Water Quantity

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Objectives
  1. Water Distribution
  2. Water Cycle
  3. Groundwater


Properties

Water has some very unique properties

  1. Water expands as it freezes
  2. Water is a liquid over a wide temperature range
  3. High specific heat
  4. Universal solvent
  5. High surface tension

Water Distribution

Water Distribution
Distribution of Freshwater


Water Cycle

Next section describes the individual parts of this diagrams


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Definitions

  • Precipitation - Water falling from the atmosphere in the form of rain, snow, etc.
  • Evaporation - Transport of water from surface to atmosphere as a vapor
  • Transpiration - Water loss from plant leaves
  • Evapotranspiration - Evaporation plus transpiration
  • Streamflow - Movement of water in a natural channel such as a river
  • Runoff - Flow of water over land
  • Infiltration - Downward movement of water into soil and porous rock
  • Storage - Water can be stored in: oceans, lakes and rivers, atmosphere, snow and ice, soil and groundwater




Groundwater


Groundwater


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Definitions

  • Aquifer - A saturated layer of rock that can transmit significant quantities of water
  • Aquitard - A layer of rock incapable of transmitting significant quantities of water
  • Confined Aquifer - An aquifer between two aquitards
  • Unconfined Aquifer - An aquifer with no aquitards above it
  • Water Table - The top of the saturated water layer in an unconfined aquifer
  • Vadose Zone - Unsaturated layer above an unconfined aquifer



Human impact on the water cycle

Groundwater pumping

Effects of groundwater pumping on water table. Top figure shows a normal pumping condition.

Pumping over water too fast from an unconfined aquifer draws down the water table. Hazards include:

  • Wells going dry
  • Subsidence
  • Salt water intrusion
  • Contamination

Pumping too much water from confined aquifers can lead to aquifer depletion and subsidence.

Water scarcity

Types of water scarcity:

  • Drought - lack of rainfall
  • Dry climate
  • Water Stress - too many people using the same resource
  • Water wastage - system leaks, inefficiency, other losses
  • Desiccation - drying of soil

Note that each of these require different solutions (see below).

A very useful way of categorizing the above water scarcity is as follows:

Types:

  • Blue water - Water in lakes, rivers, and aquifers
Blue water is used directly
  • Green water - Water in the soil
Green water is important for crop production
  • Real scarcity - due to lack of rainfall or too many people sharing the same resource
  • Apparent scarcity - sufficient water but the water gets wasted due to inefficiency and losses

We then have four groups of scarcity: real blue water, apparent blue water, real green water, and apparent green water.

Solutions - different types require different solutions.

For example: Apparent blue water scarcity can be reduced by reducing system leaks and reducing wasteful water usage.

Apparent green water scarcity can be reduced by soil conservation measures.

Flooding

The opposite problem is too much water creating floods.

Floods are natural phenomena and beneficial. They leave silt for farmland and recharge groundwater. Ancient Egyptian civilization would not have existed if it was not for annual floods of the Nile River.

The following effects of human actions lead to increased flooding:

  • Drainage of wetlands, reducing natural storage
  • Channelization of rivers, changing the streamflow
  • Building on floodplains - the area in which a river naturally floods
  • Urbanization, concrete and asphalt prevents infiltration and increases runoff
  • Deforestation and removal of vegetation, has the same effect on infiltration and runoff

Other issues

Public vs. private water supply

There is a debate whether water supply should be owned by private companies or public governments

One side claims that private companies are more efficient.

The other side says that water supply is a basic service which should not be driven by profit

Transboundary water issues - whose water?

Problems occur when upstream countries withdraw too much water or release large amounts of water. Problems can be seen, for example, in the Nile, Ganges, and Mekong Rivers.



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Activity
Investigate what drinking water source is used in your home town. Be as specific as possible. What, if any, treatment is done to this water before it reaches your home?