Theenvironmental impact of irrigation relates to the changes in quantity and quality ofsoil andwater as a result ofirrigation and the subsequent effects on natural and social conditions inriver basins and downstream of anirrigation scheme. The effects stem from the alteredhydrological conditions caused by the installation and operation of the irrigation scheme.

Amongst some of these problems is the depletion of undergroundaquifers throughoverdrafting. Soil can be over-irrigated due to poordistribution uniformity ormanagementwastes water, chemicals, and may lead towater pollution. Over-irrigation can cause deep drainage from rising water tables that can lead to problems of irrigationsalinity requiringwatertable control by some form ofsubsurface land drainage. However, if the soil is under-irrigated, it gives poorsoil salinity control, which leads to increasedsoil salinity with the consequent buildup of toxicsalts on the soil surface in areas with highevaporation. This requires eitherleaching to remove these salts or a method ofdrainage to carry the salts away. Irrigation withsaline orhigh-sodium water may damage soil structure owing to the formation ofalkaline soil.

Anirrigation scheme draws water from groundwater,rivers,lakes, oroverland flow, and distributes it over a certain area.Hydrological, or direct, effects of doing this^[1] include reduction in downstream river flow, increasedevaporation in the irrigated area, increased level in thewater table asgroundwater 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,^[2] or in other cases modifies the atmospheric circulation, delivering rain to different downwind areas.^[3] Increases or decreases in irrigation are a key area of concern inprecipitationshed studies, that examine how significant modifications to the delivery of evaporation to the atmosphere can alter downwind rainfall.^[4]

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 ofwaterlogging andsoil salination occur directly on the land being irrigated. Theecological andsocioeconomic consequences take longer to happen but can be more far-reaching.

Someirrigation schemes usewater wells for irrigation. As a result, the overall water level decreases. This may causewater mining, land/soilsubsidence, 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.^[5] In other words, irrigated land produces 2.5 times more product than non-irrigated land.

The reduced downstream river flow may cause:

• reduced downstream flooding
• disappearance of ecologically and economically important wetlands or flood forests^[6]
• reduced availability of industrial, municipal, household, and drinking water
• reduced shipping routes. Water withdrawal poses a serious threat to theGanges. In India,barrages control all of the tributaries to the Ganges and divert roughly 60 percent of river flow to irrigation^[6]
• reduced fishing opportunities. TheIndus River inPakistan 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 endangeredIndus river dolphin, one of the world's rarest mammals. Fish populations, the main source ofprotein and overall life support systems for many communities are also being threatened^[6]
• reduced discharge into the sea, which may have various consequences likecoastal erosion (e.g. inGhana^[7]) and saltwater intrusion in delta's and estuaries (e.g. inEgypt, seeAswan Dam). Current water withdrawal from the riverNile for irrigation is so high that, despite its size, the river does not reach the sea in dry periods.^[6] TheAral Sea has suffered an "environmental catastrophe" due to the interception of river water for irrigation purposes.

Increased groundwater recharge stems from the unavoidabledeep 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 likesprinkler irrigation anddrip irrigation or by well-managedsurface 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 fromwells
• 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 longerleached andsoil salinity problems develop
• stagnant water tables at the soil surface are known to increase the incidence of water-borne diseases likemalaria,filariasis,yellow fever,dengue, andschistosomiasis (Bilharzia) in many areas.^[9] Health costs, appraisals of health impacts, and mitigation measures are rarely part of irrigation projects.^[10]
• to mitigate the adverse effects of shallow water tables and soil salinization, some form ofwatertable control,soil salinity control,drainage anddrainage 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" (seeMethemoglobinemia).

Owing to drainage of surface and groundwater in the project area, which waters may be salinized and polluted byagricultural chemicals likebiocides andfertilizers, 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 (seeAswan 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.^[11] 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.^[12] In 1998, uncertain calculations and heavy sediment greatly affected the reservoir's ability to fulfill its flood-control function properly.^[13] 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^[14]

Downstream water users often have no legal water rights and may fall victim to irrigation development.

Pastoralists andnomadic 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.

• InBaluchistan,Pakistan, the development of new small-scale irrigation projects depleted the water resources ofnomadic tribes traveling annually between Baluchistan andGujarat orRajasthan, India^[15]
• After the closure of theKainji dam,Nigeria, 50 to 70 percent of the downstream area of flood-recession cropping was lost^[16]

Irrigation projects may reduce the fishing opportunities of the original population and the grazing opportunities for cattle. Thelivestock pressure on the remaining lands may increase considerably because the ousted traditional pastoralist tribes will have to find theirsubsistence and existence elsewhere,overgrazing may increase, followed by serioussoil erosion and the loss ofnatural resources.^[17]
TheManatali reservoir formed by theManantali dam inMali intersects the migration routes of nomadic pastoralists and destroyed 43000 ha ofsavannah, probably leading to overgrazing and erosion elsewhere. Further, the reservoir destroyed 120 km² offorest. The depletion of groundwater aquifers, which is caused by the suppression of the seasonal flood cycle, is damaging the forests downstream of the dam.^[18][19]

When more groundwater is pumped from wells than replenished, storage of water in theaquifer is beingmined, and the use of that water is no longer sustainable. As levels fail, it becomes more difficult to extract water, and pumps will struggle to maintain the design flow rate, which may consume more energy per unit of water. Eventually, extracting groundwater may become so difficult that farmers may be forced to abandon irrigated agriculture.
Some notable examples include:

