Welcome to this lecture on catchments and the water balance.

A catchment, is the area the drains to a river.

It's surrounded by the water divide,

which separates different catchments from each other.

If we have a mountain here and it rains,

we see that water that falls in this side of the mountain,

ends up on this side,

and water that falls in this side of the mountain,

flows in this direction.

So what we see is that there is this point where the water flows into one direction,

and on the other side of this point,

water flows into the other direction.

This is the water divide.

In mountainous terrain, this is always located at the highest point in the landscape.

So if we have to rivers,

we can still determine the water divide between these two rivers,

and see in which direction the water flows,

whether it ends up in catchment one,

or in catchment two.

Here in Lunghin Pass,

we have a very special situation.

This is Europe's only three way water divide.

To the east, water flows into the Inn,

and then via the Danube ends in the Black Sea.

To the north west, water flows into the Julia,

and then via the Rhine,

ends up in the North Sea.

And to the south, the water flows into Die Maira,

then to Lake Como,

and then via de Pau ends up in the Mediterranean.

Now the catchment and the water divide are based on the surface topography,

but we have to be careful,

because we may assume that water does fall on this left side of the mountain,

to end up in the stream on the left.

But what may happen is that water flows along these layers of geology,

and instead, end up in this river here on the right.

So once we have determined the location of the water divide,

we can determine the catchment area of the stream at that point.

So here in red, we have the catchment area of the Aare where it flows into the Rhine.

It's a fairly large catchment area,

and it covers a large part of Switzerland.

However, if we would move upstream to brook,

the catchment area of the Aare, again in red,

is much smaller, but we also see the catchment area of des Rois in yellow,

and the Limmat in blue.

And if we would go to Zurich,

and we determined to catchment area for the Limmart there, it's even smaller.

And there we also have the catchment area of the Sihl,

shown here in green.

Sometimes we can see very clearly,

the water from these two rivers mixing.

So here on the right, we have the water coming from

the Limmat out of Lake Zurich and it's very clear.

But on the left we have water coming from the Sihl,

and after several days of heavy precipitation,

it contains a lot of sediment and looks very murky.

Once we have determined the catchment area,

we can calculate the water balance.

The water balance describes how much water is flowing into our catchment,

how much water is going out of a catchment,

and a change in the amount of water that's stored in our catchment.

You can think of the water balance,

as being very similar to the accounting for your household.

You have money coming into your bank account,

you have money leaving,

and as a result there is a change in

the amount of money that is stored in your bank account.

So if we have a catchment,

there is precipitation coming in,

there is water going out through evaporation and to runoff,

and the change in storage,

is the change in the amount of water stored in groundwater,

in soil water, the amount of water stored in lakes,

and wetlands, as well as in glaciers.

We can write the water balance as,

what comes in equals what goes out and the change in storage.

What goes in is the precipitation,

what goes out is the evaporation and the runoff,

and the change in storage are these changes that we talked about.

But we have to be careful because,

water can also come in from other catchments,

for example through pipelines.

Over all of Switzerland,

on average over a given year,

there's 1,456 millimeters of precipitation,

484 millimeters of evaporation,

380 millimeters of runoff coming from other countries,

and 1,296 millimeters leaving Switzerland through runoff.

So we can simplify this and say,

that on average over all of Switzerland,

there is 1,500 millimeters of precipitation per year.

1,000 millimeters of runoff per year,

to two thirds and one third of 500 millimeters per year of evaporation.

You may wonder what is this one millimeter.

Well, one millimeter, is one liter per square meter.

So if we have one meter of water,

and pour it out over an area that is one meter by one meter,

or one meter squared,

it would form a layer that is one millimeter thick.

So one millimeter is one liter per square meter.

In this video lecture,

we've talked about catchments and the water balance.

In the coming lectures,

we will talk about how we measure these different components of the water balance,

and how they vary across Switzerland,

and with the seasons,

and also introduce you to some of the very famous research catchments in Switzerland.

We look forward to seeing you there.