The Drainage Basin System

Question 1

What is a drainage basin?

Answer 1

An area of land that is drained by its land and tributaries.

Question 2

What is the end of a river basin marked by?

Answer 2

A boundary known as the watershed.

Question 3

What kind of system is the drainage basin?

Answer 3

An open system

Question 4

What is the water balance equation expressed as?

Answer 4

Precipitation = total runoff x evapotranspiration +/- storage (in soil and rock).

Question 5

What does it mean if the runoff percentage is high?

Answer 5

Most of the precipitation is transferred straight to the river - little is stored or lost on the way. Flooding is likely.

Question 6

Why might there be a low runoff percentage?

Answer 6

Because of a heavily fostered drainage basin or a drainage basin with permeable rocks.

Question 7

Why would the drainage basin system be different in winter?

Answer 7

Because deciduous tress cover the basin.
During this season, the trees lack leaves, meaning less water can be absorbed onto the leaves - so less evapotranspiration.

Also, there will be a decrease in interception, resulting in more precipitation entering the river in a shorter space of time which increases the likelihood of a flood.

Question 8

Why might increased precipitation lead to a rise in the water table?

Answer 8

More precipitation leads to more infiltration (because of the greater volume of water) which is then soil water, and the ground is saturated.

Because the soil is now unable to absorb more water, it becomes overland flow and makes its way to the river where it becomes channel flow and the water table rises.

Question 9

What happens if the water table rises?

Answer 9

Water cannot infiltrate to the soil or rocks underground because the ground is saturated.

Therefore, precipitation must travel as overland flow to the stream which increases its volume and causes flooding.

Question 10

What happens if the temperature has been below 0*C for several weeks and it has been snowing (infiltration)?

Answer 10

The ground freezes. So the precipitation is unable to infiltrate through the soil to reduce runoff - which is particularly a problem in higher areas and increases the likelihood of flooding.

Question 11

What halogens if there is there is an increase in sub-zero temperatures at the bottom of the slope?

Answer 11

There will be an increased flood risk.

This is because when the temp is below 0C, the precipitation falls as snow, but when the temp rises to about 0C, of the snow melts into liquid water.

Because this happens in a short time, there is too large of a volume to infiltrate into the soil and it becomes saturated.

This means that melted snow travels as overland flow to the stream in a large volume, increasing the likelihood of a flood.

Question 12

Why should a land owner not consider chopping down trees and using the land for arable crops?

Answer 12

Because there will be an increased risk of flooding.

This is because there are fewer trees to intercept precipitation. Thus, a larger volume of water is able to travel to the river in a shorter space of time (causing flooding).

Question 13

How would the hydrology of a slope be affected by chopping down trees for arable crops?

Answer 13

More infiltration because all of the water will try to infiltrate through to the water table via the soil layers because the water will go directly from the cloud to the ground with no precipitation being intercepted then evaporation from that vegetation.

Question 14

Define water balance.

Answer 14

Within a drainage basin, the balance between inputs and outputs.

Question 15

Soil moisture recharge?

Answer 15

Precipitation is greater than potential evaporation.

The soil water starts to fill up.

Question 16

Soil moisture surplus?

Answer 16

Precipitation is greater than potential evaporation.

Soil water is full which gives a surplus of soil moisture for plant use and runoff into streams.

Question 17

Soil moisture use?

Answer 17

As it warms up, potential evapotranspiration is greater than precipitation.
The water store is gradually used by plants.

Question 18

Soil moisture deficit?

Answer 18

There’s a deficit of soil water, so plants must have adaptations to survive for long periods in dry conditions, otherwise they will wilt.
Alternatively the land must be irrigated.

Question 19

When is there soil moisture recharge?

Answer 19

In the winter months.

There is more precipitation and it exceeds evapotranspiration so the soil waters fill up.

Question 20

When if there a soil moisture use?

Answer 20

At the beginning of summer months (may - july). Evapotranspiration exceeds precipitation, so the soil moisture begins to be utilised before the soil dried out and there is a soil deficit.

Question 21

Why is the water balance important?

Answer 21

Ira important if you want to understand how runoff changes throughout the year as it shows the equilibrium in the drainage basin between the inputs and outputs, and changes in ground storage.

Question 22

Define field capacity.

Answer 22

The amount of water the soil can hold.

Question 23

Why does the field capacity remain the same throughout the year?

Answer 23

Because no matter how much precipitation there ism the amount of water the soil can hold (field capacity) won’t change.

Question 24

From a soil moisture graph, how would you be able to tell that the temperature have only changed a little?

Answer 24

Because the potential evapotranspiration remains very similar.

Question 25

Define bankfull.

Answer 25

The maximum discharge that a river channel is capable of carrying without flooding.

Question 26

Define base flow.

Answer 26

The normal day-to-day discharge of the river.

It is the consequence of slow moving soil throughflow and groundwater seeping into the channel.

Question 27

Define discharge.

Answer 27

The amount of water in a river flowing past a particular point (expressed as cumecs).

Question 28

Define river regime.

Answer 28

The variability in a river’s discharge throughout the course of a tear in response to precipitation, temperature, evapotranspiration and drainage basin characteristics.