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Welcome!

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My name is Hubert Savenije and I am a Hydrologist

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Hydrology is the science of water.

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It tries to describe and understand how water
moves over and through the Earth and

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to find out what the physical processes are
that drive the movement of water.

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More simply, you could say that

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"Hydrology is the science of the origin and
fate of water on Earth"

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But if you ask me, I would just say that

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“Hydrology is Beautiful”.

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Even if it rains, or especially when it rains.

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Because everything we do and because everything
living on Earth depends on water, there are

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large and important scientific questions that
demand our attention.

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Such as: Where does the water come from?

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How much water is there available for development,
for feeding hungry mouths and for a healthy

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environment?

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How can we better protect ourselves from water
threats, such as floods and droughts?

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How can we maintain a healthy environment
in a rapidly developing world?

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How does climatic change propagate into the
hydrological behaviour and water resources

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availability?

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In which way do we, as human beings, influence
the hydrology, and is there an interaction?

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And many people (particularly politicians)
want us to tell when we can expect a flood.

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But that, unfortunately, is a question we
cannot answer.

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We can communicate the probability of an extreme
event to occur,

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but not easily the moment when it will occur.

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Prediction is difficult in hydrology, particularly
when it relates to the future!

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The question “where the water comes from”
has inspired the great philosophers.

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The early Greek philosophers indeed believed
that river water and groundwater were fed

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by rain, but they also liked to compare the
Earth to the human body.

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Leonardo Da Vinci, whom we all know as the
painter of the Mona Lisa,

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was not only a great artist, he was also an
engineer, a writer and a scientist.

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Leonardo wanted to understand what he painted.

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To be able to paint water and clouds realistically,
he tried to better understand how it behaved.

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He developed instruments and devices to measure
flow velocity, wind velocity, air humidity

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etc. and he came to incredibly accurate insights.

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He even designed sluice gates that are still
in use in The Netherlands!

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However, he respected the classical ideas
of Plato so much, that he thought that water

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was pumped through the Earth in analogy to
the human body and

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he thought that sea water was pumped through
the Earth to appear again on the top of mountains.

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It was more than 100 years later till the
Frenchman Perrault could demonstrate that

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precipitation was sufficient to maintain the
flow of the river Seine

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which was a revolutionary idea because the
famous English scientist Halley (the one from

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the comet) still believed that much of the
river water originated in caves where moist

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air condensed against the cold mountain rock.

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The world’s water resides in a system of
interacting stocks and fluxes.

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Stocks are represented by boxes; and fluxes
by arrows.

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These boxes and arrows have very different
sizes and magnitudes

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By far the largest stock of water is in the
oceans and seas. But this water is saline.

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The largest stock of fresh water is in the
polar ice. But this is hardly accessible.

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Then there is a huge stock of water deep under
the ground: in large alluvial plains and even

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under the Sahara.

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But this water is often fossil, not being
replenished at a human time scale.

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The only sustainable stocks of water are the
amounts that are regularly renewed

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These stocks generally lie close to the surface.

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We have come to indicate these stocks and
fluxes by the colours blue and green.

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We distinguish the light blue water, which
we can see on the surface,

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from the deep blue water which feeds the surface
water from underground.

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An equally large resource is the green water
(a term coined by Prof Malin Falkenmark),

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which is the water stored in the soil and
used by plants to produce biomass.

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This is the water that feeds the world population
by agricultural products and which sustains

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our biomass based economies.

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Part of the precipitation does not become
blue or green water, but evaporates back to

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the atmosphere directly.

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I name this white water.

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Here we see an overview of the magnitudes
of these stocks and fluxes expressed per unit

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surface area.

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They are approximate values, but they clearly
show that some fluxes are very large (the

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atmospheric flux) and some are very small
(deep blue).

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Conversely, we see very large stocks (the
oceans), whereas some are very small (the

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white stock).

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The right column shows the ratio between the
stocks and the fluxes, which represents the

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residence time.

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Let’s look at these balances in an analytical
way.

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The water balance is the most basic equation
in hydrology. It implies conservation of mass.

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It shows that if there is an imbalance between
inflow and outflow, that there then should

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be an increase of the storage over time.

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Or: the time derivative of the storage is
the difference between inflow and outflow.

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In hydrology, the inflow can be an inflow
of water, but also precipitation on a surface.

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Outflow can be the river discharge, but also
the evaporation from the surface.

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An interesting property of such water balance
systems is that if we divide the storage by

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the outflow, we obtain a number with a time
dimension.

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This number represents the average time that
a water particle resides in the stock.

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More correctly is it to say that this ratio
of stock to flux is the time scale of the process.

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If we now look again at our global water resources
table, then we see that the Oceans have the

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largest residence time.

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Not surprising. A water particle, once it
ends up in the ocean, has to wait on average

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28000 years before it may again travel to
the land.

