Module #27

Human Alteration of Water Availability

Module Introduction:

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• We have seen that the availability of water around the world can be unpredictable.
• In the Klamath River, for example, years of low precipitation caused an insufficient water supply for human needs. As a result, humans have learned to live with variations in water availability in several ways.
• We can channel the flow of flood waters with levees and dikes, block the flow of rivers with dams to store water, divert water from rivers and lakes and transport it to distant locations, and even obtain fresh water by removing the salt from salt water.
• In this module we will look at each of these water distribution methods and examine their costs and benefits.

Module #27: Human Alteration of Water Availability

Essential Knowledge

5.5 Irrigation Methods (Modules 27, 28)

• The largest human use of freshwater is for irrigation (70%).
• Types of irrigation include drip irrigation, flood irrigation, furrow irrigation, drip irrigation, and spray irrigation.
• Waterlogging occurs when too much water is left to sit in the soil, which raises the water table of groundwater and inhibits plants’ ability to absorb oxygen through their roots.
• Furrow irrigation involves cutting furrows between crop rows and filling them with water. This system is inexpensive, but about 1/3 of the water is lost to evaporation and runoff.
• Flood irrigation involves flooding an agricultural field with water. This system sees about 20% of the water lost to evaporation and runoff. This can also lead to waterlogging of the soil.

Essential Knowledge

5.5 Irrigation Methods Continued (Modules 27, 28)

• Spray irrigation involves pumping groundwater into spray nozzles across an agricultural field. This system is more efficient than flood and furrow irrigation, with only 1/4 or less of the water lost to evaporation or runoff. However, spray systems are more expensive than flood and furrow irrigation, and also requires energy to run.
• Drip irrigation uses perforated hoses to release small amounts of water to plant roots. This system is the most efficient, with only about 5% of water lost to evaporation and runoff. However, this system is expensive and so is not often used.
• Salinization occurs when the salts in groundwater remain in the soil after the water evaporates. Over time, salinization can make soil toxic to plants.
• Aquifers can be severely depleted if overused for agricultural irrigation, as has happened to the Ogallala Aquifer in the central United States.

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Essential Knowledge

6.9 Hydroelectric Power (Modules 27, 38)

• Hydroelectric power can be generated in several ways. Dams built across rivers collect water in reservoirs. The moving water can be used to spin a turbine. Turbines can also be placed in small rivers, where the flowing water spins the turbine.
• Tidal energy uses the energy produced by tidal flows to turn a turbine.
• Hydroelectric power does not generate air pollution or waste, but construction of the power plants can be expensive, and there may be a loss of or change in habitats following the construction of dams.

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Environmental Themes: Climate Change

• Climate change will not only alter global temperatures but also alter precipitation patterns. Some places will get wetter, others drier.
• What externalities are driving these changes and what patterns of inequity do they pose?

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Controlling Surface Water

Levees can be natural or built to prevent flooding along rivers, but present their own challenges:

• The fertility of the floodplain is reduced without floodwater to deposit sediment and nutrients.
• Levees may prevent flooding in one location, but in doing so force the flood water somewhere else.
• Use of levees encourages development in the floodplain, which occasionally may still flood.

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Levee: An enlarged bank built up on each side of a river.

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A floodplain is a generally flat area of land next to a river or stream.

Sea Walls and Ocean Water

• Similar to levees, sea walls (sometimes called dikes) are used to control ocean water.
• As sea levels rise due to climate change (due to the melting of glaciers and thermal expansion), more nations will need to invest in sea walls to protect cities and areas in low elevations.
• Sea walls can also protect against storm surges.

‘Living Shorelines’ are becoming an increasingly popular method of controlling rising sea levels and provide the same function as dikes with the added benefit of ecosystem services including habitat to expand biodiversity.

