The Water Cycle

• The water cycle is the continuous movement of water between the oceans, atmosphere, land surface, and living organisms.
• Evaporation occurs when heat energy from the sun causes water to change state from liquid to gas (water vapor).
• Transpiration refers to the process by which plants release water into the air through their leaves.
• Condensation takes place as water droplets form around tiny particles such as dust or salt crystals in the atmosphere.
• Condensation involves the conversion of water vapor back into liquid form as it cools and becomes saturated with moisture.
• Precipitation includes rainfall, snowfall, sleet, hail, and dew formation.
• Transpiration refers to the process by which plants release moisture into the air through their leaves.
• Runoff is the flow of excess water over the ground's surface.
• Infiltration is the process whereby precipitation enters the soil.
• Infiltration describes the downward movement of precipitated water into soil pores and cracks.
• Runoff is the flow of excess water that does not soak into the ground but instead flows overland towards rivers and lakes.
• Runoff is the flow of excess water overland due to heavy rainfall or melting ice/snow.
• Groundwater recharge occurs when water seeps into underground aquifers, replenishing them.
• The water cycle consists of four main processes: evaporation, transpiration, condensation, and precipitation.
• Oceanic Water:

  • largest water store on Earth (97% of total hydrosphere)
  • covers 72% of the Earth's surface
  • falling pH (becoming more acidic)
  • salty (difference between ocean water & fresh water)
• Cryospheric Water:

  • water stored on Earth in solid form (e.g. ice)
  • e.g. permafrost (water is frozen into the soil)
  • occurs as ice sheets, glaciers, sea ice, ice caps, etc.
• Terrestrial Water:

  • surface water (e.g. rivers, lakes)
  • freshwater on the Earth
  • biological water (stored in biomass e.g. plants & animals)
  • also occurs as groundwater, soil water, etc.
• Atmospheric Water:

  • the atmosphere
  • can be held in all three states
  • affects global warming (water vapour acts as a GHG and controls Earth's temperature)
• Evaporation rates depend on:

  • the temperature of the air
  • transpiration rates
  • the amount of solar energy available
  • the availability of water
  • the humidity of the air
• Condensation rates depend on:

  • surface temperature (below freezing = water sublimates to form hoar frost)
  • availability of surfaces for the water molecules to condense onto (e.g. condensation nuclei = pollen, pollution particulates, etc.)
  • excess water in the air to be converted to liquid at the dew point temperature
• Precipitation rates depend on:

  • condensation rates (the direct cause of all precipitation)
  • changes in adiabatic cooling (changes in the state of water due to pressure) e.g. the volume of air increases and lowers pressure with no change in heat
• The reasons it rains:

  • relief rainfall
  • convectional rainfall
  • frontal rainfall
• Relief rainfall occurs when warm, moist air is forced to rise over high land so it cools and condenses, forming clouds. It then rains and the air descends, warms, and becomes drier again.
• Convectional rainfall occurs when the sun heats the land and the air above, causing it to rise, cool and condense to form clouds. Rain then occurs.
• Frontal rainfall occurs because warm air is less dense than cool air so when they meet they don't mix; the warm air rises above the cool air, condenses via adiabatic cooling, and rains.
• Factors affecting the water cycle:

  • storm events
  • seasonal changes
  • farming practices
  • land use changes
  • water abstraction
• Storm events can affect the water cycle by bringing increased heavy precipitation in a short amount of time, leading to an increase in both channel flow and surface runoff as the soil is quickly saturated and infiltration rates slow down. Flash floods occur along a stream or in low-lying urban areas where there is impermeable concrete or tarmac. They can occur within 6 hours of intense rainfall.

  E.g. Boscastle flood, August 2004.
• In August 2004, Boscastle experienced 60mm of rainfall in two hours. The ground was already saturated from above-average rainfall during the previous two weeks. The drainage basin there had steep slopes and impermeable slate which increased runoff. The village of Boscastle is located at the confluence of three rivers - Valency, Jordan, and Paradise - and therefore when approx. two billion litres of water rushed down the valley and overflowed them, Boscastle experienced flash flooding.
• • Seasonal changes in the UK can bring frequent storms (rainfall is low and evaporation rates are high) but can also influence vegetation growth, increasing rates of interception/transpiration. The warm air holds moisture but the soils are dry, encouraging infiltration (/overland flow when baked).
  • Winter brings more rainfall and a likelihood of snow, but the cold air cannot hold much moisture so storms are uncommon. Interception and transpiration rates are reduced by dead vegetation and soils may become saturated, increasing overland flow. River channels are more likely to have high flows.
• Farming practices affect the water cycle because farmers can control local water cycles through irrigation or land drainage. Soils covered with plants have higher infiltration rates and therefore reduced runoff.

  If desertification occurs the capacity of the soils to retain water is much reduced, or lost completely when the soils are sealed.
• Land use changes, such as deforestation and urbanisation, can affect the water cycle drastically.

  Urbanisation occurs when vegetated ground is replaced by impermeable tarmac and concrete. Water cannot infiltrate the soil so overland flow is increased, which increases the likelihood of flooding. Soil water and groundwater stores are reduced.

  Deforestation affects the water cycle as the removal of trees leads to surface runoff and soil erosion as interception and photosynthesis rates are reduced, also reducing soil water stores.
• Water abstraction is the process of removing water from a river or aquifer. Aquifers can become depleted and contaminated from an influx of saltwater if the water table drops below sea level. An example of this occurring is the chalk aquifer in London. Water abstraction can also lead to low river flow conditions, which become less reliable as the water table sinks.

  Groundwater also helps sustain reservoirs that are often highly productive ecosystems or resources for tourism. Irrigation is the main cause of groundwater exploitation in agricultural areas (e.g. the Po River in Italy).
• • Peak Precipitation: the highest amount of precipitation that falls during a rainfall event.
  • Runoff: the amount of water in the river
  • Cumecs: 1 cumec = 1m³/s
  • Baseflow: the expected flow of the river without rain
  • Peak Discharge: the highest amount of water in the river
  • Bankfull Discharge: the river's maximum capacity is exceeded
  • Lag Time: the time between peak rainfall and peak discharge

  
• Gradient: the steeper the slope the quicker the lag time.
• Antecedent rainfall: the soil is already saturated with water so infiltration is reduced and it is much easier for floods to occur.
• Rock type/soil type: permeable ground material can absorb a lot more water, decreasing the lag time.
• Vegetation: more vegetation will intercept more water and lead to a lower peak discharge/shorter lag time.
• Drainage density: a high density (more watercourses) speeds up water transfer and collects greater quantities of water faster in its many tributaries, so the lag time will be shorter.
• Deforestation: decreases interception, which increases peak discharge, soil erosion and flood risk.
• Bangladesh is highly prone to flooding because of its low-lying and flat topography. 80% of its people are exposed to this flood risk.

  The country's national adaptation plan encourages climate-resilient housing built above flood levels that can conserve water and energy through techniques like rainwater storage. Development organisations are also expanding flood protection work.