Urban Precipitation (AQA A Level Geography)

• Urban areas experience higher rates of precipitation compared to rural regions because of:
  • Pollution - urban air holds more condensation nuclei in the form of dust and dirt particulates
  • Warmer air - the warmer the air, the more moisture it holds
• Surface runoff (overland flow) is dominant as much of the surfaces are impermeable or designed to shed water - the camber of a road, funnels water to drains
• There is also less vegetation to intercept precipitation, which means less evapotranspiration (EVT) rates to cool the air and reduce humidity
• As a result, more drainage is needed to remove surface water quickly

Image showing contrast between urban and rural runoff

• The urban form impacts the drainage system in the following ways:
  • Inputs are increased
    • Precipitation is increased due to pollution and temperature
    • Extra water is artificially imported into the system
• Outputs are unbalanced
  • Less vegetation reduces EVT
  • Impermeable surfaces increases runoff
  • Wastewater discharge is an adjusted output into a river or sea (sewage discharge is treated but there will still be liquid waste added to the system)
• Transfers are increased
  • Runoff is contaminated through dust and pollutants from industry and transport
  • Artificial drainage systems are added above and below ground (e.g. guttering and storm drains etc.)
• Stores are decreased overall
  • Impermeable surfaces reduce ground infiltration and stores are reduced
  • Rivers are diverted, dredged or channelised which increases capacity 
  • Reservoirs and ponds are permanent stores but vulnerable to evaporation
  • Depression (puddles) stores are temporary
  • Lakes and ponds in parks and homes are often drained
• These changes become more pronounced across the urban pathway, with the urban CBD system seeing the greatest impacts, compared to the rural-urban fringe system

• Urban hydrographs are 'flashy'
• They show a rapid rise in discharge over a short period of time 
  • Short lag time
  • High peak discharge
  • Fast return to base flow
• Caused by:
  • Low infiltration rates
  • High surface runoff

Comparison between rural and urban hydrographs - rural hydrographs are 'flat', whereas urban are 'flashy'

• Flash floods in urban areas are increased through:
  • Heavier rates of precipitation
  • Increased thunderstorms
  • High rates of impermeable surfaces
  • Channelling of surface runoff to drains
  • Building on floodplains to meet demand for housing
  • Culverts, rivers and drains blocked with debris
  • Narrow channels under bridges constricting flow