Environmental impact of irrigation

Environmental impacts of irrigation are the changes in quantity and quality of soil and water as a result of irrigation and the ensuing effects on natural and social conditions at the tail-end and downstream of the irrigation scheme.
The impacts stem from the changed hydrological conditions owing to the installation and operation of the scheme.
An irrigation scheme often draws water from the river and distributes it over the irrigated area. As a hydrological result it is found that:

• the downstream river discharge is reduced
• the evaporation in the scheme is increased
• the groundwater recharge in the scheme is increased
• the level of the water table rises
• the drainage flow is increased

These may be called direct effects.

The effects thereof on soil and water quality are indirect and complex, waterlogging and soil salination are part of these, whereas the subsequent impacts on natural, ecological and socio-enonomic conditions is very intricate.

Irrigation can also be done extracting groundwater by (tube)wells. As a hydrological result it is found that the level of the water descends. The effects may be water mining, land/soil subsidence, and, along the coast, saltwater intrusion.

Irrigation projects can have large benefits, but the negative side effects are often overlooked^[1][2]

Reduced downstream river discharge

The reduced downstream river discharge may cause:

• reduced downstream flooding
• disappearance of ecologically and economically important wetlands or flood forests^[3]
• 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^[3]
• reduced fishing opportunities. The Indus River in Pakistan faces scarcity due to 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 in the world. 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^[3]
• reduced discharge into the sea, which may have various consequences like coastal erosion (e.g. in Ghana^[4]) and salt water 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, in dry periods the river does not reach the sea.^[3] The Aral sea has suffered an "environmental catastrophe" due to the interception of river water for irrigation purposes.

Increased groundwater recharge, waterlogging, soil salinity

The 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 be obtained with sophisticated techniques like sprinkler irrigation and drip irrigation, or by precision land levelling for surface irrigation, in practice the losses are commonly in the order of 40 to 60%. This may cause:

• 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 are 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.^[5] Health costs, appraisals of health impacts and mitigation measures are rarely part of irrigation projects, if at all.^[2]
• 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 built up of those nutrients in the ground water aquifer. High nitrate levels in drinking water can be harmful to humans particularly for infants under 6 months where it is linked to 'blue-baby syndrome' (see Methemoglobinemia).

Case studies:

1. In India 2.189.400 have been reported to suffer from waterlogging in irrigation canal commands. Also 3.469.100 ha were reported to be seriously salt affected here,^[6][7]
2. In the Indus Plains in Pakistan, more than 2 million hectares of land is waterlogged.^[8] The soil of 13.6 million hectares within the Gross Command Area was surveyed, which revealed that 3.1 million hectares (23%) was saline. 23% of this was in Sindh and 13% in the Punjab.^[8] More than 3 million ha of water-logged lands have been provided with tube-wells and drains at the cost of billions of rupees, but the reclamation objectives were only partially achieved.^[9] The Asian Development Bank (ADB) states that 38% of the irrigated area is now waterlogged and 14% of the surface is too saline for use^[10]
3. In the Nile delta of Egypt, drainage is being installed in millions of hectares to combat the water-logging resulting from the introduction of massive perennial irrigation after completion of the High Dam at Assuan^[11]
4. In Mexico, 15% of the 3.000.000 ha if irrigable land is salinized and 10% is waterlogged^[12]
5. In Peru some 300.000 ha of the 1.050.000 ha of irrigable land suffers from this problem (see Irrigation 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%, Irak 50%, Egypt 30%. Irrigation-induced salinity occurs in large and small irrigation systems alike^[13]
7. FAO has estimated that by 1990 about 52 x 10⁶ ha of irrigated land will need to have improved drainage systems installed, much of it subsurface drainage to control salinity^[14]

Reduced downstream drainage and groundwater quality

• The downstream drainage water quality may deteriorate owing to leaching of salts, nutrients, herbicides and pesticides. This may negatively affect the health of the population at the tail-end and downstream of the irrigation scheme, as well as the ecological balance. The Aral sea, for example, is seriously polluted by drainage water.
• The downstream quality of the groundwater may deteriorate in a similar way as the downstream drainage water and have similar consequences.

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 sea shore (see Aswan dam).

Affected downstream water users

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

Pastoralists and nomadic tribes may find their land and water resources blocked by new irrigation developments without having a 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 Rajastan, India^[15]
• After the closure of the Kainji dam, Nigeria, 50 to 70 per cent of the downstream area of flood-recession cropping was lost^[16]

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.^[17]
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.^[18][19]

Groundwater mining with wells, land subsidence

When more groundwater is pumped from wells than replenished, storage of water in the aquifer is being mined. Irrigation from groundwater is no longer sustainable then. The result can be abandoning of irrigated agriculture.
The hundreds of tubewells installed in the state of Uttar Pradesh, India, with World Bank funding have operating periods of 1.4 to 4.7 hours/day, whereas they were designed to operate 16 hours/day^[20]
In Baluchistan, Pakistan, the development of tubewell irrigation projects was at the expense of the traditional qanat or karez users^[15]
Groundwater-related subsidence^[21] of the land due to mining of groundwater occurred in the USA at a rate of 1m for each 13m that the watertable was lowered^[22]
Homes at Greens Bayou near Houston, Texas, where 5 to 7 feet of subsidence has occurred, were flooded during a storm in June 1989 as shown in the picture^[23]

Simulation and prediction

The effects of irrigation on watertable, 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^[24]

See also

• Environmental issues with agriculture
• Environmental impacts of reservoirs
• Irrigation in viticulture

References

Further reading

• 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

External links

• Download of simulation and prediction model SaltMod from: [8]
• Download of simulation and prediction model SahysMod from: [9]
• "SaltMod: A tool for interweaving of irrigation and drainage for salinity control": [10]
• "Modern interferences with traditional irrigation in Baluchistan": [11]