Module 1 — The Hydrosphere on Planet Earth and the Water Cycle in Nature

• Every science lesson in this unit begins from one observation: Earth is the only planet in the solar system that we know has large amounts of liquid water on its surface, and Earth is also the only planet we know to be full of life.
• Scientists who search for life on Mars, on the moons of Jupiter and Saturn, or on planets around other stars always begin by searching for water.
• The reason is simple: water is the medium in which the chemical reactions of life take place (you will study this in Module 5), it dissolves the substances that cells need (Module 2), it resists rapid temperature change (Module 3), and it can carry materials, gases and heat from place to place (Modules 1 and 7).
• That is the answer to the book's "Think!" question: scientists look for water on other planets because wherever liquid water exists, the basic conditions for life as we know it may exist; water is important because it is the solvent, the reactant, the transport medium and the temperature stabiliser of living things.

• The word environment comes from the French word environ, meaning "what surrounds us".
• The environment is everything that surrounds a living organism and affects it.
• It has two kinds of components:

• Biotic (living) factors: humans, animals, plants and micro-organisms.
• Abiotic (non-living) factors: light, air, water and soil (we can add temperature, pressure, wind, salinity and so on).

• The environment is a whole system: you cannot remove one part without affecting the others.
• It is built from physical, chemical and biological elements, and it is influenced by many variables that surround humans and other organisms.
• Notice how this matches the idea of integrated science: a river is at the same time a physics problem (flow, pressure, heat), a chemistry problem (dissolved salts, pH, gases) and a biology problem (fish, algae, bacteria).

• An ecosystem is a community of living organisms that interact with one another and with the non-living components of their surroundings, and that adapt to changing conditions.
• Examples of ecosystems around us: aquatic ecosystems (seas, oceans, lakes, rivers, ponds, wetlands), deserts, and the various kinds of forests.
• In this unit we study the aquatic ecosystem; the next unit studies the atmosphere that surrounds and interacts with it.

The natural environment consists of four interconnected spheres:

• These four spheres are in continuous change, they affect one another, and they are affected by human activities.
• For example, burning fuel (human activity) adds carbon dioxide to the atmosphere; some of it dissolves in the ocean (hydrosphere), changes its acidity, and harms shell-forming animals (biosphere) that build their shells from calcium carbonate taken from the water (a lithosphere-type material).
• One event, four spheres.
• Keep this picture in mind: almost every question in this book can be answered by following matter and energy from one sphere to another.

• The hydrosphere is all the water found on Earth: in oceans, seas, lakes, rivers, streams, glaciers and ice caps, groundwater, soil moisture, the atmosphere (water vapour and clouds) and inside living organisms.
• The hydrosphere is a feature that distinguishes planet Earth from the other planets of the solar system.

• Water covers about 70 % of Earth's surface. (The remaining ~30 % is land.) This is why Earth is called "the blue planet".
• The distribution of that water is very uneven:

• Look carefully at the numbers: although water covers most of the planet, the part that is easily available to humans is only about one hundredth of the total.
• This single fact is the foundation of the "included issues" listed at the start of the unit: water pollution, sustainability of water resources, water resources management and population growth. Every pollution event that spoils fresh water damages a tiny and precious fraction of the hydrosphere, and a growing population shares this same small fraction.

Common error.

• Students sometimes say "97 % of the water is fresh because it is in the oceans".
• The oceans are salt water.
• Only about 3 % of Earth's water is fresh (1 % liquid and accessible + 2 % frozen).

• Problem. A model of Earth holds 2000 litres of "water" distributed in the same percentages as the real hydrosphere (97 % salt water, 2 % ice, 1 % accessible fresh water).
• A town consumes 0.5 litre per day from the model's accessible fresh-water store.
• How much water is in each store, and how many days would the town's consumption take to use up the whole accessible fresh store if nothing were replaced?

Solution.

