The environmental impact of irrigation relates to the changes in quantity and quality of soil and water as a result of irrigation and the subsequent effects on natural and social conditions in river basins and downstream of an irrigation scheme. The effects stem from the altered hydrological conditions caused by the installation and operation of the irrigation scheme.
Amongst some of these problems is the depletion of underground aquifers through overdrafting. Soil can be over-irrigated due to poor distribution uniformity or management wastes water, chemicals, and may lead to water pollution. Over-irrigation can cause deep drainage from rising water tables that can lead to problems of irrigation salinity requiring watertable control by some form of subsurface land drainage. However, if the soil is under-irrigated, it gives poor soil salinity control, which leads to increased soil salinity with the consequent buildup of toxic salts on the soil surface in areas with high evaporation. This requires either leaching to remove these salts or a method of drainage to carry the salts away. Irrigation with saline or high-sodium water may damage soil structure owing to the formation of alkaline soil.
Direct effects
An irrigation scheme draws water from groundwater, rivers, lakes, or overland flow, and distributes it over a certain area. Hydrological, or direct, effects of doing this include reduction in downstream river flow, increased evaporation in the irrigated area, increased level in the water table as groundwater recharge in the area is increased and flow increased in the irrigated area.
Likewise, irrigation has immediate effects on providing moisture to the atmosphere, inducing atmospheric instabilities, and increasing rainfall downwind,M. H. Lo and J. S. Famiglietti, Irrigation in California's Central Valley strengthens the southwestern U.S. water cycle, Geophysical Research Letters, Volume 40, Issue 2, pages 301–306, 28 January 20132 or in other cases modifies the atmospheric circulation, delivering rain to different downwind areas.O. A. Tuinenburg et al., The fate of evaporated water from the Ganges basin, Journal of Geophysical Research: Atmospheres, Volume 117, Issue D1, 16 January 2012 Increases or decreases in irrigation are a key area of concern in precipitationshed studies, that examine how significant modifications to the delivery of evaporation to the atmosphere can alter downwind rainfall.P. W. Keys et al., Analyzing precipitation sheds to understand the vulnerability of rainfall dependent regions, Biogeosciences, 9, 733–746, 2012 PDF
Indirect effects Indirect effects are those that have consequences that take longer to develop and may also be longer-lasting. The indirect effects of irrigation include the following: Waterlogging Soil salination Ecological damage Socioeconomic impacts
The indirect effects of waterlogging and soil salination occur directly on the land being irrigated. The ecological and socioeconomic consequences take longer to happen but can be more far-reaching.
Some irrigation schemes use water wells for irrigation. As a result, the overall water level decreases. This may cause water mining, land/soil subsidence, and, along the coast, saltwater intrusion.
Irrigated land area worldwide occupies about 16% of the total agricultural area, and the crop yield of irrigated land is roughly 40% of the total yield.Bruce Sundquist, 2007. Chapter 1- Irrigation overview. In: The earth's carrying capacity, Some related reviews, and analysis. On line: In other words, irrigated land produces 2.5 times more product than non-irrigated land.
Adverse impacts
Reduced river flow The reduced downstream river flow may cause: reduced downstream flooding disappearance of ecologically and economically important wetlands or flood forests.World Wildlife Fund, WWF Names World's Top 10 Rivers at Greatest Risk, online: http://www.ens-newswire.com/ens/mar2007/2007-03-21-01.asp reduced availability of industrial, municipal, household, and drinking water reduced shipping routes. Water withdrawal poses a serious threat to the Ganges. In India, barrages control all of the tributaries to the Ganges and divert roughly 60 percent of river flow to irrigation. reduced fishing opportunities. The Indus River in Pakistan faces scarcity due to the over-extraction of water for agriculture. The Indus is inhabited by 25 amphibian species and 147 fish species, of which 22 are found nowhere else. It harbors the endangered Indus river dolphin, one of the world's rarest mammals. Fish populations, the main source of protein and overall life support systems for many communities are also being threatened. reduced discharge into the sea, which may have various consequences like coastal erosion (e.g. in GhanaTimberlake, L. 1985. Africa in Crisis - The Causes, Cures of Environmental Bankruptcy. Earthscan Paperback, IIED, London) and saltwater intrusion in delta's and estuaries (e.g. in Egypt, see Aswan Dam). Current water withdrawal from the river Nile for irrigation is so high that, despite its size, the river does not reach the sea in dry periods. The Aral Sea has suffered an "environmental catastrophe" due to the interception of river water for irrigation purposes.
