Draining Away

Globally the amount of water stays the same but if you consider the river system of a particular area…

…the amount of water in the river will vary depending on changing local conditions. 

The area from which a river collects its water is called its drainage basin.  Water is added to the drainage basin when it rains, but the amount added depends on how much rain.  At the same time water is leaving the drainage basin.  Some flows down the river; some leaves by evaporation before it even reaches the river.    

Let’s have a look at how these inputs and outputs vary across the British Isles.

Most rain occurs on the west side of both Britain and Ireland, so the drainage basins in these areas get plenty of input.

Temperatures in Britain and Ireland are warmer towards the south.  These areas will have more evaporation which, along with river flow, takes water out of the drainage basin.

So, the biggest inputs are in the west and the lowest evaporation loss is in the colder north, which means the north-west has the most water available for us to use.

But where do most people live?

The shaded areas have an average of more than 250 people per square kilometre. You can see that there is high population over the whole of the south-east, the area with the lowest amount of water input.  And even in other parts of the country, large cities, with people concentrated into a relatively small area, can have issues with supplying enough water to the population.

And its not just households that need a water supply. Join me again next week.