The term used to describe a man-made system whereby water is spread from its natural source (such as a lake or river) over a much larger geographic range to aid in agricultural production is “irrigation.” The invention of “irrigation” has been fundamental the growth of human societies by ensuring that a community can farm enough food to support an ever-growing population. It was first developed roughly six thousand years ago in China and Mesopotamia.  

A “sawah” is an irrigated, or flooded, field used to grow rice. Sawahs are commonly found in Southeast Asia and Oceania. Rice is instrumental to sustaining the massive population of east and south Asia and much of it is grown in flooded fields in countries like Vietnam, Thailand, and Indonesia.

The term “desertification” is used to describe the process by which previously fertile lands become arid and unusable for farming. This process has happened in many places throughout the world, famously in Ancient Greece and Ancient Mesopotamia. The primary cause of desertification is usually over farming.

Desertification is the process by which deserts expand or arise. Many phenomena can lead to increased risk of desertification including: drought, over farming, and deforestation. 

Of the options, only one is not really directly related to desertification. Ocean acidification, although a very real and pressing problem, is not as directly related to desertification as the others. 

“Desertification” is the name given to the process by which previously fertile agricultural lands are made infertile made by human practices. “Salinization,” which refers to the buildup of salt deposits through irrigation and evaporation, is an example of desertification.

Although you might not have known when exactly the “American Drought Relief Service” was created you should be able to guess based on information that you do know. The Dust Bowl drought, which occurred in the first half of the 1930s was the most severe drought in American history and led to widespread famine and forced migration. The Drought Relief Service was created to try to alleviate this problem.

“Degraded agricultural land” is land that was once fertile but has been made infertile by mismanagement and human intervention. It is most common in densely populated areas of the world where large-scale farming has been common for a long period of time. For this reason, densely populated Western Europe is the correct answer. While all of these answers may contain some amount of degraded agricultural land, Western Europe contains the highest percentage of such land.

“Salinization” occurs frequently in arid climates. People use water to “irrigate” a previously untenable plot of land and make it usable for agricultural purposes. This allows them to work new plots of land for a period of time, but the water in this newly irrigated land evaporates much quicker than elsewhere and soon the earth is covered in residues of “salt” that render the land infertile.

“Salinization” occurs when soil in an arid climate has been made available for agricultural production using irrigation. The water evaporates quickly off the newly irrigated land, leaving residues of salt lying in the earth. Over time this causes the land to become infertile. This process is known as “salinization.”

Even though North America is a developed region, it is not safe from desertification. Nearly 74% of North America's drylands are affected by desertification. The increasing rate and intensity of wildfires in the western part of North America is due to how climate change is affecting this region.

Africa does struggle with the affects of desertification and is just behind North America with 73% of drylands affected. Though there is less land affected, the economic toll for some countries in Africa is much greater.

Certain parts of Asia are also very affected by desertification. However, it is on a much smaller scale in the vast expanse of this region.

Europe and South America are the least affected by desertification.