Water Quantity

Objectives

Water Distribution Water Cycle 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

All of these properties are due the fact that water undergoes Hydrogen Bonding. Water, expanding as it freezes, can cause water pipes to break in cold weather if precautions are not taken. This effect has an interesting consequence for ecology (in cold climates). If water was a normal substance lakes would freeze from the bottom up, causing organisms like fish to die. But lakes actually freeze from the top down.

High specific heat is why water is used for fighting fires. Water can absorb a lot of heat before it becomes steam. High surface tension causes the effect shown in the picture.

Water Distribution

Water Distribution

Distribution of Freshwater

Water Cycle

The text describes the individual parts of this diagram

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

Aquifers

Groundwater is not a river or lake underground. Even though they exist they are relatively rare. When geologists and environmental scientists refer to groundwater they are talking about water which is stored inside rock. All rocks contain small holes called pores. In some types of rock these pores can hold water. The schematic below shows a typical groundwater arrangement.

Groundwater

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 Recharge - Infiltration of water into an aquifer either from above or the side Well - Dug or drilled hole used to extract groundwater When above we say "significant" we mean pumpable (see next section).

Groundwater pumping

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.

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

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 collapses
• Salt water intrusion - see figure below

Showing the situation which leads to saltwater intrusion. This happens when a well on the land side draws in water from the saltwater aquifer.

Pumping too much water from confined aquifers can lead to:

• Subsidence
• Aquifer depletion - removing water faster than the recharge rate. Note some aquifers have very long recharge times.

Case Study: Ogallala Aquifer

Depletion of the Ogallala Aquifer in the USA. This area is dominated by highly irrigated agriculture

The Ogallala Aquifer is an aquifer located in the Great Plains of the United States (see map). It is one of the largest aquifers in the world. Most of the area is under intense agriculture, especially as center-pivot irrigation systems. This irrigation removes more water from the aquifer than the aquifer receives from recharge. This has caused the top level of the aquifer to fall as much as 90 m in some areas.

Water scarcity

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).

Water Stress Map. Notice that the stress is a function of both amount of water and population density

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.

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?