Why You Should Give a Dam

• Dams are built to restrict the flow of streams and rivers. We build dams for many reasons including water consumption, flood control, recreation and to generate electricity.
• Dams have enormous environmental impacts including disruption of wildlife. Fish ladders (and sometimes fish cannons) are often used to alleviate this problem for migrating fish like salmon.
• The largest hydroelectric dam in the world is the Three Gorges Dam on the Yangtzee River in China.
• Dam: A barrier that runs across a river or stream to control the flow of water.
• Reservoir: The water body created by a damming a river or stream.

Fish ladder: A stair-like structure that allows migrating fish to get around a dam.

Dam Resources!

(I’m having too much dam fun)

Resources to help understand the environmental impact of dams:

• THE WILD: A river runs through it…once again
• Native Plants, Healthy Plant: The Dam Show
• Patagonia: DamNation

Case Study:

Egypt’s Aswan Dam

Ecological Issues:

• Sedimentation: the dam prevents the Nile River from depositing sediments that historically enriched the flood plains along the river. This produces sedimentation behind the dam itself → increased coastal erosion downstream.
• Waterlogging: the irrigation provided by the dam has left groundwater levels artificially high → waterlogging (saturation) of soil.
• Increased soil salinity: irrigation water contains salts that build up over time, increasing soil salinity and harming plant growth.

Sedimentation is a problem common to all dams. If not treated regularly, the dam will either collapse or become a water fall as sedimentation pushes the reservoir up and over the dam.

Waterlogging

• As such, water logging is closely related with permeability (ie. percolation).
• Soils with a high percolation rate are easily leached, losing nutrients from the upper layers of soil where roots are most abundant.
• While water is necessary for plant growth, waterlogged soils are problematic for most plants especially in agriculture.
• Waterlogged soils will present anaerobic conditions and limit gas exchange with the atmosphere. Left untreated, this can result in the asphyxiation of roots, the development of harmful fungal growth and ultimately the death of the plant.

• When soil is saturated, all available pore space is filled with water and the soil is said to be waterlogged.

Transporting Surface Water

• Aqueducts carry water from one location to another
• Aqueduct: A canal or ditch used to carry water from one location to another.
• In the United States, both Los Angeles and New York City depend on aqueducts to meet their daily water needs.
• The consequences of water diversion can be severe as seen in the Aral Sea.

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The Aral Sea, on the border of Kazakhstan and Uzbekistan, was once the world’s fourth largest lake. Since the two rivers that fed the lake were diverted, its surface area has declined by 60 percent and the lake has split into two parts: the North and South Aral seas.

Impacts on the Aral Sea

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The water diverted for irrigation helped support agriculture and “made the desert bloom”, however…

• The Aral Sea lost 60% of its original surface area and the remaining water in the Aral became increasingly salty → habitat destruction and the loss of a fishing industry.
• The Aral sea split into two parts → habitat fragmentation and reduction of gene flow.
• The exposed sediments → salt-laden dust affecting crops and human health.
• The local climate altered due to changes in the surface area of the lake → reduced evaporation → reduced precipitation.

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You can learn more here:

World of Change: Shrinking Aral Sea

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Meeting Human Water Needs

Desalination converts salt water into fresh water and can occur in two major ways: distillation or reverse osmosis.

• Distillation: A process of desalination in which water is boiled and the resulting steam is captured and condensed to yield pure water.
• Reverse osmosis: A process of desalination in which water is forced through a thin semipermeable membrane at high pressure.

Desalination: The process of removing the salt from salt water. Also known as Desalinization.

Desalination Technologies

Salt water can be converted into fresh water in one of two ways:

• (a) Distillation uses heat to convert pure water into steam that is later condensed, leaving the salt behind.
• (b) Reverse osmosis uses pressure to force pure water through a semipermeable membrane, leaving the salt behind.
• Desalination is especially prevalent in the Middle East which has the lowest amount of naturally available freshwater.

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Global Water Availability

• The amount of water available per person varies tremendously around the world. North Africa and the Middle East are the regions with the lowest amounts of available fresh water.

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Module Review:

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• In this module, we have seen that humans have developed numerous methods of altering the distribution of water including the use of levees, dikes, dams, and aqueducts.
• We also learned that desalination technologies can be used to convert undrinkable salt water into drinkable fresh water.