5.1 The Global Water Cycle

Question 1

5.1a The global hydrological cycle

closed system

Answer 1

The global hydrological cycle is the circulation of water around the earth. It is a closed system of linked processes so there are no external inputs or outputs.
For this reason the amount of global water is finite and constant.
The only thing that does change is the state in which the water exists (liquid, vapour, ice).
The proportions of global water held in each stat vary over time with changes in climate

Question 2

5.1a The global hydrological cycle

drivers

what are the main drivers of the global hydrological cycle?

Answer 2

The global hydrological cycle is driven by two sources

1. solar energy: in the form of heat
2. gravitation potential energy: causes rivers to flow downhill and precipitation to fall to the ground

Question 3

5.1b Stores and fluxes

oceans

Answer 3

Oceans, 96.9%, 0%, 3,600 years

Question 4

5.1b Stores and fluxes

atmosphere

Answer 4

Atmosphere, 0.001%, 0.04%, 10 days,

Question 5

5.1b Stores and fluxes

biosphere

Answer 5

Biosphere (vegetation and soil moisture) 0.001% total water, 0.05% freshwater, 2-50 weeks

Question 6

5.1b Stores and fluxes

cryosphere

Answer 6

Cryosphere (icecaps) 1.9% total water, 68.7% freshwater, 15,000 years depending on size

Question 7

5.1b Stores and fluxes

groundwater

Answer 7

Groundwater, 1.1% total water, 30.1% freshwater, up to 10,000 years for deep groundwater, 100 to 200 years for shallow groundwater

Question 8

5.1b Stores and fluxes

surface water

Answer 8

Surface water (rivers and lakes), 0.01% total water, 1.2% freshwater, 2 weeks to 10 years

Question 9

5.1b Stores and fluxes

fluxes

what are fluxes?

Answer 9

Fluxes are the rates of flow between stores

The greater fluxes occur over the oceans

Question 10

5.1b Stores and fluxes

flows

Answer 10

Flows are the transfers of water from one store to another (in km cubed per year)

Question 11

5.1b Stores and fluxes

oceans and atmosphere fluxes

Answer 11

evaporation 400,000

precipitation 370,000

Question 12

5.1b Stores and fluxes

atmosphere and land masses

Answer 12

evaporation 60,000

precipitation 90,000

Question 13

5.1b Stores and fluxes

landmasses and oceans

Answer 13

surface runoff 30,000

Question 14

5.1c The global water budget

non-renewable stores

Answer 14

Fossil water is ancient, deep groundwater made from pluvial (wetter) periods in the geological past

The crysophere is areas of the Earth where water is frozen into snow or ice. The cryosphere is non-renewable but during another glacial period more water will once again be locked in glaciers and ice sheets.

Question 15

5.1c The global water budget

key figures

Answer 15

All water

97. 5% in oceans
98. 5% freshwater

Freshwater
69% in ice caps and glaciers
30% in groundwater
1% as easily accessible surface water

Question 16

5.2a The hydrological cycle

inputs

what is the main input of the hydrological cycle and its key features?

Answer 16

The main input is precipitation

1. amount: this has a direct impact on drainage discharge and the fluxes within it
2. intensity: greater intensity means greater likelihood of flooding
3. seasonality: the drainage basin system operated at different flow levels at different times of the year
4. form: affects entry of water into the drainage system
5. distribution: significant for very large basins where tributaries start in different climate zones

Question 17

5.2a The hydrological cycle

precipitation types

Answer 17

Orographic:
when air is forced to rise over a barrier, such as a mountain, it cools and condensation takes place forming rain. The leeward (downwind) slope receives relatively little rain, which is known as the rain shadow effect.

Frontal:
This happens when warm air, which is lighter and less dense, is forced to rise over cold, denser air. As it rises, the air cools and its ability to hold water vapour decreases. condensation occurs and forms clouds and rain form

Convectional:
This type of rainfall is common in tropical areas and in the UK during summer. When the land becomes hot, the ir above it becomes warmer, expands and rises. As it rises, the air cools adits ability to hold water vapour decreases. Condensation occurs and clouds develop. If the air continues to rise, rain will fall.