• The hundreds oftube wells installed inUttar Pradesh, India, with World Bank funding, have operating periods of 1.4 to 4.7 hours/day. In contrast, they were designed to operate 16 hours/day^[20]
• InBaluchistan,Pakistan, the development of tube well irrigation projects was at the expense of the traditionalqanat orkarez users^[15]
• groundwater-related subsidence^[21] of the land due to mining of groundwater occurred in the United States at a rate of 1m for every 13m that the water table was lowered^[22]
• Homes at Greens Bayou nearHouston,Texas, where 5 to 7 feet of subsidence has occurred, were flooded during a storm in June 1989 as shown in the picture^[23]

The effects of irrigation on the water table, soil salinity, and salinity of drainage and groundwater, and the effects ofmitigative measures can be simulated and predicted using agro-hydro-salinity models likeSaltMod andSahysMod^[24]

1. In India, 2.19 million ha of land has been reported to suffer from waterlogging in irrigation canal commands. Also, 3.47 million ha were reported to be seriously salt-affected,^[25][26]
2. In theIndus Plains inPakistan, more than 2 million hectares of land are waterlogged.^[27] The soil of 13.6 million hectares within the Gross Command Area was surveyed, which revealed that 3.1 million hectares (23%) were saline. 23% of this was inSindh and 13% in thePunjab.^[27] More than 3 million ha of water-logged lands have been provided with tube-wells and drains at the cost of billions of rupees. Still, the reclamation objectives were only partially achieved.^[28] TheAsian Development Bank (ADB) states that 38% of the irrigated area is now waterlogged and 14% of the surface is too saline for use^[29]
3. In theNile delta ofEgypt, drainage is being installed in millions of hectares to combat the water-logging resulting from the introduction of massive perennial irrigation after the completion of the High Dam atAssuan^[30]
4. In Mexico, 15% of the 3 million ha of irrigable land is salinized, and 10% is waterlogged^[31]
5. InPeru some 0.3 million ha of the 1.05 million ha of irrigable land suffers from degradation (seeIrrigation in Peru).
6. Estimates indicate that roughly one-third of the irrigated land in the major irrigation countries is already badly affected by salinity or is expected to become so in the near future. Present estimates for Israel are 13% of the irrigated land, Australia 20%, China 15%,Iraq 50%,Egypt 30%. Irrigation-induced salinity occurs in large and small irrigation systems alike^[32]
7. FAO has estimated that by 1990 about 52 million ha of irrigated land will need to have improved drainage systems installed, much of itsubsurface drainage to control salinity^[33]

• The downstream drainage water quality may deteriorate owing toleaching of salts,nutrients,herbicides andpesticides with high salinity and alkalinity. There is the threat of soils converting into saline oralkali soils. This may negatively affect the health of the population at the tail-end of the river basin and downstream of the irrigation scheme, as well as the ecological balance. TheAral Sea, for example, is seriouslypolluted by drainage water.
• The downstream quality of the groundwater may deteriorate in a similar way as the downstream drainage water and have similar consequences

Irrigation can have a variety of negative impacts onecology andsocioeconomy, which may be mitigated in a number of ways.These include siting the irrigation project in a location that minimizes negative impacts.^[34] The efficiency of existing projects can be improved and existing degradedcroplands can be improved rather than establishing a new irrigation project^[34]Developing small-scale, individually owned irrigation systems as an alternative to large-scale, publicly owned and managed schemes.^[34] The use ofsprinkler irrigation andmicro-irrigation systems decreases the risk ofwaterlogging anderosion.^[34]Where practicable, using treatedwastewater makes more water available to other users^[34]Maintainingflood flows downstream of the dams can ensure that an adequate area is flooded each year, supporting, amongst other objectives,fishery activities.^[34]

It often takes time to accurately predict the impact that new irrigation schemes will have on theecology andsocioeconomy of a region. By the time these predictions are available, a considerable amount of time and resources may have already been expended in the implementation of thatproject. When that is the case, the project managers will often only change the project if the impact would be considerably more than they had originally expected.^[35]

Frequentlyirrigation schemes are seen as extremely necessary forsocioeconomic well-being especially indeveloping countries.One example of this can be demonstrated from a proposal for anirrigation scheme inMalawi. Here it was shown that the potential positive effects of theirrigation project that was being proposed "outweighed the potential negative impacts". It was stated that the impacts would mostly "be localized, minimal, a short term occurring during the construction and operation phases of the Project". In order to help alleviate and prevent major environmental impacts, they would use techniques that minimize the potential negative impacts. As far as the region's socioeconomic well-being, there would be no "displacement and/or resettlement envisioned during the implementation of the project activities". The original primary purposes of the irrigation project were to reduce poverty, improve food security, create local employment, increase household income and enhance the sustainability of land use.^[36]

Due to this careful planning, this project was successful both in improving the socioeconomic conditions in the region and ensuring that land and water are sustainable into the future.

• Environmental issues with agriculture
• Environmental impacts of reservoirs
• Alkali soils
• Irrigation in viticulture
• Routing (hydrology)
• Indian Council of Forestry Research and Education

• T.C. Dougherty and A.W. Hall, 1995. Environmental impact assessment of irrigation and drainage projects. FAO Irrigation and Drainage Paper 53.ISBN 92-5-103731-0. On line:http://www.fao.org/docrep/v8350e/v8350e00.htm
• R.E. Tillman, 1981. Environmental guidelines for irrigation. New York Botanical Garden Cary Arboretum.
• A comparative survey of dam-induced resettlement in 50 cases by Thayer Scudder and John Gray

• Download of simulation and prediction model SaltMod from:[9]
• Download of simulation and prediction model SahysMod from:[10]
• "SaltMod: A tool for the interweaving of irrigation and drainage for salinity control":[11]
• "Modern interferences with traditional irrigation in Baluchistan":[12]

• Irrigation
• Environmental issues with water
• Environmental issues with soil
• Environmental impact of agriculture

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