Nutrient Cycles

Lessons

Senior 3 · Biology

Nutrient Cycles

4 lessons

Lesson 1 of 4

The Water Cycle

25 min

One forest, two great rivers

Nyungwe forest stands on the high ridge that divides Africa's two greatest river basins. Rain that falls on one side of the ridge ends up in the Congo River and the Atlantic Ocean; rain on the other side flows to the Nile and the Mediterranean. Much of that rain was water that evaporated from lakes and rose from the leaves of the forest itself only days before. The same water molecules go round and round: this is the water cycle.

In Senior 2 you learned that decomposers return mineral salts to the soil, while energy flows one way through an ecosystem. This unit follows four substances that cycle: water, carbon, nitrogen and phosphorus.

Definition

Nutrient (biogeochemical) cycle: the movement of a chemical element or substance from the non-living environment into living things and back again, over and over.

Water cycle (hydrological cycle): the continuous movement of water between the oceans, lakes, air, land and living things, driven by the energy of the sun.

Evaporation: the change of liquid water into water vapour at the surface of seas, lakes, rivers and wet ground. Transpiration: the loss of water vapour from plants, mainly through the stomata of the leaves.

Condensation: the change of water vapour into tiny liquid droplets when air cools, forming clouds, mist and dew. Precipitation: water falling from clouds as rain, hail or snow.

Infiltration: water soaking into the soil. Surface runoff: water flowing over the ground into streams and rivers. Groundwater: water stored in the soil and rocks underground.

The water cycle drawn

Figure 1: The water cycle. Every arrow shows the direction in which the water moves.

The sun heats lakes, rivers and the sea; water evaporates. Plants add water vapour by transpiration.
Warm, moist air rises and cools; the vapour condenses into droplets, which form clouds.
The droplets join and fall as precipitation.
On land, some rain infiltrates the soil and becomes groundwater, which feeds springs, wells and rivers; some flows over the surface as runoff; and some is taken up by plant roots.
Rivers and groundwater return the water to lakes and the sea, and the cycle continues.

How living things take part

Plants take up water through their roots and lose most of it by transpiration: a forest returns a large share of the rain to the air, where it can fall again as rain. Plants also use water in photosynthesis.
Animals drink water and lose it in urine, sweat and breath.
All organisms release water in aerobic respiration: $\mathrm{C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O}$ .
Roots and leaf litter hold soil open and loose, so rain soaks in instead of running off.

Worked example 1: how much of Earth's water can we use?

Of all the water on Earth, 97.5% is salt water. Of the fresh water, about 99% is ice or groundwater, so less than 1% of the fresh water is in lakes and rivers. Imagine all Earth's water shrunk to 1,000 litres. How much would be (a) fresh water, and (b) fresh water in lakes and rivers?

(a) Fresh water is $100\% - 97.5\% = 2.5\%$ : $\frac{2.5}{100} \times 1000 = 25$ litres.
(b) Less than 1% of that: $\frac{1}{100} \times 25 = 0.25$ litres.

Answer: of 1,000 litres, only 25 litres would be fresh, and less than a quarter of a litre, about one glass, would be in lakes and rivers. This is why protecting catchments such as Nyungwe matters.

Worked example 2: rain on a hillside catchment

A hillside catchment of $20\ \text{km}^2$ receives $1300\ \text{mm}$ of rain in a year. When it was forested, 20% of the rain ran off; after the forest was cleared for farms, 45% ran off. (Illustrative figures.) Calculate (a) the volume of rain, and (b) the increase in runoff.

Convert units: $1300\ \text{mm} = 1.3\ \text{m}$ ; $20\ \text{km}^2 = 20 \times 1\,000\,000 = 20\,000\,000\ \text{m}^2$ .
(a) Volume: $1.3 \times 20\,000\,000 = 26\,000\,000\ \text{m}^3$ .
Runoff, forested: $0.20 \times 26\,000\,000 = 5\,200\,000\ \text{m}^3$ . Cleared: $0.45 \times 26\,000\,000 = 11\,700\,000\ \text{m}^3$ .
(b) Increase: $11\,700\,000 - 5\,200\,000 = 6\,500\,000\ \text{m}^3$ , more than double.

