Chapter 6 of 17 · Water and the Weather
Chapter 5. Runoff and infiltration
What happens to a raindrop once it has touched the ground, and what decides whether it leaves along the surface or goes down.
Three fates for one and the same drop
A raindrop that touches the ground has just finished one journey, and it begins another. That second journey is the one we look at least and the one that decides most: it is what fills rivers, wells and reservoirs, and it is what explains why two neighbouring villages do not have the same water.
Three fates present themselves, and Météo-France sums them up in one sentence about the cycle: "part of the water runs off, feeding the rivers", while the other part "builds up in glaciers or soaks into the ground". To those two must be added a third fate, already known from the first chapter: part of the water that has fallen goes straight back into the air by evaporation, without ever flowing or going down.
This chapter deals with the first two, because the third has already been dealt with. To run off is to slide along the surface, following the slope. To infiltrate is to enter the ground and go down between the grains and the cracks. One and the same shower always does both at once, and the proportion between the two is anything but incidental: it decides the flood on one side, the water reserve on the other.
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.
- Name the three possible fates of water that has fallen on the ground.
- Tell a permeable material apart from an impermeable one, and give two examples of each.
- Describe what an underground body of water is, and say what stops it from going down any further.
- Cite four factors that tip a shower towards runoff rather than towards infiltration.
- Predict, for a piece of ground described in one sentence, whether the water will leave mainly along the surface or mainly downwards.
What this chapter assumes is already known
Two prerequisites, installed by earlier chapters of this volume: evaporation on the one hand, the measurement of a depth of rain and its equivalence in litres per square metre on the other.
Permeable, impermeable: the property that decides everything
A material is permeable when water can pass through it, and impermeable when it cannot. This is not a question of hardness or of appearance: sand is loose and permeable, clay is loose and impermeable, cracked limestone is hard and permeable, sound granite is hard and impermeable.
What makes a material permeable is the voids it contains, and above all the fact that those voids connect with one another. Sand is made of grains that leave connected spaces between them: the water goes down from one to the next. Clay is made of tiny flat particles that stick to one another: the voids exist but they are too fine and too poorly connected to let water through at any useful rate.
This property can be checked at home in a few minutes, and the experiment in this chapter is devoted to it. It also explains an everyday observation: after the same shower, a beaten-earth path keeps its puddles for hours whereas a patch of sand never had any.
Remember, finally, that permeable ground can stop being permeable for a time. When its voids are already full of water, it is saturated, and the water arriving afterwards has nowhere to go: it runs off as if the ground were impermeable. That is what explains why moderate rain falling on already sodden ground sometimes produces more runoff than a thunderstorm falling on dry ground.
Where the water that soaks in goes
Water entering the ground does not stop at the first layer. Eaufrance describes the mechanism: after rain, part of the water enters the pores and cracks of the soil, it is drawn down by gravity through the soil and it soaks into the subsoil until it is stopped by an impermeable bedrock.
That impermeable bedrock is the key to the whole arrangement. Without it, the water would go on downwards indefinitely and there would be neither groundwater, nor springs, nor wells. With it, the water stops and builds up above it, filling all the empty spaces of the rock lying over it.
That porous or cracked rock which holds the water has a name: it is the aquifer. And the water filling its voids is the groundwater. Eaufrance defines groundwater as the "reserves of water stored in the porous and permeable rocks of the subsoil", and states that the water "builds up in all the empty spaces of the aquifer".
The body of groundwater nearest the surface is called the water table. Eaufrance describes it in two sentences: "Water tables are those found near the surface. Their shallow depth makes them easily accessible for human activities." It is therefore the one reached by digging a well, and the one that feeds most springs. Remember that this accessibility has an exact reverse side: what is easy to reach from below is easy to reach from above, and whatever is spread on the surface joins it.
Groundwater is not still. It flows very slowly, following the slope of the bedrock, and it eventually comes out somewhere: at a spring, in the bed of a river, or straight into the sea. The next chapter will show that this return to the river is what makes a watercourse still flow in the middle of summer, long after the last rain.
The words to remember
Five words. The last three form a set: the rock, the water it holds, and what stops it.
- Runoff
- The flow of water over the surface of the ground, following the slope. It is fast, it carries soil away and it feeds watercourses directly.
- Infiltration
- The entry of water into the ground through its pores and cracks, then its descent under the effect of gravity. It is slow and it feeds the underground reserves.
- The aquifer
- The porous and permeable rock of the subsoil, or the cracked rock, which is able to hold water and to let it circulate.
- Groundwater
- The water itself, built up in all the empty spaces of the aquifer. The water table is the body of groundwater found near the surface, and its shallow depth makes it easily accessible for human activities.
- The impermeable bedrock
- The compact layer that interrupts the descent of underground water and on which the groundwater rests. Without it there would be neither groundwater, nor springs, nor wells.
What tips the balance
What favours infiltration
- Permeable ground: sand, gravel, cracked limestone.
- Flat ground or a gentle slope, which gives the water time to enter.
- Slow, steady rain, which brings no more than the ground can take in.
- A cover of plants: roots open passages and leaves slow the fall of the drops.
- Ground that is dry to begin with, whose voids are available.
What favours runoff
- Impermeable ground: clay, sound rock, and above all tarmac, concrete, roofs.
- A steep slope, which carries the water away before it has been able to enter.
- Intense, brief rain, which brings water faster than the ground takes it in.