1. Salt water = 0.97 × 2000 = 1940 L.
2. Ice = 0.02 × 2000 = 40 L.
3. Accessible fresh water = 0.01 × 2000 = 20 L.
4. Check: 1940 + 40 + 20 = 2000 L ✔.
5. Days = 20 L ÷ 0.5 L/day = 40 days.

• Interpretation. The calculation shows why replacement matters.
• In nature the fresh store is constantly refilled by the water cycle; if the cycle were stopped (or if we polluted or over-used the store faster than it refills), the store would run out.
• Section 2 explains the cycle.

• Within the range of temperatures found on Earth, water can exist in all three states of matter: solid (ice, snow, hail), liquid (rain, rivers, oceans) and gas (water vapour).
• Water on or near Earth's surface is in a constant state of change between these three states, and it moves continuously from one place to another along many different pathways: from the sea to the air, from the air to the land, from the land back to the sea, and through living organisms.

• This continuous movement forms an almost closed system called the water cycle in nature (the hydrological cycle).
• It describes the continuous movement of water around the planet.

Why "almost closed"?

• (A favourite exam question.) Because the total amount of water on Earth stays essentially constant: water is neither created nor destroyed on a large scale by the cycle; it is only transferred between reservoirs (ocean, atmosphere, land, ice, living things) and changes state.
• The system exchanges energy with the outside (it receives energy from the Sun) but practically no matter.
• Only tiny amounts of water vapour escape from the top of the atmosphere into space, and tiny amounts are added from space or from volcanic activity, so we say "almost" closed rather than completely closed.

• The cycle is not just a plumbing diagram.
• The book emphasises that the water cycle is a system that can alter Earth's surface physically, chemically and biologically:

• Physically — flowing water erodes rock, carries sediments, shapes valleys and deltas (for example the Nile Delta), and freezing water breaks rocks.
• Chemically — water dissolves minerals, causes rocks to weather, and carries dissolved salts to the sea.
• Biologically — water supports life, enters and leaves organisms, and controls where ecosystems can exist.

• Two forces drive the cycle: solar energy, which provides the heat to evaporate water and drives winds, and gravity, which pulls rain, snow and runoff down to lower levels.
• Without the Sun the cycle would stop within days; without gravity there would be no rain falling and no rivers flowing.

• 1.
• Evaporation. Liquid water at the surface of oceans, lakes, rivers, wet soil and plant leaves absorbs heat from the Sun and changes into water vapour.
• Evaporation happens at any temperature (not only at the boiling point), but faster when the water is warmer, the air is drier, the wind is stronger and the surface is larger.
• About 80–85 % of the water vapour in the atmosphere comes from the oceans.
• Evaporation absorbs energy (the latent heat of vaporization — Module 3), which is why it cools the surface.

2. Biological activities that return water to the atmosphere. Living things are part of the cycle. The book highlights:

• Respiration — in cellular respiration glucose is oxidised: C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O + energy. Water vapour is released in the exhaled breath of animals.
• Excretion — organisms eliminate the wastes of metabolism: carbon dioxide, water vapour and nitrogenous wastes. The three main nitrogenous wastes are ammonia (typical of fish and many aquatic animals), urea (typical of mammals, amphibians and sharks) and uric acid (typical of birds, reptiles and insects). Water leaves the body with these wastes (urine, sweat).
• Transpiration in plants (explained in detail below).

• 3.
• Condensation. As moist air rises it cools (see the temperature fall with altitude in Module 9).
• Cool air cannot hold as much vapour, so the vapour condenses into tiny liquid droplets (or ice crystals) on dust and salt particles.
• Billions of droplets together form a cloud.
• Condensation releases the energy that was absorbed during evaporation.

• 4.
• Precipitation. When cloud droplets grow too heavy to stay suspended they fall as precipitation: rainfall and snowfall (also hail and sleet).
• Precipitation is the main way in which fresh water reaches the land.

5. Runoff and infiltration. Water falling on land follows two paths:

• Surface runoff — it flows over the ground into streams, rivers and lakes and finally back to the sea.
• Infiltration — it soaks through the pores of the soil and sedimentary rocks (sandstone and limestone are typical porous rocks) and, moving slowly downward, collects as groundwater (an aquifer). Groundwater feeds wells and springs and flows slowly toward rivers and seas, so it may take years or centuries to return.

