Agricultural water use, environment and health
Agriculture may have distinct negative externalities in terms of water quantity and quality. Pasture and crops take up 37 percent of the Earth’s land area. Agriculture is the largest water user and the main source of nitrate pollution of ground and surface waters, as well as the principal source of ammonia pollution. It is also a major contributor to the phosphate pollution of waterways and of release to the atmosphere of the greenhouse gases methane and nitrous oxide. Land degradation, Stalinization, over-abstraction of groundwater and the reduction of genetic diversity in crop and livestock affect the basis of agriculture’s own future. The vanishing Aral Sea is a clear example of the irreversible impacts of excessive withdrawals. The irrigation sector is coming under increasing public scrutiny as amenity and ecosystem values are lost while the expected economic and social benefits of irrigation systems are not fully realized. Competition between urban dwellers and agriculture is also a growing issue and may worsen environmental pressure. In developed countries, environmental concerns have been a key driver for modernizing irrigation systems.
Reclamation of wetlands has historically made a major contribution to agricultural growth. Because of the presence of water during a large part of the year and in view of the relative fertility of their soils, many wetlands have a good potential for agricultural use. However, this use leads to serious environmental damage, which has been recognized by the adoption of the Convention on Wetlands (Ramsar, 1971) to protect wetlands. The developing countries still have some 300 million ha of wetlands that may be suitable for crop production but only a relatively small percentage is currently used to this end. Where no alternative additional land resources are available to exploit, wetlands will inevitably be converted to crop production. This is the case in many parts of sub-Saharan Africa, where the nutritional situation is bad and wetlands represent an attractive opportunity for agricultural development.
Unwise use of wetlands may result in environmental degradation. Draining of wetlands has often been carried out under the wrong assumption that wetlands are useless and worthless. Sustainable use of wetlands can be achieved by selecting crops adapted to wetland conditions, using appropriate water and soil management technologies and planning wetland development and management carefully within the global framework of the watershed area. Wetlands of particular international or national importance on account of their significance in terms of ecology, botany, zoology or biodiversity should be protected from any agricultural use and from the influences of agricultural activities in upstream areas.
Water pollution, habitat degradation and massive water withdrawals can deprive fishing communities of their livelihood and push them into food insecurity. The resulting environmental impacts affecting fishery resources in inland waters can be devastating. Even in estuarine and coastal zones at the lower end of river basins, fishery resources are impacted by pollution, habitat degradation and upstream water withdrawal and use. It is increasingly recognized that agriculture also has positive externalities, including environmental services and products. The multifunctional nature of agriculture is increasingly acknowledged and encouraged, so that farmers are seen not only as commodity-producers but also appreciated as self-employed citizens, stewards of the landscape and stakeholders in vibrant communities. Trade-offs between food security and the environment can be further reduced through already available or emerging technologies and land-management practices. By using more sustainable production methods, the negative impacts of agriculture on the environment can be attenuated. Agriculture can play an important role in reversing negative impacts by, for example, environmentally sound water use, biological treatment of waste, enhancing the infiltration of water to reduce flood runoff, preserving agricultural and natural biodiversity, and storing carbon in soils.
With rising demands for water, quality concerns over water have risen rapidly. Pollutant loads have increased enormously, and at the same time the amounts of water available for dilution are decreasing. The situation is particularly alarming in developing nations while, in developed countries, the enforcement of water quality measures has resulted in improved water-quality conditions for most rivers. Water quality poses a serious threat to the sustainability and the safety of food produced by intensive farming systems upon which global food security has become increasingly dependent. Security and stability in food supplies in this century will be closely linked to success in water quality control. Organic matter, if free of pathogens, can actually be beneficial to irrigated agriculture (see Box 7), but water contamination with hazardous chemicals makes it unusable for food production.
It is estimated that poor drainage and irrigation practices have led to waterlogging and salinization of about 10 percent of the world’s irrigated lands, thereby reducing productivity. In particular, mobilization of resident salts is a widely-occurring phenomenon in irrigated river basins in arid regions. Waterlogging and salinization in large-scale irrigation projects are often the result of unavailable drainage infrastructure that was not included in the engineering design to make projects look economically more attractive. These problems are generally associated with large-scale irrigation development under arid and semi-arid conditions, as in the Indus, the Tigris-Euphrates and the Nile river basins. The solutions to these problems are known but their implementation is costly.
