1.2- Drainage Basins As Open Systems

Question 1

What is a drainage basin?

Answer 1

The area of land drained by a river and its tributaries

Question 2

What separates one drainage basin from another?

Answer 2

An imaginary line called a watershed

Question 3

The drainage basin is considered to be an open system with

Answer 3

Inputs, and outputs of energy and stores and transfers of water

Question 4

Drainage basins come in a variety of scales from the very small to continental e.g.

Answer 4

The Mississippi Basin

Question 5

The drainage basin is part of the hydrological cycle- it starts when

Answer 5

Precipitation (input) first lands on the surface and finishes when water leaves the drainage basin either by evapotranspiration (output) or as the river enters the sea through runoff (output)

Question 6

Precipitation is an input- explain the main points

Answer 6

• water enters drainage basin as precipitation
• plants and trees may intercept some water, where it may be stored before being evaporated
• it takes time for water to drip through leaves or down the stem (stemflow) to the ground surface
• it is stored as puddles, flows over the ground as overland flow or infiltrates the soil
• some water is taken up by plants before being transpired

Question 7

What are the 2 main outputs?

Answer 7

1- evapotranspiration (total output of water from the drainage basin directly back into the atmosphere)

2- runoff (all the water that enters a river channel and eventually flows out the drainage basin)

Question 8

As water travels through the drainage basin it can stop, for a while, in a number of

Answer 8

Stores

Question 9

What are the main stores?

Answer 9

• interception
• surface water
• soil water
• groundwater
• channel storage (water in the river)

Question 10

Explain how interception is a store

Answer 10

• when precipitation lands on leaves and branches of vegetation
• some vegetation is a better interception store than some others
• deciduous broad leaves intercept more precipitation than the needles of a coniferous tree

Question 11

Explain how surface water is a store

Answer 11

• refers to when water accumulates on the ground, often in the form of puddles
• while common in a man-made environment, they are naturally rare in natural environments
• water usually infiltrates the ground at a greater rate than it rains- water can only build up on the surface after a long period of rain, an intense rainstorm or an impermeable surface (either man-made e.g. concrete or natural e.g. frozen surface)

Question 12

Explain how soil water is a store

Answer 12

• simply water stored within the soil
• it occupies the pore spaces between soil particles
• coarse, sandy soils absorb more water rapidly than clay soils which have tiny pore spaces
• when water passes through the soil,the transfer is known as throughflow

Question 13

Explain how groundwater is a store

Answer 13

• simply water in the ground (rocks) beneath the surface

- some rocks are able to store a lot of water, especially if they are porous like chalk = AQUIFERS

Question 14

What are the main transfers/flows?

Answer 14

• stemflow
• infiltration
• overland flow
• channel flow

Question 15

Explain stemflow

Answer 15

• water making its way from the leaves of vegetation to the ground
• this drips/falls from leaf or branch to another until it eventually reaches the ground
• water also flows down stems of grasses and in very heavy storms, it can flow straight down the trunks of trees

Question 16

Explain infiltration

Answer 16

• water’s ability to sink into the soil
• speed at which this happens is called the infiltration rate/ capacity
• if there is intense or prolonged then the infiltration rate may not be fast enough to absorb all the water

Question 17

Explain overland flow

Answer 17

• if water is unable to infiltrate it may run off the surface as overland flow flowing across a large surface area (sheetflow) or concentrated into smaller channels called rills
• overland flow on agricultural land is not common in the UK as much of the land is covered by vegetation, although it can sometimes be seen in winter when the soils are bare
• in urbanised areas, particularly on roads, overland flow is extremely common often exacerbating flooding

Question 18

Explain channel flow

Answer 18

• all of those flows lead water to the nearest river or deep groundwater storage
• the river then transfers the river by channel flow
• the amount of water that leads the basin in this way is called runoff

Question 19

Any changes to inputs, transfers or stores by people or natural means will have a knock-on effect somewhere else in the system- for example

Answer 19

• deforestation in a river basin will result in less interception and transpiration and eventual increase in channel flow
• a drought may cause a reduction in runoff and decrease in other stores and transfers such as infiltration and soil water storage and ground water store = depleted aquifers

Question 20

The amount of water in the drainage basin will vary over time depending on the balance between

Answer 20

Inputs and outputs

Question 21

The water balance in a drainage basin shows the balance between

Answer 21

Inputs (precipitation) and the outputs (runoff and evapotranspiration) together with changes in groundwater storage

