Thursday, July 08, 2010

Water irrigation

Share some of the writings from different interesting courses in University over the years. Here from the continuation course Human cultivation of nature in Human Ecology, translated from Swedish-

Ecological and societal problems that may arise
with agricultural irrigation

School of Global Studies, University of Gothenburg
Department of Human Ecology
Anna Rosengren

Fresh water is a scarce resource in many areas around the world which has resulted in modification of waterways to get the water where it can do some good. The reverse is also true, sometimes there is too much water in an area which needs to be removed. Sweden, for example, did this a long time ago, when we drained our wetlands to be fertile soil. In other areas, the scarcity of water results in long channels which lead water to areas where they want to grow crops. Cunningham (2005) tells us that two-thirds of the world's water consumption goes to irrigation. The irrigated farmland has increased from 8 million hectares to 250 million hectares since 1800 (Thomas, 1997). In 2000, 40% of world's food consumption came from the 17% of the arable land which were irrigated (Postel, 2000). Irrigation is important for agriculture, but usually use too much water to be sustainable, when irrigation is often very ineffective; as much as half of the water can be lost in evaporation when using trenches with water around crops (Cunningham, 2005). An example from Postel (2000); 3000 tons of water was used for a while for every tonne of wheat produced in Saudi Arabia, which are three times as much as it usually is calculated for the wheat harvest, this was not sustainable. Countries may instead choose to buy their grain, to save water. Cunningham (2005) means that other ways to reduce water consumption can be with improved irrigation systems. Many farms which have been around for centuries, when passed to modern irrigation has been destroyed by water collections on the field or salinization. McCully (2001) tells that more farms are damaged by salinization than new farms are founded. Salinisation is a direct consequence of irrigation in arid environments where evaporation is high and permeability in soil low (Thomas, 1997). When cities are spreading and old farmland is replaced with urbanized land, agricultural water is then often in conflict with industries and thirsty cities (McCully, 2001).

Ecological problems
The ecological implications are endless when talking irrigation. Irrigation is often to drain the water from a large area to a smaller, and the natural flow of ground disappears. Instead, the water is concentrated in the excavated trenches. This means that the soil becomes dry, light and can blow away; the rain wash away the earth. The consequences from an irrigated field can result in large adjacent areas around the fields, when the water are taken from these areas. If the water is collected in a pond or lake, and affect water outlet obstructed, the course downstream can have negativ effects; it may happen that the river bed is dry most of the year which prevents migratory fish. But of course the problems are more than that; downstream areas may be completely without water, affecting both the surrounding vegetation and also animals which use to come to the river to drink. Dams are particularly bad when they are not allowed to overflow into the surrounding nature, and often have vertical edges. At the straight edges, the vegetation has difficulty becoming established, especially if the water level fluctuated during the year, then it's no chance for vegetation to establish itself in a pond edge. Fluctuations in a controlled lake can be several meter in various parts of the year and affects the vegetation at the water. Vegetation which is an important habitat for the tadpoles, fish babies and insect larva which are particularly important for larger aquatic animals in the water. When the water depth changes, attached eggs on the vegetation can dry out or get too deep under the water.

When tall vegetation is removed and replaced with vegetation with shallow roots, such as arable land or grazing, land becomes more volatile. Drainage of wetlands may have the same effect. McCully (2001) argues that agriculture has caused that the sediments to the water has risen from 7 million tonnes to 45 billion tons. The removal of wetlands is also a loss of the natural purification of water. McCully gives some other examples; approximately 90% of the wetlands of the Rheine in Germany has been drained and used as farmland, half of U.S's wetlands have transformed to agriculture land, and in the Mississippi River area, the figure is as high as 75%. Removal of the wetlands trees along river, means a greater risk of flooding when trees no longer stand as a barrier, both on site and areas downstream (McCully, 2001).

Problems with salt accumulation of arable land means that science tries to breed plants that are more salt tolerant. If the field plants have a higher salt tolerance and is doing better, it means that the surrounding nature can be negatively affected if the saline water spreads over larger areas. The vegetation along the field is disadvantaged and when the water retention properties of the soil deteriorates, it becomes even more difficult to get water to stay on the field.

