Water Cycle

• Transfers in the water cycle include: precipitation, evaporaion, infiltration and percolation.
• 96.5% of water is stored in the hydrosphere
• 0.001% of water is stored in the atmosphere
• 1.7% of water is stored in the cryosphere
• 1.7% of water is stored in the lithosphere
• Only 2.5% of the Earth's water is freshwater
• Just over 30 per cent of all freshwater is stored in rocks deep below the ground surface forming vast underground reservoirs called aquifers.
• Aquifers most commonly form in rocks such as chalk and sandstone, which are porous (contain pores – air pockets) and permeable (allow water to pass through).
• Soils vary enormously in their capacity to store and transfer water – this is the soil water budget.
• The upper level of saturated rock is called the water table. This rises and falls in response to groundwater flow, water abstraction by people or by recharge
• Aquifers in the deserts of Africa, the Middle East and Australia are called fossil aquifers and were formed thousands of years ago when the climate in those regions was much wetter
• Saline aquifers exist where seawater has infiltrated into the rocks, often due to the over-abstraction
• Precipitation = Transfer of water from the atmosphere to the ground. It can take the form of rain, snow, hail, dew
• Sublimation = Transfer from a solid state (ice) to a gaseous state (water vapour) and vice versa.
• Infiltration = Transfer of water from the ground surface into soil where it may then percolate into underlying rocks
• In the last ice age, a large amount of water was 'locked up' as snow and ice, so the sea levels fell by over 100m compared to the present day
• Cloud Formation:

  • Solar radiation heats the Earth’s surfaces.
  • Earth’s surfaces heat the air around them as they release heat energy absorbed, some liquid evaporates.
  • Warmer air is less dense, so it is able to rise.
  • As air rises it expands and cools adiabatically.
  • As it cools it reaches its dew point temperature and condenses onto condensation nuclei.
  • At high altitudes, liquid water freezes to form ice crystals, and when these become too heavy, they fall as precipitation.
• If you look at a satellite photograph of the Earth, you will see that clouds are very unevenly distributed, as is associated precipitation. The driving force behind cloud formation and precipitation is the global atmospheric circulation model and differential heating
• In the mid latitudes, cloud formation is mostly driven by the convergence of warm air from the Tropics and cold air from the Arctic. The boundary of these two distinct air masses – the polar front – results in rising air and cloud (and rain) formation. These are driven by the jet stream and establish the weather in UK
• 95% of ice is locked up in the ice sheets covering Antarctica and Greenland
• Inputs in the hillslope water cycle include precipitation
• Outputs in the hillslope water cycle include evapotranspiration and streamflow
• Stores in the hillslope water cycle include interception, surface storage, soil water, groundwater and the river channel
• Flows in the hillslope water cycle include evapotranspiration, precipitation, infiltration, percolation, throughflow
• Deforestation impacts the hillslope water cycle as it reduces interception and infiltration
• Seasonal change impacts the hillslope water cycle as frozen ground can interupt water transfers and affect the magnitude of the stores
• Urbanisation can impact the hillslope water cycle, as impermeable surfaces reduce infiltration. Trees will also be cut down, and water will flow quickly through pipes to nearby river channels.
• In wetter seasons, precipitation exceeds evapotranspiration, meaning there is a water surplus and soil moisture increases
• In drier seasons, evapotranspiration exceeds precipitation, and so soil moisture is lost and not replaced by precipitation
• A drainage basin is the area of land that is drained by a river and its tributaries
• The edge of a river basin is marked by a boundary called the watershed.
• In a hydrological sense the drainage basin is an open system. However, for planning purposes, it is often considered to be a closed system – the principles of cause and effect are contained and do not spread outside its area.
• Precipitation = water enters the drainage basin as precipitation. Here it may be intercepted by plants or tree where it may be stored before being evaporated
• Groundwater Flow = Feeds rivers through their banks and bed. A slow method of transfer which continues to supply water well after an individual rainfall event
• Infiltration = Infiltration capacity is extremely important and varies according to soil type and antecedent conditions. When it is exceeded, water is unable to be absorbed and flows over the surface.
• Overland Flow = When water is unable to infiltrate it runs off the surface as overland flow, flowing across a large surface area (sheetflow) or concentrated into small channels called rill
• Throughflow = Occurance depends on the depth and texture of the soil, coarse soil absorbs and transfers water through discrete 'pipes' caused by animal activity. These have a low field capacity
• Clay soils drain slower and have a high field capacity
• The water balance is the balance between inputs and outputs of water
• The water balance can be expressed as:

  Precipitation = Total runoff + Evaporation +/- Storage