Hectare for hectare, irrigated land is far more productive than rainfed land, and the expansion of irrigated acreage over the past 30 years has played an important part in the gains in food production. At the height of the Green Revolution in the 1970s, irrigated land was expanding at a global rate of more than 2 percent per year (94). Since then, annual growth in irrigated hectares has declined to about 1 percent, mostly because of the great expense involved in developing irrigation systems, as well as the increased competition for water supplies.
Despite these barriers of cost and limited water availability, agriculture experts expect continued growth in irrigated land to meet future food needs and expand export agriculture, at least in developing countries. The Food and Agriculture Organization of the United Nations (FAO) projects that irrigated land in developing nations (excluding China) will increase at roughly 0.8 percent per year, expanding from 123 million hectares in 1990 to some 146 million hectares in 2010. Egypt, Mexico, and Turkey anticipate particularly rapid growth in irrigated acreage (95).
Along with the benefits of higher and more reliable yields, irrigation brings risks of increased infectious diseases such as malaria and schistosomiasis through habitat disruption. By providing habitat for disease agents, irrigation channels and the dams or impoundments that supply them can greatly extend the range and transmission of some infectious diseases (96). The association of schistosomiasis with irrigation is a case in point. Worldwide prevalence of the disease has risen over the past five decades, due mostly to the expansion of irrigation systems in hot climates (97). Clear links to increases in schistosomiasis have been documented in irrigation projects such as the Mwea project in Kenya, where schistosomiasis accounts for 18 percent of all deaths (98). The slow-moving water in the irrigation canals and drainage ditches provides ideal habitat for the snail that is the intermediate host of the schistosomiasis organism. Calm waters along reservoir banks, thick with vegetation, are also heavily colonized by these snails and are an ideal transmission site for the disease because human activity is usually centered near the lakeside as well (99)(100). In the upper region of Ghana, schistosomiasis prevalence tripled in the late 1950s and early 1960s when a large number of agricultural impoundments were constructed. In these areas, infection rates were as high as 50 percent (101).
In all, more than 30 diseases have been linked to irrigation, and the health toll from these can be quite significant regionally (102). Asian paddy-rice agriculture is frequently associated with malaria and Japanese encephalitis, in addition to schistosomiasis. In Central and South America, irrigated farming may expose farmers to filariasis as well as malaria and other mosquito-borne diseases (103). In Africa, in addition to the suite of mosquito-borne diseases and schistosomiasis associated with irrigation canals, dam spillways have provided breeding sites for the black fly that carriers river blindness (onchocerciasis) (104). Although it is impossible to quantify the additional toll of diseases related specifically to irrigation systems, it is fair to say that in agricultural areas these systems are important contributing factors to the overall burden of water-related diseases.
Other health risks emerge from the use of wastewater from municipal and industrial sources to irrigate crops. Irrigating with raw or partially treated sewage can conserve water and fertilize crops economically by capturing nutrients that would normally be wasted. This irrigation method is also an effective way to prevent contamination of nearby waterways with the disease organisms and nutrients that sewage contains a considerable health benefit (105). The most serious drawback of using sewage for irrigation is its role in transmitting infectious diseases both to agricultural workers and the general public. Two worm-related infections that are endemic in many developing countries, one from the Ascaris worm and the other from the Trichuris worm, are commonly associated with wastewater irrigation. Eating uncooked vegetables that have been irrigated with such water can effectively transmit these worms, which colonize the small and large intestines. Farmworkers in fields irrigated with wastewater may also become infected with hookworm, whose debilitating effects include anemia (106).
Wastewater irrigation has also been linked to transmission of enteric diseases such as cholera and typhoid, even in areas where these diseases are not endemic. For example, a 1970 cholera outbreak in Jerusalem was blamed on consumption of vegetables irrigated with wastewater. The seasonal pattern of typhoid infections in Santiago, Chile, in the 1970s and 1980s was tied to consumption of salad crops and vegetables grown outside the city on sewage-irrigated farms. In both instances, sanitation levels were fairly good, so the normal routes of cholera and typhoid transmission (contaminated drinking water and poor personal hygiene) were not the culprits (107). Other gastrointestinal diseases such as dysentery, giardiasis, and even infectious hepatitis may be similarly transmitted through contaminated vegetables (108).