BOX 7 USE OF WASTEWATER FOR IRRIGATION
The cost of disposing of urban wastewater is all too often externalized against the aquatic environment and downstream users in rivers, estuaries and coastal zones and hardly, if ever, appears in the benefit and cost accounts. However, wastewater is recognized as a resource, particularly in water-scarce regions. If the polluter actually pays, wastewater is free or has only a low cost, is reliable in time and close to urban markets. In addition to direct benefits to farmers who would otherwise have little or no water for irrigation, wastewater improves soil fertility and reduces water contamination downstream. The total land irrigated with raw or partially diluted wastewater is estimated at 20 million hectares in fifty countries, somewhat below 10 percent of total irrigated land in developing countries. For irrigation use, wastewater should be subject to primary and secondary treatment, but in poor countries that is often not the case and raw sewage is applied. Disadvantages and risks related to use of insufficiently treated wastewater concern the exposure of irrigation workers and food consumers to bacterial, amoebic, viral and nematode parasites as well as organic, chemical and heavy metal contaminants. In a context of prevailing poverty, such water is used in the informal, unregulated sector, but sanitary concerns preclude the export of the products and, at least partially, the access to local food markets. Governments and the development community promote efforts to lead wastewater reuse into sustainable channels, but countries and municipalities short of resources are slow in facing the cost of water treatment. Given water scarcity and the relatively high cost of obtaining potable freshwater for municipal uses, the use of treated wastewater in the urban context is projected to increase in the future.
The key irrigation-related vector-borne diseases are malaria, schistosomiasis and Japanese encephalitis. Irrigation development has in the past sometimes been accompanied by adverse impacts on the health status of local communities. The principal causes for these impacts are rooted in ecosystem changes that created conditions conducive to the transmission of vector-borne diseases, and drinking water supply and sanitation conditions leading to gastrointestinal conditions. The attribution of the burden of each of these diseases to irrigation, or components thereof in specific settings is complex. Only where irrigation is introduced in an arid region where the diseases previously did not occur, is the association between the resulting dramatic landscape changes and the explosive rise of disease incidence and prevalence clear-cut. In most cases, there is a complex mixture of contextual determinants of the diseases combined with a number of confounding factors. For example, in parts of Africa south of the Sahara, the transmission of malaria is so intense throughout the year that the additional risk factors from irrigation development will not add to the disease burden. Schistosomiasis, rightly equated with irrigation in Africa, is also determined by human behaviour and by the state of sanitation.
Many vector-borne disease problems in irrigated areas can be traced to absent or inadequate drainage. The various forms of surface irrigation all impose increased vector-borne disease hazards, while overhead irrigation and drip irrigation are virtually free of such hazards. Crop selection can be important. In that sense, flooded rice and sugarcane are crops that carry increased vector-borne disease risk. Irrigated agriculture often requires additional chemical inputs for crop protection, and the application of pesticides can disrupt the ecosystem balance favouring certain disease vectors; it can also contribute to an accelerated development of resistance to insecticide in disease vector species.
There are many opportunities in the planning, design and operation of irrigation schemes to incorporate health safeguards: hydraulic structures, for instance, can be designed so they provide less or no habitat for vector breeding. Improved water-management practices such as alternate wetting and drying of irrigated rice fields, rotational drying of parallel irrigation canals, flushing of canals with pools of standing water and clearing canals of aquatic weeks can reduce vector breeding. Moreover, the very infrastructural development that usually accompanies irrigation development and the economic development that follows in its wake imply improvements in access to health services and increased buying power to purchase drugs, mosquito nets and other preventative and protective tools and products.
Until the 1980s, a drinking-water-supply component was often overlooked in the context of irrigation development. While this situation has improved, the two types of water use are occasionally at odds. Highly increased chemical inputs may pollute groundwater and local communities have been known to revert to canal water because the quality of water from their pumps had deteriorated. Easy access to large quantities of water in irrigation canals for domestic needs other than drinking will contribute positively to overall hygiene. There are also important overlaps between operation and maintenance tasks for irrigation and drainage schemes and for drinking water supply and sanitation services that would allow important economies of scale to be achieved. A study carried out in three African countries (see Box 8) for instance, showed that small dams and wells acted as catalysts for change, initiating actions that generated income and allowed people to diversify diets, afford health services and better cope with hungry periods of the year.
BOX 8 INTEGRATING IRRIGATION, NUTRITION AND HEALTH
FAO assessed the impact of three small-scale irrigation projects on the health and welfare of villagers in Burkina Faso, Mali and the United Republic of Tanzania. The assessment showed that small dams and wells acted as catalysts for change, initiating actions that generated income and allowed people to better cope with the hungry periods of the year, diversify diets and afford health services. These projects encouraged production, processing and preparation of a variety of indigenous foods, nutrition education and the participation of women’s groups. In all three cases, irrigation increased food production or income by enough to provide one additional meal per day, even during the ‘hungry season’ before the harvest (FAO, 2001b). In the arid countries of Asia, there are often large areas where groundwater is brackish and people have to obtain water from irrigation canals for all uses, including domestic. A study by the International Water Management Institute in Pakistan showed that safe use of canal irrigation water is possible if households have a large water storage tank in their house and have a continuous water supply for sanitation and hygiene. The results also showed that children from households having a large storage capacity for water in the house had much lower prevalence of stunted growth than children from families lacking such a storage facility. Increasing the quantity of irrigation water available for domestic use and providing toilet facilities are the most important interventions to reduce the burden of diarrhoeal disease and malnutrition.