Increased groundwater recharge, waterlogging, soil salinity
Increased groundwater recharge stems from the unavoidable deep percolation losses in the irrigation scheme. The lower the irrigation efficiency, the higher the losses. Although reasonably high irrigation efficiencies of 70% or more (i.e., losses of 30% or less) can occur with sophisticated techniques like sprinkler irrigation and drip irrigation or by well-managed surface irrigation, in practice the losses are commonly in the order of 40% to 60%. This may cause the following issues:
rising water tables increased storage of groundwater that may be used for irrigation, municipal, household, and drinking water by pumping from wells waterlogging and drainage problems in villages, agricultural lands, and along roads - with mostly negative consequences. The increased level of the water table can lead to reduced agricultural production. shallow water tables - a sign that the aquifer is unable to cope with the groundwater recharge stemming from the deep percolation losses where water tables are shallow, the irrigation applications are reduced. As a result, the soil is no longer leached and soil salinity problems develop stagnant water tables at the soil surface are known to increase the incidence of water-borne diseases like malaria, filariasis, yellow fever, dengue, and schistosomiasis (Bilharzia) in many areas.World Health Organization (WHO), 1983. Environmental health impact assessment of irrigated Agriculture. Geneva, Switzerland. Health costs, appraisals of health impacts, and mitigation measures are rarely part of irrigation projects.Himanshu Thakkar. Assessment of Irrigation in India. World Commission on Dams. On line: http://www.dams.org/docs/kbase/contrib/opt161.pdf to mitigate the adverse effects of shallow water tables and soil salinization, some form of watertable control, soil salinity control, drainage and drainage system is needed as drainage water moves through the soil profile, it may dissolve nutrients (either fertilizer-based or naturally occurring) such as nitrates, leading to a buildup of those nutrients in the ground-water aquifer. High nitrate levels in drinking water can be harmful to humans, particularly infants under six months, where it is linked to "blue-baby syndrome" (see Methemoglobinemia).
Reduced downstream river water quality Owing to drainage of surface and groundwater in the project area, which waters may be salinized and polluted by agricultural chemicals like biocides and fertilizers, the quality of the river water below the project area can deteriorate, which makes it less fit for industrial, municipal and household use. It may lead to reduced public health. Polluted river water entering the sea may adversely affect the ecology along the seashore (see Aswan Dam).
The detention of sediments behind the dams can eliminate the natural contribution of sediments, which is critical to surface water irrigation diversions. Sedimentation is an essential part of the ecosystem that requires the natural flux of the river flow. This natural cycle of sediment dispersion replenishes the nutrients in the soil, which will, in turn, determine the livelihood of the plants and animals that rely on the sediments carried downstream. The benefits of heavy sedimentation deposits can be seen in large rivers like the Nile River. The sediment from the delta has built up to form a giant aquifer during flood season and retains water in the wetlands. The wetlands created and sustained due to built-up sediment are a habitat for numerous species of birds.<r/r Ellen Wohl, “The Nile: Lifeline in the Desert”, A World of Rivers p. 98f> However, heavy sedimentation can reduce downstream river water quality and can exacerbate floods upstream. This has been known to happen in the Sanmenxia reservoir in China. The Sanmenxia reservoir is part of a larger man-made project of hydroelectric dams called the Three Gorge Project.<ref></Allen Wohl, “The Chang Jiang: Bridling a Dragon”, A World of Rivers p 275, p.283. Calculating the amount of sediment that will be carried downstream to the Sanmenxia reservoir is difficult to estimate.</ref>
In 1998, uncertain calculations and heavy sediment greatly affected the reservoir's ability to fulfill its flood-control function properly.Ellen Wohl, “The Chang Jiang: Bridling a Dragon”, A World of Rivers p284 This also reduces the downstream river water quality. Shifting more towards mass irrigation installments to meet more socioeconomic demands is going against the natural balance of nature, and use water pragmatically- use it where it is found.Donald Worster, “ Thinking Like a River” in The Wealth of Nature: Environmental History and the Ecological Imagination (New York: Oxford University Press, (1993), p133
Affected downstream water users
Downstream water users often have no legal water rights and may fall victim to irrigation development.
Pastoralists and nomadic tribes may find their land and water resources blocked by new irrigation developments without having legal recourse.
Flood-recession cropping may be seriously affected by the upstream interception of river water for irrigation purposes. In Baluchistan, Pakistan, the development of new small-scale irrigation projects depleted the water resources of nomadic tribes traveling annually between Baluchistan and Gujarat or Rajasthan, IndiaModern interferences in traditional water resources in Baluchistan. In: Annual Report 1982, pp. 23-34. ILRI, Wageningen, The Netherlands. Reprinted in Water International 9 (1984), pp. 106- 111. Elsevier Sequoia, Amsterdam. Also reprinted in Water Research Journal (1983) 139, pp. 53-60. Download from: , under nr. 10, or directly as PDF: After the closure of the Kainji dam, Nigeria, 50 to 70 percent of the downstream area of flood-recession cropping was lost.C.A. Drijver and M. Marchand, 1985. Taming the floods. Environmental aspects of the floodplain developments of Africa. Centre of Environmental Studies, University of Leiden, The Netherlands.
Lost land use opportunities Irrigation projects may reduce the fishing opportunities of the original population and the grazing opportunities for cattle. The livestock pressure on the remaining lands may increase considerably because the ousted traditional pastoralist tribes will have to find their subsistence and existence elsewhere, overgrazing may increase, followed by serious soil erosion and the loss of natural resources.Ecosystems Ltd., 1983. Tana delta ecological impact study. Nairobi, Kenya.