The water cycle

Question 1

What are the four major physical systems where water is stored?

Answer 1

Lithosphere - Water stored in the Earth’s crust.

Hydrosphere - All the water on Earth is stored in the oceans, rivers, lakes, groundwater, and the atmosphere.

Cryosphere - Frozen water, in snow and ice.

Atmosphere - Water stored in the air.

Question 2

What is the hydrosphere?

Answer 2

All the water on Earth stored in the oceans, rivers, lakes, groundwater, and the atmosphere.

Question 3

What is the global distribution of groundwater aquifers?

Answer 3

• Over 30% of all freshwater is stored in rocks deep below the ground surface forming vast underground reservoirs (aquifers, most commonly form in rocks such as chalk and sandstone which are porous and permeable)

Question 4

How does climate change, change the magnitude of water the water cycle stores?

Answer 4

180,000 years ago (at the peak of the last Ice Age), around 1/3 of the Earth’s land area was covered by glaciers. With water locked up, the magnitude of this store increased significantly.
With less liquid water reaching the oceans, sea levels fell by over 100m compared with the present day.
- 3 million years ago, during warmer periods - ocean levels were 50m higher than today as the amount of water stored as snow and ice declined.

Question 5

How does precipitation occur at the equator?

Answer 5

• At the Equator, high temperatures result in high rates of evaporation
• The warm, moist air rises, cools and condenses to form clouds and heavy rainfall in a low-pressure zone (ITCZ - Intertropical Convergence Zone)
• Seasonally, this zone moves north and south

Question 6

How does cloud formation occur at mid-latitudes?

Answer 6

• Convergence of warm air from the Tropics and cold air from the Arctic
• The boundary of these two air masses - the polar front - results in rising air and cloud formation
• Strong upper-level winds (jet stream) drive these unstable weather systems across the mid-latitudes, establishing the conditions experienced in the UK.

Question 7

How do clouds form?

Answer 7

1. Cooling air that contains water vapour will cause water vapour molecules to slow down in the air.
2. AS they slow, some molecules aren’t able to maintain vapour form so they cluster in the air to form tiny liquid droplets - typically needing a particle to condense upon, condensation nuclei.
3. A cloud is composed of tiny water droplets suspended in the air - if they become large enough, they may be a visible cloud.
4. Warmer air can contain more water vapour.

Question 8

How do cryospheric processes change the magnitude of water cycle stores?

Answer 8

Melting of all the polar ice sheets could result in a 60m rise in sea level, adding water to the ocean store.

Question 9

What is the impact of Arctic ice shrinking?

Answer 9

• Since 1980, the summer extent of Arctic sea ice has been shrinking at a rate of 12.6% per decade
• By 2035, the Arctic may be ice-free in the summer
• With less reflective surface ice, more solar radiation is absorbed by the darker water (positive feedback)

Question 10

How does convectional rainfall occur?

Answer 10

1. The sun’s energy is concentrated in areas around the Equator, causing rapid, and high levels, of evaporation.
2. Large quantities of water vapour, combined with rapid condensation, create heavy towering storm clouds such as cumulonimbus.

e.g. Brazil and Indonesia (areas near the Tropics)

Question 11

How does frontal rainfall occur?

Answer 11

1. Warm air meets cold air, it is forced to rise because it is less dense.
2. As the warm air rises, it begins to cool and as a result condenses to form clouds.
3. Along the “front” a variety of clouds form, causing moderate to heavy rainfall.

e.g. UK (warm air from the tropics mixes with the cooler arctic and polar air masses)

Question 12

How does orthographic rainfall occur?

Answer 12

1. Moist warm air blown in across the ocean, by the prevailing wind, is forced to rise over areas of steep relief.
2. As the air rises, it cools and condenses - cold air cannot hold as much warm air so precipitation occurs.
3. Air descends the other side of the mountain, usually warming as it loses altitude and, therefore has a greater capacity to hold moisture.
4. As a result, there is little rain on the far side of the mountain (=rain shadow)

e.g. West Coast of the UK, Cumbria, and the Lake District

Question 13

What is the soil water budget?

