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CTB3300WCx Introduction to Water and Climate

GWC 4 Runoff Generation

Hubert Savenije

Welcome! My name is Hubert Savenije and I am a hydrologist.

The most visible and most intriguing part of the hydrological

cycle is the runoff. very person has at some stage looed in

fascination at flo"ing "ater# or been inspired by the beauty or

violence of a stream $lo"ing "ater seems alive. %nd it is an

inseparable part of our life. Water has been essential to create

and support our societies and sometime it threatens it "ith its

destructive force. %s "e sa" earlier# it too philosophers and

scientists some time before they reali&ed that river runoff iscompletely dependent on precipitation. %t first they thought

that there "as not enough precipitation to feed the rivers# but

in fact there is a lot more than that. 'lobally# less than ()* of

the precipitation leads to runoff.

'lobally# less than ()* of the precipitation leads to runoff. %s

precipitated moisture travels through the terrestrial cycle# it is

step"ise partitioned into other flu+es. The first partitioning

point is the surface. If rain falls on "ater or on saturated land#

then it runs off directly to the open "ater. We call this

saturation e+cess overland flo". %bove land# rainfall is

intercepted by, leaves# the litter on the ground and the ground

itself -"here it may pond and from there it evaporates bac to

the atmosphere. The typical time scale of this process is

bet"een an hour and a day. If the ponding is large enough#

"ater may flo" overland and find its "ay to the stream. We

call that infiltration e+cess overland flo"# or Hortonian

overland flo"# after the great %merican hydrologist Horton.

This overland flo" is very fast# "ith a time scale of hours. The

remainder infiltrates into the soil# from "here it can transpire

or percolate into the ground"ater. /uring a rainfall event# the

soil can become so saturated that pocets of saturation occur

under the ground at relatively shallo" depth. If the terrain is

sloped# then this sub0surface pocet may connect and

generate subsurface runoff through a net"or of preferential

channels. These channels have been created by the "ater

itself# maing use of root channels# animal burro"s and

fissures. In mountainous and hilly area# this is a dominant

mechanism# "hich "e call storage e+cess subsurface flo" or

shortly interflo"2. The "ater that percolates to the

ground"ater subse3uently feeds the very slo" ground"ater

flo"# that sustains the base flo" of the river. $inally#

ground"ater can be e+changed "ith neighbouring areas or

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may rise to the unsaturated soil by capillary rise. The many

storages and thresholds in this system are the reason "hy the

rainfall0runoff processes is highly non0linear# although the

individual processes can often very "ell be described by linear

processes# just lie "e sa" "ith the ground"ater seepage.

So the fast surface runoff is the precipitation minus the

interception and the infiltration. The fast sub0surface runoff is

the infiltration minus the transpiration# the soil evaporation

and the percolation. %nd the base flo" e3uals the percolationminus the capillary rise.

It is hard to distinguish these different runoff components

once the "ater has entered the river. 5ut the "ater can reveal

its origin "hen "e loo at its colour and composition. If it is

very turbid# and flo"ing fast# then it stems from surface runoff.

If it flo"s fast and is mostly clear# then it stems from rapid

subsurface flo". If the "ater is clear and the flo" is 3uiet# then

it consists of seepage flo" from 'W. $rom the chemical

elements in the "ater# the isotope composition and the

turbidity "e can deduct the origin of the "ater# but also the

time it spent in the catchment before it came to runoff 5ut

that goes beyond this introductory course.

/epending on the climate and the physical properties of the

landscapes in the catchment# rivers have different runoff

signatures. Here "e see clearly different signatures from t"o

neighbouring catchments. 6n average# the 7hine has about ten

times more discharge than the Meuse. 8ot only is the runoff of

the 7hine larger# it is also less variable over the year. What

maes these patterns so different9 Is it climate9 :It2s the

landscape# the climate is not so different;. The mountainous

part of the 7hine# "ith its sno" cover and glaciers maes that

the runoff during the dry summer months is still substantial#

"hereas the runoff of the Meuse in de dry summer months is

very lo".

5ut climate can be very important. Whether a river is

ephemeral# intermittent or perennial depends primarily on the

climate.

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