Chapter 8 of 17 · Water and the Weather
Chapter 7. The water cycle, whole
Putting the previous chapters together into a single loop, understanding why it has neither beginning nor end, and seeing what it keeps and what it separates.
Six mechanisms, a single loop
Each previous chapter dealt with one piece, and each one stood on its own. This chapter does only one thing: it puts them end to end, and it shows that they close a loop. Nothing new is introduced here, apart from the vocabulary that names the whole.
Météo-France defines what we obtain like this: "the water cycle is a natural cycle continually in motion". Two words count. Natural: no machine, no intervention, no energy other than that of the Sun and of gravity. Continually: it never stops, it has no dead season, and it is working while you read this sentence.
One warning before we start, and it holds for the whole chapter. The arrow diagram of the water cycle is one of the most copied images there is, and it is also one of the most misleading, because it gives the impression of a single circuit in which every drop makes the same round. The reality is a tangle of loops of very different sizes and lengths, some of them lasting a few hours and others thousands of years. This chapter keeps the loop, and it adds what the arrow does not say.
The objectives of this chapter
Each one is observable. By the end of the chapter, either you do it, or you know which section to read again.
- List in order the stages of the water cycle, naming the change of state at each one.
- Name the great reservoirs in which water stays, and say which one holds the most.
- Explain why a cycle has neither beginning nor end, and why the order in which it is set out is arbitrary.
- Explain why rain is not salty when it comes largely from the oceans.
- Reconstruct a complete journey, from ocean to ocean, passing through at least one underground reservoir.
What this chapter assumes is already known
This chapter is a chapter of synthesis: it assumes everything that precedes it. The three links below are the ones it draws on most directly.
The loop, stage by stage
Let us begin with the oceans, because they hold most of the water and because Météo-France recalls that water covers "nearly three quarters of the surface" of the globe. This choice of a starting point is a convenience of narration, and nothing more: the next section will explain why.
First stage, evaporation. According to Météo-France, "the heat of the Sun allows part of the water contained in the oceans, the rivers and the lakes to evaporate". Change of state: liquid to gaseous, at ordinary temperature, without boiling, as chapter 1 established.
Second stage, evapotranspiration. To the vapour coming from water surfaces is added that from the transpiration of plants. A forest thus sends back into the air a considerable share of the water its roots have taken from the soil. Change of state: liquid to gaseous, here too.
Third stage, the formation of clouds. This vapour "rises into the atmosphere and turns into tiny droplets or ice crystals in suspension to form the clouds". Change of state: gaseous to liquid, or gaseous to solid. Chapter 3 gave the four mechanisms that set it off.
Fourth stage, precipitation. "When the droplets are too heavy, they fall to the ground in liquid form (rain) or solid form (snow and hail) according to the temperature of the air." Chapter 4 set out the growth and the nomenclature in detail.
Fifth stage, the sharing at ground level. "Part of the water from precipitation runs off, feeding the rivers and the large rivers that flow into the oceans. The other part builds up in glaciers or soaks into the ground, reaching underground rivers or water tables, which themselves make a delayed contribution to surface flows." Remember the word delayed, for it carries the whole of chapter 6 on its own: the share that soaks in does come back to watercourses, but weeks or months later. Chapters 5 and 6 dealt with this stage in full.
And the loop closes: the water of the large rivers rejoins the ocean, from which it evaporates again. No stage has been added to what the previous chapters had already established separately.
| Reservoir | In which state | How water enters it | How it leaves |
|---|---|---|---|
| The oceans and the seas | Liquid, salty | By large rivers, by direct precipitation, by the melting of ice. | By evaporation, and by that alone. |
| The atmosphere | Gaseous, plus droplets and crystals in suspension | By the evaporation of water surfaces and by the evapotranspiration of plants. | By precipitation. |
| Glaciers and ice sheets | Solid | By snowfall that builds up and packs down. | By melting, and by sublimation. |
| Groundwater | Liquid | By infiltration through permeable soils. | By springs, by discharge into watercourses, by abstraction. |
| Watercourses and lakes | Liquid | By runoff, by tributaries, by discharge from groundwater. | By flow towards the outlet, and by evaporation. |
| Living things and soils | Liquid | By the uptake of roots, by drinking, by the soil holding water. | By transpiration, by respiration, by drainage. |
Why this cycle has no beginning
The previous section began with the oceans. It could have begun with a cloud, with a body of groundwater or with a puddle: the account would have been the same, shifted by one step. This is the property that defines a cycle, and it deserves to be stated clearly because it is the source of a great deal of confusion.
In a chain, there is a first stage and a last. In a cycle there is neither: each stage is at once the consequence of the previous one and the cause of the next, endlessly. Asking where the water cycle begins makes no more sense than asking where a circle begins.
A second property follows from the first. A cycle does not consume its matter: it moves it about. The water falling today is not new water. It has already been ocean, cloud, river, groundwater, sap, and it will be so again. The total quantity of water on the planet does not depend on how much rain falls this year.
This conservation has a corollary that surprises people and is nevertheless exact: to speak of losing water or running short of water never means that the matter has disappeared. It means that it is not in the place wanted, at the moment wanted, in the state wanted. The next chapter will show that this is exactly the problem the small water cycle sets out to solve.
