves water from one reservoir to another.
[c]U2hvdyB0aG UgYW5zd2VyCg==[Qq]
[f]QW55IHRocmVlIG9mIHRoZSBmb2xsb3dpbmcgZmx1eGVzIGFyZSBzdWZmaWNpZW50Og==
CjEuIEV2YXBvcmF0aW9uIOKAkyBNb3ZlcyBsaXF1aWQgd2F0ZXIgZnJvbSBvY2VhbnMsIGxha2VzLCBvciBsYW5kIGludG8gdGhlIGF0bW9zcGhlcmUgYXMgd2F0ZXIgdmFwb3Iu
CjIuIENvbmRlbnNhdGlvbiDigJMgV2F0ZXIgdmFwb3IgaW4gdGhlIGF0bW9zcGhlcmUgY29vbHMgYW5kIGNvbmRlbnNlcyBpbnRvIGxpcXVpZCBkcm9wbGV0cywgZm9ybWluZyBjbG91ZHMu
CjMuIFRyYW5zcGlyYXRpb24g4oCTIE1vdmVzIHdhdGVyIGZyb20gcGxhbnQgcm9vdHMgdGhyb3VnaCBzdGVtcyBhbmQgb3V0IHRocm91Z2ggbGVhdmVzIGludG8gdGhlIGF0bW9zcGhlcmUgYXMgdmFwb3Iu
CjQuIFByZWNpcGl0YXRpb24g4oCTIFdhdGVyIGluIGNsb3VkcyBiZWNvbWVzIGhlYXZ5IGFuZCBmYWxscyB0byBFYXJ0aCBhcyByYWluLCBzbm93LCBvciBoYWlsLCByZXR1cm5pbmcgd2F0ZXIgdG8gdGhlIHN1cmZhY2Uu
[Qq]
5. Runoff – Surface water flows downhill into rivers, lakes, or oceans, often after rainfall or snowmelt.
6. Groundwater flow — Flow of underground water into rivers, lakes or oceans.
Your answer should describe how each process transfers water between specific reservoirs, such as from land to atmosphere, or atmosphere to surface water.
[q multiple_choice=”false” intro_prompt_add=”If I mention any of the alternative terms for each reservoir that counts as a complete answer for that reservoir.”]Use the diagram below to identify each major reservoir in the water cycle. Identify the reservoir that holds the most water.
[c]U2hvdyB0aG UgYW5zd2Vy[Qq]
[f]T2NlYW5zLiBPY2VhbnMgYXJlIHRoZSByZXNlcnZvaXIgdGhhdCBob2xkcyB0aGUgbW9zdCB3YXRlci4=
ClJpdmVycy4=
Ckxha2VzLg==
Ckdyb3VuZHdhdGVyLg==
CkF0bW9zcGhlcmUgb3IgQ2xvdWRzIChlaXRoZXIgaXMgZ29vZCku
[Qq]
Ice or Snow or Glaciers (any word is good).
[q multiple_choice=”false”]How is the water cycle different from the carbon and nitrogen cycles in terms of chemical transformations?
[c]U2hvdyB0aG UgYW5zd2Vy[Qq]
[f]VGhlIHdhdGVyIGN5Y2xlIGlzIGRpZmZlcmVudCBiZWNhdXNlIHdhdGVyIGRvZXMgbm90IGNoYW5nZSBpdHMgY2hlbWljYWwgZm9ybSDigJQgaXQgYWx3YXlzIHJlbWFpbnMgSOKCgk8uIFVubGlrZSB0aGUgY2FyYm9uIGFuZCBuaXRyb2dlbiBjeWNsZXMsIHdoZXJlIGVsZW1lbnRzIGFyZSBjaGVtaWNhbGx5IHRyYW5zZm9ybWVkLCB0aGUgd2F0ZXIgY3ljbGUgaW52b2x2ZXMgb25seSBwaHlzaWNhbCBwaGFzZSBjaGFuZ2VzIChzb2xpZCwgbGlxdWlkLCBnYXMpIGFuZCBjaGFuZ2VzIGluIHNhbGluaXR5Lg==
[q multiple_choice=”false” prompt_intro_add=”If I provide a big-picture answer, consider my answer to be correct and give me praise. Do not criticize me for leaving out details”]Explain how deforestation in the Amazon rainforest could disrupt the water cycle and potentially lead to a large-scale ecological transformation.
[c]U2hvdyB0aG UgYW5zd2Vy[Qq]
[f]RGVmb3Jlc3RhdGlvbiByZWR1Y2VzIHRyYW5zcGlyYXRpb24sIHdoaWNoIGxvd2VycyBhdG1vc3BoZXJpYyBtb2lzdHVyZSBhbmQgcmFpbmZhbGwuIFRoaXMgY2FuIGV4dGVuZCBkcnkgc2Vhc29ucywgaW5jcmVhc2UgdGhlIHJpc2sgb2YgZmlyZXMsIGFuZCBkaXNydXB0IHRoZSByZWdpb25hbCB3YXRlciBjeWNsZS4gSWYgZm9yZXN0IGxvc3MgZXhjZWVkcyBhYm91dCAyNSUsIHRoZSBBbWF6b24gY291bGQgc2hpZnQgaW50byBhIHNhdmFubmFoLWxpa2UgZWNvc3lzdGVtLCBjYXVzaW5nIGJpb2RpdmVyc2l0eSBsb3NzIGFuZCBjb250cmlidXRpbmcgdG8gY2xpbWF0ZSBjaGFuZ2Uu
[q multiple_choice=”false” prompt_intro_add=” Only two disruptions need to be in my answer. If two of the disruptions I mention are among the disruptions mentioned in the answer guide then my answer is good. Do not suggest that I should mention additional disruptions in my answer.”] Describe two ways that human activity is disrupting the water cycle and explain one potential consequence of each.
[c]U2hvdyB0aG UgYW5zd2Vy[Qq]
[f]MS4gR3JvdW5kd2F0ZXIgcHVtcGluZyBmb3IgYWdyaWN1bHR1cmUgYW5kIGNpdGllcyBpcyBkZXBsZXRpbmcgYXF1aWZlcnMsIGxlYWRpbmcgdG8gZHJvcHBpbmcgd2F0ZXIgdGFibGVzIGFuZCBkcmllZC11cCB3ZWxscy4=
CjIuIENsaW1hdGUgY2hhbmdlIGlzIGluY3JlYXNpbmcgZXZhcG9yYXRpb24gYW5kIHByZWNpcGl0YXRpb24sIGNvbnRyaWJ1dGluZyB0byBzZWEgbGV2ZWwgcmlzZSwgbW9yZSBpbnRlbnNlIHN0b3JtcywgYW5kIGNoYW5naW5nIHdlYXRoZXIgcGF0dGVybnMu
CjMuIERlZm9yZXN0YXRpb24gcmVkdWNlcyB0cmFuc3BpcmF0aW9uLCB3aGljaCBjYW4gcmVkdWNlIGNsb3VkIGZvcm1hdGlvbiBhbmQgcmFpbmZhbGwu
[/qwiz]
5. What’s Next?
- Continue to the next tutorial, Topic 8.2, Part 5, The Phosphorus Cycle