land use changes affecting water cycle

Question 1

How does deforestation affect the water cycle?

Answer 1

• when forests removed, new vegetation generally has fewer leaves and shallower roots meaning it uses less water than the forest it replaces
• this results in less water evaporating from the land surface into the atmosphere; more water runs off land and stream flow is increased
• study - 20% = little effect, 50-100% = large increase

Question 2

how does soil drainage affect the water cycle

Answer 2

• Subsurface drainage removes excess water from the soil profile and is carried out usually through a network of perforated tubes installed 60-120cm below the soil surface. These tubes are known as ‘tiles’.

Question 3

advantages of soil drainage

Answer 3

○ As the soil has more access to air it is able to become warmer quicker which can make possible earlier sowing of seeds, with greater likelihood of improved germination
○ Large numbers of animals can be allowed to graze the land, once again without compacting the soil
○ Heavy machinery can work on the land without danger of compaction (and so leading to increased overland flow)

Question 4

disadvantages of soil drainage

Answer 4

○ The insertion of drains artificially increases the speed of throughflow in the soil. Much more water reaches watercourses more quickly than before drainage. This can increase the likelihood of flooding and increase the range of flows in rivers. It is interesting to note that before the drainage of many floodplains in the UK from the eighteenth century onwards, rivers were more navigable than today; the annual flow regime was much more even.
○ The dry topsoil can be subject to wind erosion if not properly protected. Soil loss by wind erosion has mainly been documented for sandy and peaty soils in the eastern and middle counties of England
○ Another major concern with regard to land drainage is nitrate loss. It can lead to eutrophication. Water draining from fields finds its way into local watercourses. There it enriches ponds, etc., with nitrogen or phosphorus. It causes algae and higher forms of plant life to grow too fast. This disturbs the balance of organisms present in the water and the quality of the water concerned.

Question 5

how can water abstraction affect the water table

Answer 5

• The extraction of water from rivers or groundwater
• An aquifer is an underground formation of permeable rock that stores groundwater e.g. chalk and sandstone
• 30% of all freshwater is stored in aquifers
• Aquifers can produce useful quantities of water when tapped by wells. Intensive use of aquifers results in depleting the storage, disturbing the natural equilibrium established over centuries. In some saline aquifers exist where seawater has infiltrated the rocks.
• Many aquifers are being exploited unsustainably
  ○ Some of the most acute problems with over-abstraction have been found in chalk stream systems, where up to 95% of the flow is derived from underground aquifers.
  ○ The catchments of chalk streams provide underground reservoirs of generally high quality groundwater which can be abstracted for public supply
  ○ This abstraction has dramatically reduced the flow in many chalk streams and, in some cases, completely dried up sections of these important rivers, particularly during dry summers when public demand is at its highest
  ○ This also has an economic impact on local communities, resulting from the inability to fish, enjoy river views due to encroaching vegetation or undergo other recreational activities

Question 6

how has urbanisation affected the water cycle

Answer 6

• most urban surfaces impermeable
  ○ Surface water storage - less surface water storage as surfaces are impermeable
  ○ Soil water storage - less surface water storage as surfaces are impermeable
  ○ Surface runoff - increased surface run off
  ○ Overland flow - increased overland flow
  ○ Evapotranspiration - decreased evapotranspiration
  ○ Groundwater - less water stored in groundwater

Question 7

how has irrigation affected the water cycle?

Answer 7

Water for irrigation can be diverted from surface stores such as lakes, reservoirs and rivers, or from groundwater stores .
Consequences of irrigation can include:
* Salinisation
* Loss of aquifer capacity
* Greater summer rainfall