• 6.
• Storage. Water can be stored for different lengths of time: a few days in the atmosphere; weeks to months in rivers and soil; years in lakes; decades to thousands of years in deep groundwater; thousands to hundreds of thousands of years in polar ice and deep oceans.

Sea water is heated → evaporates → vapour rises and cools → condenses to clouds → wind carries clouds over land → precipitation → part runs off to rivers, part infiltrates to groundwater, part is absorbed by plants and animals → plants transpire and animals respire and excrete → vapour returns to the atmosphere; rivers and groundwater return to the sea. The cycle then repeats.

Which process, if stopped, would disturb the balance of the cycle most directly?

• Evaporation.
• If evaporation stopped, the atmosphere would receive no new vapour, there would be no clouds and no rain, and the cycle would collapse.
• Transpiration, runoff and infiltration are also important but are parts of the cycle that can partly be replaced by other pathways; evaporation is the key step that lifts water into the atmosphere.

Transpiration is the process by which plants release excess water in the form of water vapour from the plant surfaces that are exposed to air.

• Most transpiration occurs through microscopic openings called stomata (singular: stoma), which are found mainly on the surfaces of leaves and also on green stems.
• Each stoma is bordered by two guard cells that open and close the pore.
• A small amount of water also escapes directly through the cuticle (the waxy layer) and through small pores in woody stems (lenticels).

1. Cooling the plant. Evaporation of water absorbs a large amount of heat (the latent heat of vaporization of water is high because of hydrogen bonds — Module 3). The leaf therefore loses heat and its temperature falls. This protects leaf tissue and enzymes from overheating, just as sweating cools a person.
2. The "transpiration pull" that lifts water. Water evaporating from the leaf cells is replaced by water pulled from the leaf's xylem vessels. Because water molecules stick together (cohesion, through hydrogen bonds) and to the vessel walls (adhesion), a continuous column of water is pulled upward from the roots through the xylem tissue to the upper parts of the plant, carrying dissolved minerals with it. This "pulling force" is a suction created by evaporation at the top; it can lift water to the tops of trees tens of metres tall without any pump.

• Additional consequences (consistent with the book and your later lessons): transpiration helps bring minerals to the leaves, maintains the water content of cells, and contributes a large amount of vapour to the atmosphere.
• Over land, particularly forests, transpiration accounts for a large part of the vapour that returns to the air.
• When forests are cut (deforestation), local rainfall often falls, which is one reason why plants are described as a part of the water cycle.

(You will use the humidity effect again in Module 11.)

• Problem. A cultivated field contains 40 000 plants per hectare.
• On a sunny day each plant transpires 0.25 litre.
• (a) How much water (in litres and in cubic metres) returns to the air from 5 hectares in 10 sunny days?
• (b) How much heat is absorbed from the plants and air if the latent heat of vaporization of water is taken as 2.26 × 10⁶ J/kg and 1 litre of water has mass 1 kg?

Solution. (a) Plants in 5 ha = 5 × 40 000 = 200 000 plants. Water per day = 200 000 × 0.25 = 50 000 L. For 10 days = 500 000 L = 500 m³ (since 1000 L = 1 m³). (b) Mass = 500 000 kg. Heat = m × L = 500 000 × 2.26 × 10⁶ = 1.13 × 10¹² J.

Interpretation. The same field that supplies food also acts as a large natural "air conditioner": about 1.1 trillion joules of heat are absorbed during evaporation, which cools the local air, and 500 m³ of vapour enters the atmosphere to form future clouds.

• Problem. A lake receives 30 million m³ of water per year from rainfall and river inflow.
• Evaporation removes 12 million m³ per year, and the outflow river removes 17 million m³.
• Its volume was 100 million m³ at the beginning of the year.
• What is its volume at the end of the year, and what does the change mean for the water cycle?

