Water EQ1

• Stores: stocks of water, places where water is held. For example, oceans.
• Fluxes: measurement of rate of flow between stores
• Processes: physical factors which drive fluxes of water between stores
• Hydrological cycle = closed system and no water is added to global budget and none is removed.
• Hydrological system driven by solar energy and gravitational potential energy
• Water held in different states (liquid, gas and solid) and stores which vary for both human and physical reasons.

  • In last Ice Age, more water held within cryosphere in solid form as snow and ice
  • Recent climate warming reversing this with major losses of ice in Greenland and Antarctica
  • Humans have built water storage reservoirs (on smaller scale) which has increased security of their water supply
• Main stores of water:

  • Oceans: 96.9%
  • Ice caps and glaciers: 1.9%
  • Groundwater: 1.1%
  • Rivers and lakes: 0.01%
• Freshwater store:

  • Ice caps and glaciers: 68.7%
  • Groundwater: 20.1%
  • River and lakes: 1.2%
• Blue water: Freshwater stored in rivers, streams and lakes (visible part of cycle)
• Green water: Freshwater stored in soil and vegetation (invisible part of cycle)
• Flows achieved by processes such as precipitation, evaporation, transpiration, crysopheric exchanges and runoff
• Water stores have different residence times, with the larger store having a longer residence time. Some stores non-renewable e.g fossil water and when cryosphere melts
• Residence time: Average time water molecule spends in store or reservoir
• Residence times impact turnover within water cycle system. Some ancient groundwater e.g in Sahara Desert, result of former pluvial (wetter) periods - termed fossil water and is not reachable for human use.

  • Ice core dating suggests residence time of some water in Antarctic ice is over 800,000 years
  • Major ice sheets store water as ice for very long periods so data shows average

  
• Drainage basin is subsystem within hydrological cycle and is an open system with inputs and outputs.
• Drainage basins vary enormously in size e.g drainage basin of Amazon is made up of drainage basins of tributary rivers. Tributary basins made up of even smaller basins of streams that drain into these tributaries.
• Climate influences type of precipitation and amount of precipitation overall and amount of evaporation. Climate can also in some locations have impact on vegetation type.
• Soils determine amount of infiltration and throughflow and indirectly type of vegetation
• Geology impacts subsurface processes e.g percolation and groundwater flow and hence on aquifers. Also indirectly alters soil formation.
• Relief can impact amount of precipitation and slopes can impact amount of runoff.
• Presence of absence of precipitation has major impact on amount of infiltration, interception and occurrence of overland flow and transpiration rates.
• Human factors affect DBC by changing speed of natural processes hence creating new stores and removing water out of the cycle:

  • Deforestation
  • Abstraction
  • Construction of dams
  • Building river defenses
  • Overpopulation
  • Pollution from industry and fertilisers
  • Global warming
  • Urbansation
• Water taken from aquifers at rate higher than replacement level sometimes, causing reduction in groundwater flow and at a lower water tale. Increased industry and deforestation increases groundwater storage hence increasing risk of flooding if water table reaches surface.

  E.G: In China, groundwater irrigates 40% farmland and provides 70% drinking water in north-west and north. Groundwater dropped by meter per year (1974-2000)
• Building dams increases surface water stores and evaporation which decreases downstream river flow and discharge.

  E.G Lake Nasser behind Aswan Dam in Egypt estimated to have evaporation losses of 10-16 bn cubic meters every year, which represents loss of 20-30% of Egyptian water volume from River Nile.
• Urbanisation creates impermeable surfaces decreasing infiltration and increases surface runoff and throughflow through artificial drains thus often increasing river discharge as a result.

  E.G: Urbanisation increased risk across UK; increased flooding in Maidenhead (2014), York (2015) and Manchester (2015)
• Cloud seeding is attempt to change amount or type of precipitation through dispersion of substances into air, serving as cloud condensation nuclei (hydroscopic nuclei). New tech and research claims to produce reliable results making cloud seeding a dependable and affordable water-supply practice for many regions - effectiveness still debated.

  E.G: China used this in Beijing just before 2008 Olympic Games to create rain to clear air of pollution; used in Alpine Meadows ski area in California to improve snow cover; used in 2015 Texas to decrease drought impact
• Water budgets: annual balance between inputs (precipitation) and outputs (channel flow and evaporation)
• Calculating a water budget:

  Precipitation (P) = streamflow (Q) + evaporation (E) +/- Changes in storage (S)
• Water budget shows times when water naturally enters and leaves system
• Positive water balance: when there is more than enough water
• Negative water balance: when there is not enough water
• A: More input so more water available; runs off into streams and groundwater levels topped up

  B: Evapotranspiration (EVT) increases until higher than precipitation so water drawn up from soil and starts to get used up

  C: EVT is highest and precipitation is lowest (hot weather conditions); river levels fall, plants use up soil moisture whilst crops need irrigation

  D: Soil water used up (only specially adapted plants survive)

  E: Precipitation higher than EVT so amount of soil moisture starts to increase again

  F: Soil saturated and can't hold any more moisture - field capacity

  
• River regimes: annual variation in discharge or flow of river.
• Main factors affecting river regimes:

  • Drainage basin area
  • Maximum altitude/variation in altitude
  • Geology
  • Mean annual precipitation
  • Mean discharge
  • Main land use

  
• Simple river regime - where river experiences period of seasonally high discharge, followed by low discharge. Typical of rivers where inputs depend on glacial meltwater, snowmelt or seasonal storms (e.g monsoons). Rivers within temperature climates which rise in mountainous regions where snowmelt occurs tend to be like this (e.g Rhone in France)
• Complex river regimes - where larger rivers cross several different relief and climatic zones and thus experience effects of different seasonal climatic events - true of rivers e.g Mississippi or Ganges. Human factors also contribute to complexity such as damming rivers for energy of irrigation.
• River regime reflects differences in precipitation, temperature, evapotranspiration and land use throughout river's catchment during year.
• Characteristics of drainage basin (shape, geology, soil type, land cover) and human intervention influence river regime.
• Some of world's lonest rivers are several thousand kms in length hence they may cross several climatic zones and encounter very different land uses and population densities along course. Longer the river, the more complex the variables tend to be.
• Amazon Complex River Regime:

  • South America (Peru, Venezuela, Brazil, Ecuador, and Bolivia)
  • 6308 km long
  • Drains 6 mn km^2 basin
  • Humid tropical climates based by ancient shield rock
  • Regime linked to snowmelt from Andes Mountain Range