The environmental effects of irrigation relate 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 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 and 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.
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 the provision of moisture to the atmosphere, inducing atmospheric instabilities and increasing downwind rainfall, or in other cases modifies the atmospheric circulation, delivering rain to different downwind areas. 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.
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:
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. In other words, irrigated land produces 2.5 times more product than non-irrigated land.
The reduced downstream river flow may cause:
Increased groundwater recharge stems from the unavoidable deep percolation losses occurring in the irrigation scheme. The lower the irrigation efficiency, the higher the losses. Although fairly 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:
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 natural contribution of sediments can be eliminated by the detention of sediments behind the dams 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 deposits of sedimentation 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 that are created and sustained due to built-up sediment at the basin of the river are a habitat for numerous species of birds. 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  In 1998, uncertain calculations and heavy sediment greatly affected the reservoir’s ability to properly fulfill its flood-control function  This also reduces the downstream river water quality. Shifting more towards mass irrigation installments in order to meet more socioeconomic demands is going against the natural balance of nature, and use water pragmatically- use it where it is found
Downstream water users often have no legal water rights and may fall victim to the development of irrigation.
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.
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.
The Manatali reservoir formed by the Manantali dam in Mali intersects the migration routes of nomadic pastoralists and destroyed 43000 ha of savannah, probably leading to overgrazing and erosion elsewhere. Further, the reservoir destroyed 120 km2 of forest. The depletion of groundwater aquifers, which is caused by the suppression of the seasonal flood cycle, is damaging the forests downstream of the dam.
When more groundwater is pumped from wells than replenished, storage of water in the aquifer is being mined 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 and may consume more energy per unit of water. Eventually, it may become so difficult to extract groundwater that farmers may be forced to abandon irrigated agriculture.
Some notable examples include:
The effects of irrigation on the water table, soil salinity and salinity of drainage and groundwater, and the effects of mitigative measures can be simulated and predicted using agro-hydro-salinity models like SaltMod and SahysMod
Irrigation can have a variety of negative impacts on ecology and socioeconomy, which may be mitigated in a number of ways. These include siting the irrigation project in a location that minimizes negative impacts. The efficiency of existing projects can be improved and existing degraded croplands can be improved rather than establishing a new irrigation project Developing small-scale, individually owned irrigation systems as an alternative to large-scale, publicly owned and managed schemes. The use of sprinkler irrigation and micro-irrigation systems decreases the risk of waterlogging and erosion. Where practicable, using treated wastewater makes more water available to other users Maintaining flood flows downstream of the dams can ensure that an adequate area is flooded each year, supporting, amongst other objectives, fishery activities.
It often takes time to accurately predict the impact that new irrigation schemes will have on the ecology and socioeconomy 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 that project. 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.
Frequently irrigation schemes are seen as extremely necessary for socioeconomic well-being especially in developing countries. One example of this can be demonstrated from a proposal for an irrigation scheme in Malawi. Here it was shown that the potential positive effects of the irrigation 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.
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.
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