Solution. Net change = inputs − outputs = 30 − (12 + 17) = +1 million m³. New volume = 101 million m³. The lake's level rises slightly; the extra water has been stored. If evaporation were to rise because of higher temperatures (climate change) to 14 million m³, the net change would be −1 million m³ and the lake would shrink. This shows that the water cycle is a balance of inputs and outputs, and that a change in one process changes the whole system.

• The book names three kinds of material that living organisms eliminate in excretion: carbon dioxide, water vapour and nitrogenous wastes (ammonia, urea, uric acid).
• Link each to its source:

• Carbon dioxide and water come from cellular respiration (oxidation of glucose).
• Nitrogenous wastes come from the breakdown of proteins and amino acids; ammonia is very toxic and needs a lot of water to remove it (suitable for aquatic animals), urea is less toxic (needs less water), and uric acid is almost insoluble and needs very little water (suitable for animals living in dry places, such as birds and reptiles that lay eggs on land).

• These excreted wastes dissolve in water in the surroundings, so animals both use water and return it to the environment.
• In the human body about 70 % of the body mass is water, and a person loses roughly 2–3 litres per day through urine, sweat, exhaled breath and faeces, replaced by drinking and eating.
• Every one of those litres is part of the cycle.

The unit lists six included issues. Here is how the water cycle connects to each (this helps with essay and open-ended questions):

1. Water pollution — pollutants evaporate, are washed by rain, infiltrate into groundwater or flow into rivers and seas; the cycle spreads pollution widely.
2. Climate change — a warmer atmosphere holds more vapour and evaporates more water, changing rainfall patterns, increasing floods in some regions and droughts in others, and melting ice (adds water to oceans).
3. Sustainability of water resources — resources are sustainable only if withdrawal does not exceed the rate at which the cycle renews them.
4. Conservation of biodiversity — wetlands, rivers and coral reefs depend on stable water supply and quality.
5. Water resources management — planning of dams, irrigation, recycling and desalination must respect the natural cycle.
6. Sustainability challenges with population growth — more people share the same ~1 % of accessible fresh water.

Activity: researching how rainfall is measured and how future change in the water cycle is predicted.

Aim. To find out which instruments meteorologists use to measure annual rainfall over an area, and how scientists forecast future changes in the water cycle.

• Method. Use books, a school library or reliable websites (a national meteorological authority, NASA, WMO are good examples).
• Make two short reports.

What you should find.

1. Rainfall measurement: the standard rain gauge (a funnel leading to a graduated cylinder; the depth of water collected in millimetres is the rainfall; 1 mm of rain over 1 m² equals 1 litre), the tipping-bucket gauge that records automatically each time a small bucket fills and tips, weather radar that estimates rainfall over large areas from reflected radio waves, and satellites that measure clouds and rainfall from space. The annual rainfall is the sum of the daily readings over the year.
2. Prediction of future change: scientists feed data from satellites, weather stations, ocean buoys and ground measurements into climate models (computer programs built on physical and chemical equations). The models test what happens to evaporation, rainfall and ice melt if the atmospheric carbon dioxide and temperature rise. Predictions are expressed as probabilities and ranges, not single exact values.

• Expected result / conclusion. Accurate measurements, repeated for many years, are needed in order to detect trends.
• Models predict higher evaporation and more extreme rainfall events in a warming world, so management of water resources must adapt.

• Do not confuse transpiration (plants, vapour from stomata) with evaporation (physical change from any wet surface) and excretion (removal of metabolic waste).
• "Almost closed" because matter is conserved but energy enters from the Sun.
• Mention stomata and xylem in any transpiration answer.
• When asked about "biological processes that contribute to the water cycle", answer: respiration, excretion, transpiration (and also feeding/drinking and decomposition of organisms).
• Infiltration forms groundwater; condensation forms clouds.

1. Name the four spheres and give one example of each.
2. Why can't humans use most of Earth's water?
3. List six processes of the water cycle in order.
4. State two roles of transpiration.
5. Why is the water cycle called "almost closed"?