Question 18

5.2a The hydrological cycle

flows

Answer 18

Interception: the retention of water by plants and soils which is subsequently evaporated or absorbed by the vegetation.

Infiltration: the process by which water soaks into, or is absorbed by, the soil.

Percolation: similar to infiltration, but a deeper transfer of water into permeable rocks.

Throughflow: the lateral transfer of water downslope through the soil

Groundwater flow: the very slow transfer of percolated water through pervious (permeable) or porous rocks.

Surface runoff: the movement of water that is unconfined by a channel across the surface of the ground. A.k.a. overland flow.

River or channel flow: takes over as soon as the water enters a river or stream; the flow is confined within a channel.

Question 19

5.2a The hydrological cycle

variability

what are the different types of variability?

Answer 19

secular variability happens long term, for example as a result of climate change trends

periodic variability happens in an annual, seasonal, monthly or diurnal(daily) context

stochastic variability results from random factors for example in the localisation of a thunderstorm within a basin

Question 20

5.2a The hydrological cycle

outputs

Answer 20

Evaporation: the process by which moisture is lost directly into the atmosphere from water surfaces, soil and rock

Transpiration: the biological processes by which water is lost from plants through minute pores and transferred to the atmosphere

Discharge (channel flow): into another, larger drainage basin, a lake or the sea

Question 21

5.2b Physical factors within drainage basins

drainage basins

Answer 21

A drainage basin is the area of land drained by a river and its tributaries, sometimes referred to as a river catchment. The boundary of a drainage basin is defined by the watershed

The drainage basin is a subsystem within the global hydrological cycle. It is an open system with external inputs and outputs that vary causing changes to the amount of water in the drainage basin.

Question 22

5.2b Physical factors within drainage basins

climate

Answer 22

climate has a role in influencing the type and amount of precipitation overall and the amount of evaporation ( the major inputs and outputs)

climate also has an impact on the vegetation type

Question 23

5.2b Physical factors within drainage basins

soils

Answer 23

soils determine the amount of infiltration and through flow and indirectly, the type of vegetation

Question 24

5.2b Physical factors within drainage basins

geology

Answer 24

geology can impact on subsurface processes such as percolation and groundwater flow (and, therefore on aquifers).

Indirectly geology alters soil formation

Question 25

5.2b Physical factors within drainage basins

relief

Answer 25

altitude can impact on precipitation totals

slopes can affect the amount of run-off

Question 26

5.2b Physical factors within drainage basins

vegetation

Answer 26

the presence or absence of vegetation has a major impact on the amount of interception, infiltration and occurrence of overland flow, as well as on transpiration rates

Question 27

5.2c Humans disrupting the drainage basin by accelerating processes

deforestation

Answer 27

Deforestation reduces evapotranspiration, but increases infiltration and surface runoff

Question 28

5.2c Humans disrupting the drainage basin by accelerating processes

changing land use

Answer 28

agriculture:
arable to pastoral farming leads to the compaction of soil by livestock, increasing overland flow
pastoral to arable increases infiltration by loosening and aerating the soil through ploughing

urbanisation is covered in later section

Question 29

5.2c Humans disrupting the drainage basin by accelerating processes

new water reservoirs and abstracting water

Answer 29

river management:
construction of storage reservoirs holds back river flows
abstraction of water for domestic flow and industrial use reduces river flows
abstraction of groundwater for irrigation lowers water tables

Question 30

5.3a Water budgets

Answer 30

the water budget is the annual balance between precipitation, evapotranspiration and runoff (inputs and outputs)

Question 31

5.3a Water budgets

P = E + R +/- S

Answer 31

P= precipitation
E= evapotranspiration 
R= runoff
S= discharge

Question 32

5.3a Water budgets

impact on soil water availability

Answer 32

At a local scale, water budgets can inform about available soil water (the amount of water that can be stored and air available for growing crops). this is valuable to users, such as farmers, who can use it identify when irrigation might be required, and how much

Question 33

5.3a Water budgets

water budget graphs

Answer 33

water budget graph in textbook (Hodder page 14)

stages:
water surplus
soil moisture utilisation
soil moisture deficiency
soil moisture recharge
(field capacity reached)

Question 34

5.3b River regimes

river regime

Answer 34

river regimes indicate the annual variation of discharge of a river at a particular point (measured in cumbers)

river regimes result from the impact of climate, geology and soils

Question 35

5.3b River regimes

influences on river regimes

what influences river regimes?