Answer 13

Describes the changes in the soil water store during the course of a year.

Question 14

What is the SWB pattern during wet and dry seasons?

Answer 14

Wet seasons
- Precipitation exceeds evapotranspiration = water surplus
- Ground stores fill with water = more surface runoff and higher discharge = river levels rise
Drier seasons
- evapotranspiration exceeds precipitation = ground store depletion
- some flows into the river channel but is not replaced by precipitation
Back to wet seasons
- Deficit of water at the start
- Ground stores recharged

Question 15

What is a drainage basin?

Answer 15

Area of land that is drained by a river and its tributaries.
The edge of a river basin is marked by a boundary = watershed.

e.g. Mississippi, Nile and Amazon

Question 16

What is the water balance?

Answer 16

P = Q + E +/- S

P = precipitation
Q = total runoff
E = evapotranspiration
S = storage (soil and rock)

Question 17

Where is the River Wye located? (drainage basin case study)

Answer 17

• Source in the Plynlimon Hills in mid-Wales
• Flows south-eastwards before joining the Severn Estuary at Chepstow

Question 18

What are some characteristics of the River Wye? (drainage basin case study)

Answer 18

• Total length of 215km, fifth-longest river in the UK
• An Area of Outstanding Natural Beauty and a Site of Special Scientific Interest
• Steep slopes, acidic soils and grassland
• Originally a very forested area, but has been cleared to make way for pasture - reduced interception
• Ditches have been dug to drain the land to increase the speed of water transfer, making it more productive, although this has made the river more prone to flooding.

Question 19

What are the rock types in the River Wye? (drainage basin case study)

Answer 19

• Impermeable mudstones, shales and grits
• Mainly impermeable underlying rock has led to limited groundwater flow as the soils quickly become saturated unable to absorb excess water
• Encourages overland flow, increasing flood risk downstream (Hereford has been affected)
• Western upland parts = highest rainfall totals
• East = higher temps. and evapotrans. rates

Question 20

What is discharge?

Answer 20

Measurement of runoff at a specific moment.

discharge (m3 per second = cross sectional area (m2) x velocity (metres per second)

Question 21

What is a flood hydrograph?

Answer 21

Shows the discharge of a river following a particular storm event

Question 22

How do drainage basin and precipitation characteristics lead to a ‘flashy’ hydrograph?

Answer 22

1. Basin size –> small basins often lead to a rapid water transfer
2. Drainage density –> high density speeds up water transfer
3. Land use –> urbanisation encourages rapid water transfer
4. Relief –> steep slopes lead to rapid water transfer
5. Soil water –> saturated soil results in rapid overland flow
6. Rainfall intensity –> heavy rain may exceed the infiltration capacity of vegetation and lead to rapid overland flow.

Question 23

How does a drought affect change in the water cycle? (physical variations affecting change case study)

Answer 23

Californian drought (2011-17)
- Drought causes a reduction in water stores in rivers and lakes
- Vegetation dies back or is destroyed by fire - affecting transpiration, interception and infiltration (130mil trees died)

Question 24

How does irrigation affect change in the water cycle? (human activities affecting change case study)

Answer 24

Parts of the Middle East
- Water is being abstracted from underground aquifers formed thousands of years ago
- In danger of becoming depleted, as the rate of recharge is slower than the rate of use

Question 25

How does land drainage affect change in the water cycle? (human activities affecting change case study)

Answer 25

Somerset Levels
- Vulnerable to flooding because of a network of ditches built, which move water quickly

Question 26

Why are peatlands important?

Answer 26

• They act as an important store, 584 million tonnes of co2
• As peatlands are drained, air penetrates deeper, enabling the decomposition of carbon, releasing co2