Module #26

The Availability of Water

Module Introduction:

​

• A very small amount of Earth's aboveground water is found in the atmosphere and in the form of water bodies such as streams, rivers, wetlands, and lakes.
• In this module, we will examine the major sources of freshwater on Earth.
• We will also consider the effects of unusually high and low amounts of precipitation.

Module #26: The Availability of Water

Essential Knowledge

4.6 Watersheds (Module 26)

• Characteristics of a given watershed include its area, length, slope, soil, vegetation types, and divides with adjoining watersheds.

5.10 Impacts of Urbanization (Modules 23, 26, 30)

• Urbanization can lead to depletion of resources and saltwater intrusion in the hydrologic cycle.
• Urbanization, through the burning of fossil fuels and landfills, affects the carbon cycle by increasing the amount of carbon dioxide in the atmosphere.
• Impervious surfaces are human-made structures—such as roads, buildings, sidewalks, and parking lots—that do not allow water to reach the soil, leading to flooding.
• Urban sprawl is the change in population distribution from high population density areas to low density suburbs that spread into rural lands, leading to potential environmental problems.

​

Case Study: Dams and Salmon on Klamath River

• The Klamath River stretches 250 miles from Southern Oregon to Northern California.
• For thousands of years, Native American relied on the river and the salmon it produced. Salmon are a migrating fish with eggs hatching in the upstream portion of the river and the young fish migrating downstream before returning upstream as mature adults to lay their eggs. As many as 800,000 mature chinook salmon migrate up the river each year.
• But over the past 100 years the river has been dammed for electricity and farmers have diverted water to irrigate their crops.
• As a result, salmon populations have been greatly reduced and local Native American tribes and the commercial fishing industry have suffered.

See pages 293-294 of the textbook for the full text.

Case Study: Dams and Salmon on Klamath River

• In the 1900s, the US Bureau of Reclamation drained two large lakes that supplied the water for the river and four hydroelectric dams were installed to provide power to 70,000 homes.
• While hydroelectric dams produce far less pollution than traditional power plants, they still had a significant impact and served as a barrier to salmon migrations.
• Climate change resulted in less snow in the mountains and less snowmelt to provide water, even as agricultural demand remained constant. This reduced river levels even further.
• Dispute among the stakeholders intensified as Chinook salmon became endangered and were provided protected under the Endangered Species Act.
• Ultimately the dams were removed from a combination of societal pressure and inability to maintain them. Farmers agreed to conserve water.

See pages 293-294 of the textbook for the full text.

Distribution of Water on Earth

• Fresh water represents less than 3 percent of all water on Earth, and only about three-fourths of that freshwater is surface water.
• Most of that surface water is frozen as ice and in glaciers.
• Therefore, less than 1 percent of all water on the planet is accessible for use by humans.
• Note that the majority of Earth’s freshwater is locked up as ice and glaciers, how might climate change alter this?

​

Groundwater �

• Groundwater exists in the small spaces found within permeable layers of rock and soil known as aquifers.
• Groundwater represents about 25% of all the freshwater on Earth and can be extracted for human use.
• Unconfined aquifer: An aquifer made of porous rock covered by soil out of which water can easily flow.
• Confined aquifer: An aquifer surrounded by a layer of impermeable rock or clay that impedes water flow.

Aquifer: A permeable layer of rock and sediment that contains groundwater.

Aquifers

• Unconfined aquifers are rapidly recharged by water that percolates downward from the land surface.
• Confined aquifers are capped by an impermeable layer of igneous/metamorphic rock or clay based soils, which can cause water pressure to build up underground.
• Artesian wells are formed when a well is drilled into a confined aquifer and the natural pressure causes water to rise toward the ground surface.

Artesian well: A well created by drilling a hole into a confined aquifer.

Unconfined aquifers are usually surrounded by sandy soils or permeable sedimentary rocks and can recharge more quickly than confined ones. However, they are also more vulnerable to contamination and pollution.

Groundwater Recharge�

• Groundwater has a surprisingly complex structure and dynamic relationship with precipitation and surface waters.
• Water table: The uppermost level at which the water in a given area fully saturates rock or soil.
• Groundwater recharge: A process by which water percolates through the soil and works its way into an aquifer.
• Spring: A natural source of water formed when water from an aquifer percolates up to the ground surface.

Watersheds

• NOAA: Watersheds
• Watershed studies like the one at Hubbard brook help us understand how disturbances affect ecosystem processes like erosion or nitrogen deposition.
• Watershed: All land in a given landscape that drains into a particular stream, river, lake, or wetland and eventually to outflow points such as reservoirs, bays, and the ocean.

​

Geology has a major impact on watersheds. Impermeable layers such as paved surfaces, clay or igneous rocks help to promote runoff and block infiltration of water into the soil, influencing hydrology in the watershed.

​

Watershed Geology

• Geology refers to the physical structure of the rocks and soil that underline an area.
• Geology has a major impact on watersheds.
• Impermeable geologic layers include soils with large amounts of clay or igneous rocks such as granite that have low permeability and impede groundwater recharge thereby promoting runoff and erosion.
• By contrast, sandy soils and sedimentary rocks such as sandstone or limestone are more porous and allow water to move through the ground easily, accelerating groundwater recharge and limiting runoff/erosion.

Human Impact on Groundwater

• The Ogallala aquifer, also called the High Plains aquifer, is the largest in the United States, with a surface area of about 450,000 km² (174,000 miles²).
• The aquifer has been heavily exploited for agriculture and current use is unsustainable.
• (a) The change in water level from 1950 to 2005, mostly due to withdrawals for irrigation that have exceeded the aquifer’s rate of recharge.
• (b) The current thickness of the aquifer.

