Precipitation and Excess Runoff in the Water Cycle

Question 1

Air uplift and Condensation process

Answer 1

• air rises to atmosphere, cools at saturation point
• cooling due to decrease in pressure with altitude
• expansion of air - fewer air molecule collisions
• reduction of heat energy, therefore temp decrease
• clouds forms where vapour condense to droplets at low temps

Question 2

Features of convectional rainfall

Answer 2

• common in tropics and UK summers
• air above hot land expands as it rises, cools, reducing ability to hold water vapour
• cumulus/cumulonimbus clouds form

Question 3

Features of frontal rainfall

Answer 3

• lighter air (warmer) forced to rise over dense (cooler) air
• warm air cools e.g. polar and tropical air meet over North Atlantic Ocean
• clouds and rain forms due to condensation

Question 4

Features of orographic rainfall

Answer 4

• air forces over barrier, e.g. mountain
• cools and condenses forming rain, ‘feeder-seeder’ mechanism
• high altitude clouds fall through low stratus clouds - heavier rain
• leeward slope of barrier receives little rain (rain shadow)

Question 5

Bergeron Findeisen Process

Answer 5

• 1930s
• high altitude clouds, mixture of water droplets and ice crystals at <0C
• ice crystals grow quickly and fracture by fast air currents
• SA increases, more water vapour condenses around them
• becomes too large and dense to be held by air so falls to ground and melts as they reach the warmer air

Question 6

Collision process

Answer 6

• supersized condensation nuclei provide seeds fro water droplets to form
• larger and heavier than normal sized droplets
• absorb smaller droplets as they fall and collide

Question 7

Causes of excess runoff generation

Answer 7

• snowmelt and ice ablation resulting in seasonal variations
• storm activity (prolonged rainfall) leading to saturation and overland flow
• monsoon season causes widespread flooding every year
• changes in land use, catchment urbanisation, and deforestation (human causes)

Question 8

Urbanisation and overland flow generation

Answer 8

• permeable ground becomes impermeable
• concrete and tarmac decrease effectiveness of infiltration, through flow and soil storage, increasing run off
• drain and sewer network effectively an extension of river channel network, water drained faster
• lack of interception and transpiration as a result of lack of vegetation

Question 9

Deforestation and overland flow generation

Answer 9

• forest land coverage reached all time low of 3% in UK in 1919, currently just over 10%
• in Mid Wales, infiltration rate = 67x higher under trees than sheep pasture
• 2009 & 2015, cumbrian flood due to excess overland flow on almost treeless hills

Question 10

Features of a high density forest slope

Answer 10

• evaporation and transpiration from leaves
• rainfall intercepted by trees, undergrowth or leaf litter
• dips to ground slowly
• roots aerate soil improving infiltration

Question 11

Features of a degraded forest slope

Answer 11

• rainfall directly strikes soil, soil compacts
• rainwater moves quickly over surface into rivers
• little groundwater, rain stops, river quickly becomes dry