Reduction of groundwater is a major problem in many areas where the withdrawal is larger than the replenishment. The ecological problem is that the natural springs, wetlands, streams and lakes may be reduced, and it means that large areas would be adversely affected by reduced water (Cunningham, 2005).

Societal problems
Ecological problems lead to societal problems. Sediment in rivers is threatening the water quality. Salinisation on the field and in the groundwater reduces water availability and threaten the harvest.

Cities are growing, which demands more water as well as numerous new industries. One cubic meter of water in China's industries provide more jobs and approximately 70 times more economic value than the same quantity in agriculture (Postel 2000). Postel argues that the redistribution of water between agriculture and cities may have the power to rule the world's ability to feed themselves. Many farmers may in a soon future give up the farm and sell groundwater to the city's industry and consumers. Some factories have purchased or rented rice paddies to get access to the water.

Dams built for irrigation, may stop the water to areas located downstream. Redistribution of water to crops often leads that the downstream areas are affected. It can also lead to conflict between countries, who have the legal right to the water?

Then the dam in Narmada Bargi was built, 81 000 hectare of agricultural and forest land was inundated to provide water for the 440 000 hectare of agricultural land. After seven years in operation the dam gave only water to 12 000 hectares of farmland (McCully, 2001). Dams gives many people hope of improvement and can lead to huge disappointment and dramatic conflicts. Construction dust can also have disastrous consequences if they break. Several such incidents have occurred with flooding and construction. The actual dam can be viewed as a danger to society therefore, when people live in anxiety.

Over-consumption of groundwater, when more groundwater is taken than replenished, can mean devastating societal problems. As water is sucked out of the soil in an area, it creates a depression in the ground. The land that no longer contain as much water falls, sometimes it collapses quickly and can be likened to an earthquake-disaster. Declining groundwater can also provide salt water to finds its way into the void. The result when water is affected by salt is that it is no longer useful.

Towards a better system of irrigation in agriculture
The ecological and societal problems with irrigation is enormous. The ecological system may now receive more attention. In the Murray-Darling basin in Australia, the States agreed to let 25% of the rivers' natural flow to go to maintain the system's ecological health. No increased withdrawals would be made for irrigation. 10% of the water outlet in Central Park Valley project in California is allocate to conserve fish and wildlife habitats (Postel, 2000).

In order to reduce water loss, more efficient technologies can be used. Drip irrigation consists of a network of perforated plastic tubing that is placed at or below ground level which allows the water to go where it should-to the plants roots. Water efficiency is as much as 95% compared with 50-70% of ditches or flooding, and yields can increase by 20-90% (Postel, 2000).

Department of Water Resources in California operated a network of more than one hundred automatic weather stations in key agricultural areas, each station calculates how much water will evaporate. You can then adjust irrigation accordingly.

By moving the responsibility of irrigation systems from the central government to local peasant cooperatives or other private organizations, the hope is to cut subsidies and transfer the responsibility to improve irrigation performance, when those who use the water hopefully will be using systems that are then sustainable in the long run (Postel, 2000).

Everything requires that farmers switch to methods that works. Some of these have been used already a long time, as terrace farming and agroforestry. The terrace farming water is collected on a small field, soil erosion are prevented by embankments. In agroforestry tree roots help to maintain loose soil for other crops. Rice cultivation is also an old example of keeping the resources, the soil of the rice is flooded naturally. In organic farming with supply by plant materials, compost (green manure), gives a better humus in the soil which makes the water stay longer (McCully, 2001). Distillation of salt water is a different view, but at present time there is no viable method because they are all energy consuming. One can imagine that it is possible to develop technology based on energy from solar or wind power, but we are not there right now (McCully, 2001).

Cunningham, Cunningham, Environmental Science- A global concern. The McGraw-Hill Companies, 2005
McCully. P Silenced Rivers: The Ecology and Politics of Large Dams. Zed books, London, 2001
Postel, S., Konstbevattnat jordbruk i omvandling. Tillståndet i världen 2000 (Author of Pillar of sand: can the irrigation miracle last?)
Thomas. D.S.G., Middleton N.J., Salinization: New perspectives on a major desertification issue. In The Human Impact Reader, Wiley-Blackwell (Andrew Goudie, 1997)

1 comment:

  1. I enjoyed reading your article :) PLease continue publishing helpful topics like this. Regards, from CWRenviro.

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