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Water has some very unique properties
- Water expands as it freezes
- Water is a liquid over a wide temperature range
- High specific heat
- Universal solvent
- High surface tension
Human impact on the water cycle
Pumping water from a well creates drawdown (see top of the figure below). Most of the problems with groundwater quantity are due to this drawdown.
Pumping too much water, or pumping it too fast, increases the drawdown.
Hazards to unconfined aquifers from too much pumping include:
- Wells going dry - Pumping causes drawdown to reach bottom of well
- Subsidence - Removing water from aquifer weakens the rock, which then colapses
- Salt water intrusion - see figure below
Pumping too much water from confined aquifers can lead to:
- Aquifer depletion - removing water faster than the recharge rate. Note some aquifers have very long recharge times.
Types of water scarcity:
- Drought - lack of rainfall (<70% of normal)
- Dry climate - evaporation > precipation
- 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:
- 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.
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
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.