- Bare ground, compacted or beaten down by the drops, whose pores close up at the surface.
- Ground that is already saturated, whose voids are full.
The trap of this chapter: picturing groundwater as an underground lake or a river in a cave. In the vast majority of cases there is no chamber, no visible current and no free surface: there is rock, and water in the voids of that rock, like the water in a sponge. Groundwater is a volume of wet rock. Picturing it as an underground lake leads to two serious errors: believing that it refills quickly because one sees it as a container, and believing that it is protected because one imagines it closed. Neither is true. AND ONE POINT THAT LIES OUTSIDE THE SCOPE OF THIS BOOK: the ground filters out certain particles, but it makes no water fit to drink. Water from a spring, a well or an aquifer is never drunk on the strength of where it comes from.
Worked example. Two pieces of ground, one and the same shower
Question: "A thunderstorm brings 25 mm in twenty minutes. It falls on a steep clay hillside, ploughed and with no plants on it, and on a flat sandy plateau covered with grass." Where does the water run off the most? Example guided up to the next-to-last step.
- Write down what is being looked for
We are looking to compare the share of runoff on two pieces of ground receiving exactly the same rain. Since the rain is identical, it cannot explain the difference: everything will come from the ground itself. - List the factors on the first piece of ground
Clay hillside: impermeable ground. Steep slope: the water leaves quickly. Ploughed and with no plants: bare ground, so pores closed by the beating of the drops, and nothing to slow anything down. Three factors out of three push towards runoff. - List the factors on the second piece of ground
Sandy plateau: permeable ground. Flat: the water stays where it is and has time to enter. Covered with grass: the roots open passages and the blades slow the drops. Three factors out of three push towards infiltration. - Take the intensity into account
25 mm in twenty minutes is intense rain. That factor works against infiltration on BOTH pieces of ground: even sand can only take water in at a certain rate. Some runoff is therefore to be expected everywhere, including on the plateau. - Over to you: conclude, and work out the input
Say which of the two pieces of ground runs off the more, then work out how many litres of water this same thunderstorm laid down on one hectare, that is to say on 10,000 square metres. The relation you need is in chapter 4. Check it in the answer to the self-check, question 5.
Experiment 6. Making water run off and soak in, side by side
What you need: two identical trays or two identical shallow boxes, sand or garden soil, a flat plate, a watering can or a bottle pierced with a small hole, a measuring jug, a notebook. SAFETY: this experiment is carried out with an adult's agreement, and the adult is present. It makes things wet: it is the adult who chooses the place, outdoors or in a sink, never on a slippery floor nor near an electrical appliance. Sand and soil are washed off the hands afterwards, before touching anything else.
- Prepare the two pieces of ground
Fill the first tray with sand or soil, three or four centimetres deep, without pressing it down. Leave the second empty and lay the flat plate upside down at the bottom: it will play the part of an impermeable surface, like a concrete slab. - Tilt the two in the same way
Wedge both trays with the same object under the same side, so that the two slopes are identical. Only one variable must change from one tray to the other: the nature of the surface. If the slopes differ, the experiment no longer shows anything. - Place a container at the bottom of each slope
Set a glass at the low point of each tray, so as to collect whatever flows over the surface. It is that glass which will measure the runoff. - Water them identically
Measure out the same quantity of water for both with the measuring jug, a quarter of a litre for instance. Pour slowly and steadily, from the same height and for the same length of time, with the watering can or the pierced bottle. Water poured all at once is not rain. - Measure what has arrived at the bottom
Pour the contents of each collecting glass into the measuring jug and note the two volumes. The difference between the water poured and the water collected is the share that did not run off. - Read the result
The tray with the plate collects almost everything: the water ran off. The tray of sand or soil collects little or nothing: the water soaked in. You have just reproduced, over thirty centimetres, the difference between a wooded hillside and a car park. - Do the same thing again on already wet ground
Start again on the tray of soil without letting it dry out. This time the collecting glass gathers far more. You have just shown saturation: the same ground, the same rain, and a far stronger runoff because the voids were already full.
What to remember from this chapter
- Water that has fallen on the ground has three fates: to run off along the surface, to soak in, or to go back into the air by evaporation.
- A material is permeable when its voids connect and let water through; sand is permeable, clay is not.
- Water that soaks in goes down as far as an impermeable bedrock, stops, and fills the voids of the rock lying over it.
- That rock is the aquifer, the water it holds is the groundwater, and the water table is the body nearest the surface, so the most easily accessible and the most exposed.
- Groundwater is not an underground lake: it is a volume of wet rock, which flows very slowly.
- The share that runs off rises with the slope, the impermeability, the intensity of the rain, the absence of plant cover and the prior saturation of the ground.
Self-check for chapter 5
Answer in writing, using only the knowledge given by this volume so far. The answers are gathered at the end of the volume.
- Sand and clay are both loose. Why is one permeable and the other not?
- Describe in three sentences what happens to a drop that soaks in, until it comes to a stop. Use the words aquifer, groundwater and impermeable bedrock.
- Fine rain falls for six hours on ground already sodden with water. A brief thunderstorm falls on the same ground after three weeks of drought. Say first what the question allows you to settle and what it does not, then name the factors at work in each case.
- A neighbourhood replaces a park with a car park. Cite two consequences for water, one at the surface and the other underground.
- Take up the worked example of the two pieces of ground again: conclude on the one that runs off the more, then work out the volume of water received by one hectare under 25 mm of rain.