Lesson 4-What are drainage basins and what is the water balance?

Question 1

Local water cycle

Answer 1

• In a local drainage basin system , water may be lost as an output through evapotranspiration and runoff, but more water may be gained as an input through precipitation.
• As the inputs and outputs are not balanced, it is an open system

Question 2

Global water cycle

Answer 2

• The global water cycle is a closed system.
• No water can enter or leave the system
  however energy can.
• The process that occur define how water is transferred and stored within the system.
• Water is always on the move.

Question 3

What is a drainage basin?

Answer 3

• A drainage basin (or catchment) is an area of land which is drained by a river and its tributaries.
• The boundaries are marked by the watershed.
• Drainage basins can cover many square miles/km over a number of countries

e.g. the Nile River basin drainage area is more than 3 million km2 over 12 countries with 73 % of the drainage basin in Sudan and Egypt

Question 4

Characteristics of a drainage basin?

Answer 4

• Watershed> seperate to drainage basins
• Source>start of the river
• Confluence> where two rivers meet
• Tributary>when the river splits
• Mouth>meets the sea

Question 5

What is the water balance?

Answer 5

• The water balance helps to understand the behaviour and water balance of individual drainage basins

Question 6

When does the water balance vary?

Answer 6

• Dry seasons
• Wet seasons

Question 7

Seasonal changes in water balance- In wet seasons

Answer 7

• In wet seasons, precipitation is greater than evapotranspiration which creates a water surplus.
• Ground stores fill with water which results in increased surface runoff, higher discharge and higher river levels.
• This means there is apositive water balance.

Question 8

Seasonal changes in water balance- In dry seasons

Answer 8

• In drier seasons evapotranspiration exceeds precipitation, as plants absorb water, ground stores are depleted.
• This produces a water deficit at the end of a dry season generating anegative water balance.

Question 9

What is the soil water budget?

Answer 9

• This shows the balance between precipitation (inputs) and potential evapotranspiration (outputs) of soil as a store over the year
• In summer there is more evapotranspiration than precipitation
• In winter there is more precipitation than evapotranspiration

Question 10

Soil moisture graph?

Answer 10

• Pink- Water surplus
• Yellow- Water deficit
• Orange- Precipitation
• Green- Evapotranspiration

Question 11

Soil moisture graph for an equatorial climate

Answer 11

• The temperatures vary very little throughout the year.
• This means potential evapotranspiration remails relatively constant throughout the year.
• The high rainfall fills the soils rapidly. In the short times between rainfall maxima soil water does not go into defecit and so rivers and plants have a source of water all year round

Question 12

Soil moisture graph for a tropical wet and dry climate

Answer 12

• The tropical wet/dry area experiences a dry season (July to October) where soils are reduced until there is a surplus.
• This surplus does not last long when rain stops

-There is a short period of utilisation but from December to July the soil is dry. River levels fall with some drying up.

• Vegetation has adapted to this seasonal water supply by evolving to survive drought conditions

Question 13

How does water move through the drainage basin?

Answer 13

• Above ground flows
• Below ground flows

Question 14

Above Ground Flows

Answer 14

• Throughfall
• Drip Flow
• Stem Flow
• Overland flow
• Hortonian overland flow

Question 15

Below Ground Flows

Answer 15

• Infiltration
• Throughflow
• Percolation
• Base flow/ ground water flow

Question 16

Above ground flows- Throughfall

Answer 16

Precipitation reaches the ground without ever been intercepted

Question 17

Above ground flows- Drip Flow

Answer 17

Water flows off leaves to the ground

Question 18

Above ground flows- Stem Flow

Answer 18

Water flows down tree trunks and stems to the ground

Question 19

Above ground flows-Overland Flow

Answer 19

Water flows over the ground either in a river channel (channel flow) or as sheet flow over the surface of the ground

Question 20

Above ground flows- Hortonian overland flow

Answer 20

• Water flows horizontally across land when rainfall has exceeded infiltration capacity and storage capacity.
• The water cannot enter the ground, so it runs straight off the surface (surface run off)

Question 21

Below ground flows-Infiltration

Answer 21

Water flows vertically into the ground from the surface

Question 22

Below ground flows- Throughflow

Answer 22

Water flows horizontally under the ground in the soil through lines of seepage called percaline

Question 23

Below ground flows- Percolation

Answer 23

Water flows vertically though the soil layers

Question 24

Below ground flows- Base flows/ ground water flow

Answer 24

Water that has infiltrated and percolated flows in saturated rocks (aquifers)

Question 25

Inputs and outputs of the drainage basin?

Answer 25

• Inputs-Rain, sleet, snow
• These vary throughout the year due to seasonal variation
• Outputs-Evapotranspiration, transpiration, evaporation

Question 26

Improtant flows within the water cycle?

Answer 26

• Infiltration
• Ground water flow

Question 27

Factors affecting infiltration

Answer 27

• Interception
• Soil and porosity storage
• Strom events and rainfall intensity

Question 28

Factors affecting infiltration- Interception

Answer 28

• Some precipitation will land on a bare surface, but much of it will beinterceptedby the leaves and branches of the vegetation.
• This is important, as it reduces the amount available for overland/surface flow and therefore, there is a reduction in soil erosion and flooding
  .
  -Coniferous trees intercept 22% of rainfall
  -Deciduous trees intercept 19% of rainfall

Question 29

Factors affecting infiltration- Soil porosity and storage

Answer 29

• Soil porosity is the most important factor, and is controlled by the following factors:
  -Texture of the soil (proportions of sand, silt and clays)
  -Structure of the soil (arrangement of the soil)
  -Organic content of the soil
  -Coarse-textured soils have larger pore spaces than fine-textured soils,
• Many plants utilise the water stored in the soil (vegetation storage) for growth.
  -As a consequence water is lost from the system through the process of transpiration.

Question 30

Factors affecting infiltration-Storm events and rainfall intensity

Answer 30

• Many plants utilise the water stored in the soil (vegetation storage) for growth.
  -As a consequence water is lost from the system through the process of transpiration.
• If rainfall intensity is greater than the infiltration rate then the soil has reached infiltration capacity (i.e. the soil is saturated).
  -As a result water will build up on the surface as surface storage (e.g. puddles). Much surface storage evaporates and is lost from the system.

Question 31

Flow in water cycle-Ground water flow

Answer 31

• Percolation occurs and is then be held in pore spaces in the rocks as groundwater.
• It then passes slowly into the zone of saturated rock where it can then move laterally by the process of groundwater flow.
• This is a slow movement and can feed rivers through long periods of drought.
• Porous and permeable rocks are able to store a lot of water, these are known as aquifers.