Answer 35

the size of the river and where the discharge measurements are taken along its course

the amount, seasonality and intensity of the precipitation

the temperatures, with possible meltwater and high rates of evaporation in the summer

the geology and soils particularly their permeability and porosity; groundwater noted in permeable rocks is gradually released into the river as base flow

the type of vegetation cover: wetlands can hold water and release it slowly into the river

human activities aimed at regulating a river’s discharge

Question 36

5.3c Storm hydrographs

key features

what are the key features of a storm hydrograph?

Answer 36

Once the rainfall starts, the discharge begins to rise - rising limb

Peak discharge is reached some time after the peak rainfall because the water takes time to move over and through the ground to reach the river.

The time interval between peak rainfall and peak discharge is known as lag time.

Once the input of rainwater into the river starts to decrease, so does the discharge; this is shown by the falling or recessional limb

Eventually the river’s discharge returns to its normal level, or base flow.

Question 37

5.3c Storm hydrographs

shapes of storm hydrographs

Answer 37

Storm hydrographs show discharge changes over a short period of time, often no more than a few days. The storm hydrographic plots two things: the occurrence of a short period of rain (heavy shower or storm) over a drainage basin and the subsequent discharge of a river

‘Flashy’ hydrographs have very steep limbs, especially rising limbs, a high peak discharge and a short lag time.

‘Subdued’/ ‘delayed’/ ‘flat’ hydrographs have gently inclined limbs, a low peak discharge and a long lag time.

Question 38

5.3c Storm hydrographs

flashy hydrographs

Answer 38

Short lag time, high peak, steep rising limb

Weather/climate:
Intense storm that exceeds the infiltration capacity of the soil. Rapid snowmelt as temperatures suddenly rise above zero. Low evaporation rates due to low temperatures.

Rock type:
Impermeable rocks, such as granite, which restrict percolation and encourage rapid surface runoff.

Soils:
Low infiltration rate, such as clay soils (0-4mm/h)

Relief:
High, steep slopes that promote surface runoff

Basin size:
Small basins tend to have more flashy hydrographs

Shape
Circular basins have shorter lag times

Drainage density:
High drainage density means more streams and rivers per unit area, so water will move more quickly to the measuring point

Antecedent conditions:
Basin already wet from previous rain, water table high, soil saturated so low infiltration/ percolation

Vegetation:
Bare/low density, deciduous in winter, means low levels of interception and more rapid movement through the system

Human activity:
Urbanisation producing impermeable concrete and tarmac surfaces. Deforestation reduces interception. Arable land, downslope ploughing

Question 39

5.3c Storm hydrographs

subdued hydrographs

Answer 39

Long lag time, low peak, gently sloping rising limb

Weather/climate:
steady rainfall is less than the infiltration capacity of the soil. Slow snowmelt as temperatures gradually rise above zero. High evaporation rates due to high temperatures.

Rock type:
permeable rocks such as limestone, which allow percolation and so limit rapid surface runoff.

Soils:
high infiltration rate, such as sandy soils (3-12mm/h)

Relief:
low, gentle slopes that allow infiltration and percolation.

Basin size:
Larger basins ten to have more delayed hydrographs; it takes time for water to reach gauging stations.

Shape:
Elongated basins tend to have more delayed hydrographs.

Drainage density:
Low drainage density means few streams and rivers per unit area, so water is more likely to enter the ground and move slowly through the basin.

Antecedent conditions:
Basin dry, low water table, unsaturated soils, so high infiltration/percolation.

Vegetation:
Dense, deciduous in summer, means high levels of interception and a slower passage through the system; more water lost to evaporation from vegetation surfaces.

Human activity:
Low population density, few artificial impermeable surfaces. Reforestation increases interception. Pastoral, moorland and forested land.