Fresh Water Supply is a Finite Resource

  • World’s oceans hold approx. 1,386 million km3 of water.
  • Accounts for 97.5% of global water source.
  • Only 2.5% of store is available as fresh water.
  • Almost 80% of freshwater is trapped in ice, snow, and permafrost.
  • Remaining 20% of freshwater is groundwater.
  • Only 1% accessible freshwater held in lakes, ecosystems, the atmosphere and rivers.

Case Study: Factors affecting California’s water supply

Geographical Controls on water supply:

  • Mountain chains run parallel to the coast and prevent moist air reaching inland. The prevailing moist air flow from the Pacific Ocean is forced upwards by the mountains, cools and condenses – forming relief rainfall.
  • Most rainfall falls in a coastal zone no more than 250km wide.
  • South and far east of California receive under 100mm of rainfall due to the rain shadow cast by the Sierra Nevada mountains and Coastal Ranges.
  • Surface runoff occurs over impermeable, saturated or baked surfaces – eventually reaching river channels as streamflow.
  • Some surface water infiltrates through the surface layer and eventually percolates through the rocks to become groundwater. Nearly a third of California’s fresh water comes from groundwater sources, known as aquifers.
  • High pressure systems over the Pacific Ocean block moist air currents reaching southern California.
  • Most of the major rivers are fed by snowmelt from the Sierra Nevada Mountains.
  • In recent years extended droughts have meant groundwater and surface storage levels have decreased.


Weather systems that affect California create three main problems

  • A high-pressure system over the Pacific Ocean blocks moist air currents from reaching southern California. Occasional shifts in the system allow storms and heavy rain to reach the area
  • El Nino events bring above-average runoff and flooding to the south-west, while La Nina events bring drought
  • In recent years, extended droughts have meant that groundwater and surface storage levels have decreased



  1. a) Precipitation
  • Much of California is arid with annual average precipitation of between 200-500mm.
  • 65% of precipitation is lost through Evapotranspiration, 13% flows out to sea, only 22% for human use.
  • 50% of the rain falls between November and March, seasonal shortages.
  1. b) Population
  • Has grown from 2 million people in 1900 to 37.7 million in 2007.
  • Spatial imbalance as three quarters of demand for water comes from areas south of the Sacramento – 75% of the rain falls to the north.
  • Increasing demands for water exceed natural supplies.


Californian Crisis:

  • USA’s leading economy, a major supplier of food is now at risk.
  • Wetlands have been drained, natural habitats altered, and fish stocks depleted to secure water supplies.
  • Polluted waterways, the over extraction of ground water and increasing salinity.
  • The Bay-Delta region and the Salton Sea have become environmental disaster zones; Colorado river has been reduced to a trickle as it enters the Gulf of California.


Maintaining the dream

  • The State water Project combined with the Central Valley Project provides water from the Sacramento-San Joaquin river Delta for central and southern California.
  • The Colorado river provides 60% of south California’s water via a system of dams and aqueducts.
    State Water Project Central Valley Project Colorado River
    ·         20 major dams/reservoirs

    ·         Irrigates 0.3 million hectares of farmland

    ·         Provides drinking water for approx. 20 mill Californians

    ·         22 dams/reservoirs

    ·         Irrigates 1.2 million hectares of farmland

    ·         Provides drinking water for 2 million Californians

    ·         11 major dams/reservoirs

    ·         Irrigates 1.4 million hectares of farm land

    ·         Provides drinking water for 25 million Californians.