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In the atmosphere, however, a water particle
resides only a few weeks, on average.

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And thanks to the storage in the root zone
of plants, they can survive half a year without

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rainfall, on average.

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You may wonder why the flux from land to ocean
(through the rivers) is 310 mm/a and that

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while the flux from ocean to land
(through the atmosphere) is 130 mm/a.

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How come these numbers are not equal?
Shouldn’t they be the same?

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Of course they are the same! We only have
to multiply them by the right surface area.

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Because if we look at this picture, than,
on average, the fluxes A and Q should be equal

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and opposed.

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Otherwise the storage in the ocean would either
increase or decrease without end.

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You may be wondering what happens with the
moisture you exhale.

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Does the moisture we exhale fall back as precipitation,
or does if flow back to the ocean through

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the air?

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If we look at the global water balance again,
then we see that it rains 720 mm/a on Earth,

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while the net atmospheric influx is only 310
mm/a; a factor 2.3!

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In fact a substantial part of the precipitation
finds its origin in terrestrial evaporation.

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If we equate the terrestrial precipitation
to 100%, then 40% of this water comes from

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terrestrial evaporation.

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Just look at the picture: Of the 100% precipitation
about 70% evaporates.

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A bit more than half of it returns on land
and the rest flows back to the ocean through

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the atmosphere.

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But it depends strongly on where you are on
Earth.

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This is the paper by Ruud van der Ent, which
describes this in detail.

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As a result of the dominating westerly winds
on the Northern Hemisphere, exhaled moisture

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in The Netherlands is likely to end up in
China.

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But exhaled moisture in China is likely to
end up in the Pacific Ocean.

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Here we see in red the parts of the world
where the precipitation largely consists of

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recycled moisture.

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We see that China and West Africa strongly
depend on recycled moisture.

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But where did this moisture come from?

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Here we see in red the reverse. Red are the
source areas where the chance of evaporation

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ending up on land is larger than 60%

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These are the areas where land evaporation
has a significant influence on precipitation.

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So the Amazon forest in South America; the
Great Lakes area in Africa and Eastern Europe

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are very important source areas to sustain
continental rainfall

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And land use change in these areas may have
unexpected consequences downwind.

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Although global hydrology is extremely relevant
for the analysis of human impacts on the planet,

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the natural limits of a hydrological system
are much smaller.

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The natural boundary of a hydrological system
is the watershed, catchment, or river basin

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(in increasing order of size).

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This is because precipitation falling on a
catchment has only two ways out: discharge

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through the outfall or evaporation back into
the atmosphere.

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There is no other inflow assumed to be there
than the precipitation.

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So the water balance reads that: The change
of storage over time equals the rainfall minus

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the evaporation, minus the runoff through
the outfall.

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Of course all terms in the equation need to
have the same dimensions, so if we express

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precipitation and evaporation in [L/T] then
they have to be multiplied by the size of

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the catchment area A.

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But we could also express all terms in [L/T]
and in that case the discharge and the storage

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need to be expressed per unit area.

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But is the water divide always a real divide?

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Not all runoff is generated over the surface.

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A considerable part flows to the river through
the groundwater,

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and because of the sometimes complex geology
the topographic divide and the groundwater

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divide does not have to coincide.

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Particularly in karstic or mountainous environments
this can lead to substantial errors when one

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tries to close the water budget.

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Here we see the water budgets of some of the
major river basins of the world.

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We see that they differ in size (the Mississippi
and Ob being among the largest and the Rhine

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being relatively small)

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We also see that the precipitation varies
from 1500 mm/a in the Mekong to only 220 mm/a

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in the Nile.

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The evaporation from a catchment, of course,
is always smaller than the precipitation (because

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in a catchment the precipitation is the only
inflow)

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But the proportion of evaporation to rainfall
varies a lot between catchments.

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In the Nile, the Zambezi and the Mississippi,
containing substantial semi-arid parts, the

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evaporation is more than 80% of the precipitation.

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But in more humid climates (particularly the
Orinoco) the evaporation is only 30% of the

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precipitation.

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Of course, the remainder is the runoff. And
the Orinoco where 30% of the precipitation

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evaporates, hence has a runoff ratio of 70%

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The Orinoco drains essentially tropical rainforest.

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Therefore it is interesting to see that another
catchment that generates a lot of runoff

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(60% of the precipitation) is a catchment in cold
Siberia.

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Both have insufficient solar energy to evaporate
most of the precipitation: the Orinoco because

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it rains so much, the Ob because there is
insufficient solar energy.

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So different regions and different landscapes
behave very differently all over the world.

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Hydrology is the science that wants to describe
and understand this behaviour.

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The landscape reflects this behaviour and
if we read the landscape well, we can learn

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a lot more about its properties and dynamics.

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I hope you enjoyed this part of the course
and remember:

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Hydrology is the basis of all other water-related
disciplines and of the management of our resources.