Question 22

The water balance can change with the

Answer 22

Seasons

Question 23

In the UK there is usually a _____ _____ during the winter and early springs

Answer 23

Water surplus- evaporation is greater than transpiration = high run off

Question 24

During the summer there may be a ______ _____ as temperatures rise and vegetation grows quickly- groundwater stores and water from springs may be needed to keep rivers flowing

Answer 24

Moisture deficit

Question 25

In order to gain a better understanding of the drainage basin system we can use a simple equation called the general water balance equation =

Answer 25

P= Q+E (+/- change in storage)

P= precipitation 
Q= runoff
E= evapotranspiration

Question 26

Regarding water balance, explain the events in chronological order

Answer 26

1- water surplus: Dec- Feb
2- soil moisture utilisation: March-June
3- soil moisture deficiency: July- Sept
4- soil moisture recharge: October-Dec

Question 27

What occurs in water surplus?

Answer 27

• precipitation is greater than potential evapotranspiration
• soil moisture store is full and there is a soil moisture surplus for plant use, runoff and groundwater recharge

Question 28

What occurs in soil moisture utilisation?

Answer 28

• potential evapotranspiration greater than precipitation

- the water store is being used up by plants or lost by evaporation

Question 29

What occurs between soil moisture utilisation and deficiency?

Answer 29

• soil moisture store now used up
• any precipitation is likely to be absorbed by the soil rather than produce runoff
• river levels will fall or dry up completely

Question 30

What occurs in soil moisture deficiency?

Answer 30

• deficiency of soil water as the store is used up and potential evapotranspiration is greater than precipitation
• plants just have adaptions to survive and crops must be irrigated

Question 31

What occurs in soil moisture recharge?

Answer 31

• precipitation is greater than potential evapotranspiration
• the soil water store will begin to fill again

Question 32

What occurs between soil moisture recharge and water surplus?

Answer 32

• soil water store is full
• field capacity been reached
• additional water will percolate down to the water table and groundwater stores recharged

Question 33

What does field capacity refer to?

Answer 33

Maximum amount of water soil can hold:
- a water surplus can result in saturated soil, high river levels and run-off whereas a deficit leads to dry soil, falling river levels and possibly drought

Question 34

An important aspect of the water balance equation is

Answer 34

Total runoff (expressed as a % of precipitation) = measure of total precipitation that makes its way into rivers and streams

Question 35

Runoff % will vary considerably between

Answer 35

different river basins and will depend on a range of factors

Question 36

What are the different factors that influence water flows within a drainage basin (run off variation)?

Answer 36

• vegetation and land use
• soil type and depth
• bedrock
• rainfall
• climate
• shape of the land
• conditions in river basin
• size and shape of river basin

Question 37

Explain how vegetation and land use alter run off variation

Answer 37

• amount of vegetation may vary seasonally
• type and amount of vegetation cover will affect the amount of interception storage as well as amounts of throughfall and stemflow
• vegetation tends to reduce the amount of overland flow; bare ground will increase it
• impermeable surfaces created by urban development will also reduce infiltration and increase overland flow and increasing urban sprawl will further drive this

Question 38

Explain how soil type and depth alter run off variation

Answer 38

• deeper soil able to store more water = high field capacity
• soils with smaller pore spaces in them, such as clay, will reduce infiltration

Question 39

Explain how bedrock alters runoff variation

Answer 39

• impermeable bedrock such as granite will prevent groundwater flow and encourage throughflow and overland flow
• rocks with air spaces or cracks in them will let water through
• however,if these rocks become saturated they will act as though they are impermeable

Question 40

Explain how precipitation alters runoff variation

Answer 40

• intense rainfall will tend to increase overland flows and cause a sudden rise in discharge
• gentle rain over a longer period of time allows more time for infiltration

Question 41

Explain how climate alters runoff variation

Answer 41

• the distribution of rainfall over the year affects how the drainage basin will responds
• some areas may have a very seasonal pattern with rainfall only in one season
• temperature conditions are also important as they will affect evapotranspiration and may determine whether water is stored as ice or snow

Question 42

Explain how shape of the land alters run off variation

Answer 42

• steep slopes will encourage overland flow

- if the drainage basin has many surface storage stores such as lakes, ponds and hollows, this will reduce river flow

Question 43

Explain how conditions in the drainage basin alter run off variation

Answer 43

• these may have considerable effects on water flows
• ground that is frozen or has a baked surface crust will produce more surface runoff
• if the ground is already wet, overland flow will occur sooner

Question 44

Explain how the shape and the size of the river basin alter run off variation

Answer 44

• larger basins result in more runoff
• shape of basin can affect the time taken for rainfall to reach the river channel
• a high density of tributaries can produce high flows