Lecture 22- Hydrologic Cycle & Water Resources

What are the major water reservoirs on Earth?

• Ocean - 97%

• Ice Sheets and Glaciers

• Groundwater - less than 1%

• Lakes and rivers - 0.01%

• Atmosphere - 0.001%

• Biosphere - 0.0001%

Hydrologic cycle word definition

Earth’s water cycles between these reservoirs via the hydrologic cycle

Image of hydrologic cycle

Role of the atmosphere

• Solar energy and atmospheric circulation control precipitation patterns on Earth.

• Where air rises → air moisture condenses → rain

• Where air sinks → low moisture content → dry

Role of the Atmosphere image

Surface Water

• After precipitation, water may follow several pathways

• Runoff: when water remains on top of the soil and flows downhill (eventually into streams/lakes)

• Infiltration: some of the precipitation will soak into the soil moisture zone (into subsurface)

Subsurface Water

• Upon infiltration: transpiration by plants → returns to atmosphere

• May percolate downwards

• Water percolating down can join the groundwater (saturated zone of soils & bedrock)

Two types of water resources that society uses:

• Surface water (Streams, lakes, reservoirs)

• Groundwater.

• Put together, fresh water resources account for less than 1% of earth’s water - but has to supply the needs of people and terrestrial plants/animals (most of earth’s freshwater is frozen in icecaps and glaciers)

US household water use

• over 80 gallons per person per day

• Majority used for toilet and showers

• Personal household use accounts for about 10% of this total

• The thermoelectric power (usually recycled), irrigation, public supply are the industries using the most water.

Sufarce-water withdrawals

• Total water withdrawals reflect the multiple uses of the nation’s water supplies

• Increases with population, irrigation demands

• Surface water supplies much of the West’s water use

• Dams built by Bureau of Reclamation store water in reservoirs to keep continuous supply available

• Major resources include the Rio Grande, Sacramento and Colorado rivers

Surface water case study: Colorado River

• 30+ million people depend on the water and electricity from the Colorado River system

• Colorado River Compact, 1922: Agreement that regulates water distribution in the SW US.

• CA’s Imperial Valley, east of San Diego, is desert country - rainfall averages to 3 inches per year

• Aqueduct from Colorado River flows through and permits irrigation

• The Colorado River often doesn’t make it to the sea today

Southwest water balance

• Southwest US has been drying out

• People continue to use water from reservoirs

• Lake Mead- reservoir behind Hoover Dam

• Water level has dropped 150 feet since 2000

• Minimum elevation for power generation is 950 ft

• Californio received enormous amounts of rain and snow in January and February 2017 via an atmospheric river

• The rainfall diminished the drought in 2017, but dry summers bring it back

Drought

• A period of shortage in water supply

• May be due to decreased precipitations, increased heat & evaporation, or both