Building Soils for Better Crops, Third Edition

Managing Water: Irrigation and Drainage


Worldwide, over two billion acres of virgin land have been plowed and brought into agricultural use since 1860. Until the last decades of the twentieth century, clearing new land compensated for loss of agricultural land. In the 1980s the total amount of land under cultivation began decliningfor the first time since farming reached the land between the Tigris and the Euphrates.


Deficits and excesses of water are the most significant yield-limiting factors to crop production worldwide. It is estimated that more than half of the global food supply depends on some type of water management. In fact, the first major civilizations and population centers emerged when farmers started to control water, resulting in more consistent yields and stable food supplies. Examples include Mesopotamia—literally the “land between the rivers” (the Tigris and Euphrates), the lower Nile Valley, and China. High yields in drained and irrigated areas allowed for the development of trade specialization, because no longer did everyone need to provide their own food supply. This led to important innovations like markets, writing, and the wheel. Moreover, new water management schemes forced societies to get organized, work together on irrigation and drainage schemes, and develop laws on water allocations. But water management failures were also responsible for the collapse of societies. Notably, the salinization of irrigated lands in Mesopotamia and filling up of ditches with sediments— cleaned out by enslaved Israelites among others—resulted in lost land fertility and an inability to sustain large centrally governed civilizations.

Today, many of the most productive agricultural areas depend on some type of water management. In the United States, average crop yields of irrigated farms are greater than the corresponding yields of dryland farms by 118% for wheat and 30% for corn. At a global scale, irrigation is used on 18% of the cultivated areas, but those lands account for 40% of the world’s food production. The great majority of agricultural lands in the western U.S. and other dry climates around the world would not be productive without irrigation water, and the majority of the U.S. horticultural crop acreage—especially in California—is entirely dependent on elaborate irrigation infrastructures. Even in humid regions most high-value crops are grown with irrigation during dry spells to insure crop quality and steady supplies for market outlets, in part because the soils have become less drought resistant from intensive use.

To address excess water problems, the best fields in the U.S. corn belt have had drainage systems installed, which made those soils even more productive than they were naturally. Drainage of wet fields allows for a longer growing season because farmers can get onto those fields earlier in the spring and harvest later in the fall without causing extreme compaction.

The benefits of irrigation and drainage are thus obvious. They are critical to food security as well as to the agricultural intensification needed to protect natural areas. Concerns with climate change, which is resulting in greater occurrences of deficits and excesses of precipitation, will increase pressure for more irrigation and drainage. But they also exact a price on the environment. Drainage systems provide hydrological shortcuts and are responsible for increased chemical losses to water resources. Some irrigation systems have resulted in drastic changes in river and estuarine ecosystems, as well as land degradation through salinization and sodium buildup, and have been sources of international conflict. In the case of the Aral Sea—formerly the fourth largest inland freshwater body in the world—the diversion of rivers to use for irrigated cotton farming in the former USSR resulted in a 50% decrease in the area of the sea. It also became severely contaminated with drainage water from agricultural fields.

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