17.1: Introduction

But the irrigation that nourished Mesopotamian fields carried a hidden risk. Groundwater in semiarid regions usually contains a lot of salts. … When evaporation rates are high, sustained irrigation can generate enough salt to eventually poison the crops.

—David Montgomery, 2007

Growing seasons around the world rarely have the right amount of precipitation, and deficits and excesses of water are the most significant overall yield-limiting factors to crop production. 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” Tigris and Euphrates, the lower Nile Valley and northeastern China. High yields in drained and irrigated areas allowed for the development of trade specialization because crop surpluses no longer required everyone to provide their own food supply. This led to important innovations like markets, writing and transportation. 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, often dug and maintained by enslaved peoples, resulted in lost land fertility and an inability to sustain large centrally governed civilizations.

Shortage of water. It is estimated that drought results in more crop yield losses than by all pathogens combined. It is also projected that many of the world’s agricultural regions will be drier in the future. 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 United States and in 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 supplemental irrigation during dry spells to ensure crop quality and steady supplies for market outlets.

Water in Fields

Soil conditions may vary significantly within a field, greatly influencing water infiltration and movement. Runoff with intense rainfall is common at the top of a slope or on a slope shoulder, and water tends to accumulate in depressions. Both areas may suffer during very dry periods, with the slope top or shoulder soil having low water storage and with the wet areas in depressions growing plants with shallow root systems that aren’t deep enough to access water lower in the soil when it is dry. And there may be two or more soil types within a field with different physical properties that affect water infiltration and movement. The extent of these variations may be substantial. It is estimated that these areas of year-to-year unstable yields—because of either too little or too much soil moisture—represent from about a quarter to a third of fields in the U.S. Midwest, with possible economic losses of over $500 million per year. Thus, practices such as no-till and cover cropping, and drainage of depressions, can both increase yields and decrease annual variations caused by different patterns of precipitation.

Table 17.1 Approximate Amounts of Water Needed for Food Production

Product	Gallons of water per pound
Wheat	150
Rice	300
Corn	50
Potatoes	19
Soybeans	275
Beef	1,800
Pork	700
Poultry	300
Eggs	550
Milk	100
Cheese	600
Almonds	1,900
Source: FAO

Excess of water. To address excess water problems, the best fields in the United States have had drainage systems installed, which make those soils even more productive than they were naturally. Drainage of wet fields overcomes water-logged conditions and allows for a longer growing season because farmers can get onto those fields earlier in the spring and can harvest later in the fall without causing extreme compaction. Drainage also reduces yield losses or even prevents complete crop failures when fields experience excessive precipitation during the early growing season.

The benefits of irrigation and drainage in addressing shortages and excesses of water are thus obvious. They are critical to food security as well as to the agricultural intensification needed to feed a growing global population while protecting 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 streams, rivers, lakes and estuaries. Similarly, irrigation systems can result in drastic changes in river and estuarine ecosystems, as well as in land degradation through salinization and sodium buildup, and they have been sources of international conflict.