What the cycle separates: the salt stays behind
Here is a consequence that chapter 1 allows us to deduce, and it is better deduced than remembered. A change of state concerns only the water itself. When sea water evaporates, what passes into the gaseous state is the water; the salt, for its part, does not evaporate and stays where it is.
The vapour rising from the oceans is therefore water without salt. The clouds it forms are not salty, and the rain that falls from them is not salty either. The water cycle works like an immense separation, kept going by the Sun, which draws fresh water out of a salty reserve.
It is this mechanism that makes life possible on the continents. A planet whose water was in the same place but without this sorting would be a planet of salt water, with no drinkable river and no usable groundwater. The Sun, by evaporating, constantly manufactures the fresh water the continents receive.
That same deduction explains an ordinary observation: after a plate of tap water has evaporated completely, a faint whitish deposit is left at the bottom. That deposit is what the water contained and what, for its part, could not leave into the air. Experiment 1 in chapter 1 showed it already, provided one looked at the bottom of the plate.
The words to remember
Three words that describe the whole, and no longer a piece of it.
- The water cycle
- The natural cycle, continually in motion, by which water passes from the oceans to the atmosphere, from the atmosphere to the continents, and from the continents to the oceans, changing state at each passage.
- Evapotranspiration
- The sum of the evaporation of water surfaces and soils and of the transpiration of plants. It is through it that a forest sends back into the air part of the water its roots have taken.
- A reservoir
- A place where water stays between two movements: ocean, atmosphere, glacier, groundwater, watercourse, soil, living thing. The length of the stay ranges from a few days to several thousand years according to the reservoir.
The trap of this chapter: reading the arrow diagram as if every drop made the grand tour. A drop evaporated from an ocean can fall back into the same ocean a few days later, without ever having seen a continent: that is in fact the commonest case. Another can stay caught in a glacier for thousands of years, or in deep groundwater for centuries. The cycle is therefore not ONE loop but an endless set of nested loops, of very unequal lengths. Remember the practical consequence, which is the one that counts: water drawn from deep groundwater is not replaced on the scale of a human life, whereas water drawn from a river is replaced within a few days. They are two very different acts, even though both draw on the same cycle.
Reconstructing a journey through the cycle, in four moves
- Step 1
Name the reservoir of departure and the state of the water found in it.
- Step 2
Name the mechanism that takes it out of that reservoir, and the change of state that goes with it if there is one.
- Step 3
Name the reservoir of arrival, then start again until you are back at the reservoir of departure.
- Step 4
Check that every passage has an engine: the Sun for everything that goes up, gravity for everything that comes down. A passage without an engine is a mistake in the journey.
Worked example. A journey from ocean to ocean, by way of the subsoil
Instruction: reconstruct a complete journey, from ocean to ocean, which must pass through a body of groundwater. Name each change of state. Example guided up to the next-to-last step.
- Leave the ocean
Reservoir of departure: the ocean, liquid and salty water. Mechanism of departure: evaporation, under the effect of the heat of the Sun. Change of state: liquid to gaseous. The salt stays in the ocean. - Cross the atmosphere
Reservoir: the atmosphere, water in the gaseous and invisible state. The vapour is carried by the wind over a continent, then lifted, against a relief for instance. It cools. - Form the cloud
Mechanism: condensation, when the air reaches its dew point. Change of state: gaseous to liquid, in fine droplets in suspension. The reservoir is still the atmosphere, but the water in it is now visible. - Fall
Mechanism: the droplets grow until they become too heavy, and gravity wins. Change of state: none, if it is rain. The water leaves the atmosphere for the ground. - Go down into the ground
Mechanism: infiltration, if the ground is permeable and if the rain is not too intense. The water passes through the pores and the cracks, drawn down by gravity, as far as an impermeable bedrock. Reservoir of arrival: the groundwater. - Over to you: finish the journey
It remains for you to take the water out of the groundwater and bring it back to the ocean. Name the mechanism of departure, the reservoir or reservoirs crossed, and say whether there is a change of state. Chapter 6 gives you everything you need. Check it in the answer to the self-check, question 5.
What to remember from this chapter
- The water cycle is a natural cycle continually in motion, driven by the heat of the Sun and by gravity.
- Its stages are evaporation, evapotranspiration, the formation of clouds, precipitation, then the sharing between runoff, infiltration and build-up in glaciers.
- A cycle has neither beginning nor end: the order in which it is set out is a convenience of narration.
- A cycle does not consume its matter, it moves it about. Running short of water never means that the matter has disappeared.
- Salt does not evaporate: the cycle constantly separates fresh water out of a salty reserve, and that is what makes life possible on the continents.
- It is not one loop but an endless set of nested loops: a few days for the atmosphere, several thousand years for a glacier.
Self-check for chapter 7
Answer in writing, using only the knowledge given by this volume so far. The answers are gathered at the end of the volume.
- List the stages of the water cycle in order, naming for each the change of state if there is one. Then name the great reservoirs in which water stays, and say which one holds the most.
- A pupil asks where the water cycle begins. Answer in two sentences, and say why the question is badly put.
- Rain comes largely from the oceans, which are salty, and yet it is not. Explain, drawing on chapter 1.
- Two villages draw the same quantity of water: one from a river, the other from deep groundwater. Why are these two acts not equivalent?
- Finish the journey of the worked example: take the water out of the groundwater and bring it back to the ocean, naming the mechanisms and the reservoirs.