​

Exploitation of Groundwater�

• Groundwater is a major water resource, but it is finite.
• The overexploitation of groundwater for human use can lead to severe complications, beyond using up the available water.

This map illustrates just how vast, yet limited our groundwater resources are.

Cone of Depression

• (a) When a deep well is not heavily pumped, the recharge of the water table keeps up with the pumping.
• (b) In contrast, when a deep well pumps water from an aquifer more rapidly than it can be recharged, it can form a cone of depression in the water table and cause nearby shallow wells to go dry.

Cone of depression: An area lacking groundwater due to rapid withdrawal by a well.

Cones of Depression and Climate Change

• As sea levels rise due to 1) the melting of glaciers and ice caps and 2) thermal expansion from warmer temperatures, coastlines become more vulnerable to flooding.
• Cones of depression can exacerbate the problem by generating subsidence meaning coastal communities are sinking as sea levels are rising.
• Norfolk, VA and the Chesapeake Bay region are vulnerable, but Sustainable Water Initiative For Tomorrow (SWIFT) is looking to fix that by replenishing freshwater aquifers to halt subsidence.

Subsidence: the gradual caving in or sinking of an area of land. Land subsidence occurs when large amounts of groundwater have been withdrawn from certain types of rocks, such as fine-grained sediments.

SWIFT’s Potomac Aquifer Recharge Plan

• To combat the dual threats of subsidence and sea level rise, SWIFT will recharge the Potomac Aquifer with as much as one hundred million gallons of water per day.
• This system, which can serve as a model for other communities, will not only reduce groundwater related subsidence, but restore aquifer pressure and prevent saltwater intrusion.

This specific example is probably beyond this course and the AP Exam, but still interesting and relevant!

Saltwater Intrusion

• Saltwater intrusion: An infiltration of salt water in an area where groundwater pressure has been reduced from extensive drilling of wells.
• (a) When there are few wells along a coastline, the water table remains high and the resulting pressure prevents salt water from intruding.
• (b) Rapid pumping of wells drilled in aquifers along a coastline can lower the water table. Lowering the water table reduces water pressure in the aquifer, allowing the nearby salt water to move into the aquifer and contaminate the well water with salt.

​

Case Study: Saltwater Intrusion in the Everglades

• Everglades coastal wetlands provide a number of valuable ecosystem services such as protection against storm surge, sites for recreation, and support of biodiversity through the habitat these wetlands provide for numerous species of fish and wildlife.
• But, these environments are facing many problems today. Reduced freshwater flow has increased saltwater intrusion and increased the area of the Everglades vulnerable to storm surge.
• Saltwater intrusion contaminates the drinking water supply and may lead to collapse of freshwater peat soils resulting in dramatic elevation loss leading to further penetration of saltwater and loss of coastal wetland habitat.

How are climate change and sea level rise an example of a negative environmental externality in the case of the Florida Everglades?

How does this threaten environmental equity?

Science News: A freshwater, saltwater tug-of-war is eating away at the Everglades

Case Study: Saltwater Intrusion and Wetlands

• As sea levels rise due to global climate change, saltwater intrusion will become an increasingly problematic issue in coastal wetland like the Everglades.
• The intruding saltwater poisons the plants growing in the coastal wetlands, killing them and contributing to erosion.
• The loss of organic soil from erosion and low elevation will increase risk of storm impacts.

Surface Waters

• Surface water is water that exists above ground and includes streams, rivers, ponds, and wetlands. 
• Surface water can be fresh, marine or brackish, but all surface waters form the foundations of the different aquatic biomes.
• Early human civilizations typically settled along major rivers because of their importance in transportation and access to fertile floodplains for agriculture.
• Floodplain: The land adjacent to a river.

​

Drought �

• Drought has impacts on agriculture and soil health. Crops can fail during drought leading to starvation of livestock and causing millions of people to go hungry such as in southern Africa in 2004.
• Drought → reduced plant growth → reduced root growth to hold soil in place → erosion and dust storms.
• When soil dries out, it becomes more vulnerable to erosion and can more easily be swept up in passing wind. This was seen in the Great Dust Bowl in the early 1930s.
• Many important plant nutrients (such as phosphorus) are water soluble and therefore rely on the presence of water in soil.

Flooding

• Flooding occurs when an area receives more water than it can absorb.
• Ironically, drought prone areas are at risk of flooding during heavy precipitation as the dried out soil becomes impermeable and unable to absorb water.
• Similarly, areas with large amounts of impermeable surfaces such as cities or suburban areas are at risk of flooding during heavy rainfall.
• The stormwater runoffs impermeable soil or paved surfaces and into nearby streams, rivers and ponds causing them to swell and flood.
• Like droughts, floods can lead to crop failure, erosion, property damage and loss of life.

Impermeable surface: Pavement or buildings that do not allow water penetration.

Module Review:

​

​

• In this module, we learned that fresh water makes up a tiny fraction of all water on Earth and that most of this freshwater is locked up in the form of ice and glaciers.
• Groundwater is a type of available freshwater and it can be found as either confined or unconfined aquifers that may experience fast or slow recharge. In some parts of the world the rate of water withdrawal for human use is faster than the rate of aquifer recharge.
• Surface waters such as streams, rivers, wetlands, and lakes are important sources of freshwater, and wetlands can be important for flood control.
• Finally, we learned that the climatic differences in the availability of precipitation can interact with human land use to cause dramatic floods and droughts in different parts of the world.