Interpretation: with less leaf cover and fewer roots, less rain is caught and soaked up, so more rushes downhill at once. That is why deforested hills suffer floods, landslides and soil erosion, while their streams may dry up in the dry season because less water was stored underground.

People and the water cycle

Cutting forests reduces transpiration and infiltration and increases runoff (Worked example 2).
Draining wetlands removes natural sponges that store water and release it slowly into rivers and lakes (you will meet the Rugezi wetland in Unit 4).
Paving and roofing in towns stops rain soaking in, so it runs straight into drains and streams.
Protecting catchments by planting trees, digging terraces and ditches along the contour, and keeping wetlands helps rain soak in and feed springs all year.

Warning

Common mistakes. (1) Confusing evaporation (from water surfaces) with transpiration (from plants). (2) Saying clouds are water vapour: vapour is invisible; clouds are tiny liquid droplets formed by condensation. (3) Drawing arrows the wrong way: evaporation and transpiration arrows point up, precipitation down, runoff downhill.

Earth's Water Cycle (NASA Goddard). Watch how satellites measure evaporation, clouds and rainfall, and notice that the sun's energy drives the whole cycle.

Tip

Memory hook: "up, across, down, back". Up (evaporation, transpiration), across (winds carry the vapour and clouds), down (precipitation), back (runoff, infiltration and groundwater return the water to lakes and the sea).

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Practice: the water cycle

Name the process by which water vapour leaves the leaves of plants.
What provides the energy that drives the water cycle?
Distinguish between infiltration and surface runoff.
Write the word equation for aerobic respiration and underline the product that enters the water cycle.
A 5 km² catchment receives 1,000 mm of rain in a year. Calculate the volume of rain in m³.
If 30% of the rain in question 5 runs off, what volume soaks in or evaporates?
Explain why a spring below a deforested hill may dry up in the dry season.
Explain how protecting Nyungwe forest helps to supply water to rivers far away.

Show answersHide answers

Transpiration.
Energy from the sun.
Infiltration is water soaking into the soil; surface runoff is water flowing over the ground into streams and rivers.
glucose + oxygen → carbon dioxide + water (+ energy released).
$1\ \text{m} \times 5\,000\,000\ \text{m}^2 = 5\,000\,000\ \text{m}^3$ .
$70 %$ of $5\,000\,000 = 3\,500\,000\ \text{m}^3$ .
Without trees, more rain runs off quickly in the wet season and less soaks into the ground, so less groundwater is stored to feed the spring when there is no rain.
The forest catches rain and helps it soak in; the stored water is released slowly into streams that feed the Congo and Nile basins. Transpiration from the forest also returns water to the air, which falls again as rain.

Key points

The water cycle: evaporation and transpiration → condensation → precipitation → infiltration, runoff and groundwater flow, driven by the sun.
Only about 2.5% of Earth's water is fresh, and less than 1% of that is in lakes and rivers.
Forests and wetlands help rain soak in; clearing them increases runoff, floods and erosion.

Sources and credits

20.2: Biogeochemical Cycles, from Concepts of Biology by Samantha Fowler, Rebecca Roush and James Wise (OpenStax), via LibreTexts Biology, licensed under CC BY 4.0. Adapted by Ganzaa. (section "The Water Cycle", including the shares of salt and fresh water)
Nyungwe as the watershed between the Congo and the Nile: Nyungwe National Park (African Parks and the Rwanda Development Board).
Photo: The Welcoming Mist, Dpiskho, CC BY-SA 4.0, via Wikimedia Commons.
Video: NASA | Earth's Water Cycle, NASA Goddard, embedded from YouTube.

Written by Ganzaa: Figure 1, the worked examples (the catchment figures are illustrative) and the practice questions.

1One forest, two great rivers

2The water cycle drawn

3How living things take part

4People and the water cycle

One forest, two great rivers

The water cycle drawn

How living things take part

People and the water cycle