LWU: Lesson - Impacts of Irrigation (Topic 5.5) 📖

⏳ Estimated Reading Time: 6 - 8 minutes

Describe the benefits and drawbacks of different methods of irrigation.

Effects of Irrigation

Irrigation results in different issues. Water loss through evaporation or runoff is an especially critical issue in arid regions.

Waterlogging

Field with rows of young, green plants emerging from the soil - with puddles of water between the rows of plants. — **Too much of a good thing can be too much, such as with this waterlogged soil.**

Waterlogging occurs when too much water remains on the soil surface after irrigating.

Most crop plants need oxygen for their roots to thrive and excess water fills the pore space instead of air, reducing the amount of oxygen available to the roots. If not corrected, waterlogging can end up killing off a crop. Careful water management can help avoid waterlogged soils and save the farmer money in water costs!

Essential Knowledge

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.

Salinization

Salinization can occur in drier areas as irrigation water evaporates from the soil surface after application. Water is usually not just pure water. It typically has varying levels of minerals such as calcium and magnesium which form salts. When irrigation water evaporates, the salts are left behind on the soil surface. This results in declining soil quality which can inhibit plant growth. As such, salinization is a problem that can avoided through careful irrigation management.

Essential Knowledge

Salinization occurs when the salts in the groundwater remain in the soil after the water evaporates. Over time, salinization can make soil toxic to plants.

Aquifers

What is an Aquifer?

Educational diagram illustrating the water cycle processes: precipitation, evapotranspiration, and groundwater recharge. — **Remember this image from our study of the water cycle? Aquifers are where groundwater is stored.**

Because water is such an important resource for life, most species thrive in areas where water is plentiful. Early humans located their homes and villages near water resources and, even today, most large cities are located along rivers, lakes, coastlines, and estuaries.

Most (about 63%) of the water we use comes from surface water (lakes, streams, rivers, etc.). The rest of our water comes from groundwater. There is a huge amount of water below Earth’s surface at varying depths. In fact, the U.S. Geological Service estimates that there are thousands of times more water below ground than there is in surface water. Many people do not even realize that there is water below them because it’s easier to notice surface water than groundwater.

An aquifer is the below-ground saturated area beneath the water table that holds water. Aquifers are layers of porous rock, sand, and other material through which groundwater moves and is stored. Water takes millions of years to seep into the aquifer and because of this, groundwater use is considered nonrenewable because we are using it faster than it can seep back into, or recharge, the aquifer.

Occasionally, groundwater intersects Earth’s surface. A spring or a seep can occur when there is an exit from the aquifer below the water table.

Usually, springs have a significant amount of water flowing from a small opening, whereas a seep is just that: a small amount of water seeping out of the aquifer.

A wooden walkway in a natural setting with clear spring waters and lush greenery. — **Florida is famous for springs and seeps due to its shallow water table.**

Aquifers are replenished or recharged by water that percolates down through soil and rock layers. The area above an aquifer that provides this water is called the recharge area. In drier areas, it may take hundreds of years for water to percolate down and recharge an aquifer. In wetter areas, aquifers are recharged much quicker.

Impacts on Aquifers

Explore the tabs below. Have you ever experienced one of the impacts in the tabs below? Do you know anyone who has? My sister had one of these problems! Explore the tabs to learn which problem she had!

Aquifer Depletion

Color-coded map depicting water level changes in the Ogallala Aquifer across multiple states. — **The Ogallala Aquifer underlies very rich soil in western Texas, Oklahoma, Kansas, much of Nebraska, along with eastern New Mexico and Colorado.**

In arid regions, where agriculture places a heavy demand on an aquifer for irrigation purposes, the water removal rate from the aquifer can exceed the recharge rate, causing the level of the aquifer to drop lower and lower over time.

Pumping groundwater faster than it can be recharged can lead to subsidence, lower water tables, and saltwater intrusion. Aquifer depletion due to heavy use by farmers and ranchers in the more arid areas of the Great Plains is an ongoing water issue.

90% of the water removed from the Ogallala Aquifer is used by agriculture for irrigation and livestock. This aquifer underlies western Texas, Oklahoma, Kansas, much of Nebraska and eastern Colorado.

As the water level drops, wells that draw water from the aquifer have to be drilled deeper and deeper leading to an unsustainable situation.

The U.S. Department of Agriculture is currently working with farmers and ranchers in this area to find solutions to this problem.

Impervious Surfaces

Aerial view capturing a mixed-use area with residential and commercial zones. — **Water cannot flow into the aquifer from concrete and asphalt (impervious) surfaces.**

Another impediment to the recharge rate of aquifers occurs when humans build shopping malls, houses, and other concrete and asphalt structures.

These structures are considered impervious surfaces because water cannot flow into the aquifer from these surfaces, which ultimately reduces the amount of water flowing into the aquifer.

Once removed, most of this water either evaporates or runs off instead of going back into or recharging the aquifer.

Saltwater Intrusion

Another issue associated with groundwater withdrawal is saltwater intrusion. This issue is especially prevalent near coastlines or in areas close to saltwater lakes such as the Great Salt Lake in Utah.

Saltwater intrusion occurs when so much freshwater is removed from an aquifer that nearby saltwater moves in to replace the empty pore space formally occupied by freshwater.

Fresh water that has been contaminated by salt water requires special purification methods, known as desalinization, to extract the salt and make this water drinkable again. Desalinization is expensive and usually is only used as a last resort.

Sinkholes

A large sinkhole near a residential area, damaging a wooden fence. Behind the damaged fence is a brown house with a mailbox and birdhouse, and the surrounding area appears dry with a gravel surface. — **A sinkhole in Kentucky.**

In addition to overuse of the water in the aquifer, water removal can result in subsidence, the gradual or sudden sinking of soil at the earth’s surface. Subsidence can ruin farmland or consume houses in communities that may exist close to the aquifer.

We can also create sinkholes, which are caused when underground caverns that were previously filled with water collapse.

My sister lived in Williston, Florida, known for its sinkholes. After a hurricane, a minor sinkhole formed under her house (not nearly as bad as the hole in the picture above), and it cost over $200,000 for the insurance company to fix the sinkhole.

Essential Knowledge

Aquifers can be severely depleted if overused for agricultural irrigation, as has happened to the Ogallala Aquifer in the central United States.

AP Exam Tip

You should understand the problems caused by using water for irrigation.

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