Agriculture problems - Cotton
The impact of cotton on freshwater resources and ecosystems
The pesticides have a direct toxic effect upon wildlife and, by accumulating in the biosphere, also an indirect effect. Evidence shows that this indirect effect, leading to a decrease in animal fertility, affects long-term freshwater biodiversity (Woodward, 1993).
Investigations into a case of fish-death in the USA showed that, even when pesticides are properly applied according to the technical instructions, impacts on freshwater ecosystems are still possible. In this case Endosulfan was sprayed on cotton fields. In August 1995 contaminated run-off from these fields resulted in the death of more than 240,000 fish along a 25 km stretch of a river in the State of Alabama (PANUPS 1996).
So, run-off can also lead to contamination by fertiliser of rivers, lakes and wetlands. In contrast with pesticides, fertilisers are not directly toxic but instead alter the nutrient system and in consequence the species composition of a specific freshwater ecosystem. Their most dramatic effect is eutrophication of a freshwater body - an explosive growth of algae which causes disruption to the biological equilibrium, including killing fish.
Table 2: Major impacts of cotton on freshwater ecosystems and freshwater biodiversity
|Run off from fields||Fertiliser
|Eutrophication and pollution
|Drainage||Saline drainage water
Pesticide or fertiliser contaminated drainage water
|Salinisation of freshwater
Pollution of freshwater
|China, Egypt, Uzbekistan|
|Application of pesticides||Insecticides, fungicides, herbicides and defoliants
Spray drift (e.g. aerial application)
Leakage of equipment
Contamination of adjacent wetlands, surface and ground water
Contamination of surface and ground water
|Water withdrawal for irrigation||Use of ground water
Use of surface water
|Change of water table or depletion of ground water
Degradation of wetlands and lakes
|New South Wales, Australia
Aral Sea, Yellow River Valley
|Extensive irrigation||Water logging||Raising water tables and salinisation of soil surface||Australia, Indus River Valley, Uzbekistan, Pakistan|
|Dam construction for irrigation||Regulated water flow||Habitat destruction, change of water table and change of water flow|
|Land reclamation||Change of vegetation||Habitat destruction|
Drainage and leaching
To avoid water logging and salinity of soils (see below), drainage systems are used. In some countries, in addition to the water used in irrigation for cotton production, the fields are irrigated with extra freshwater to establish a downward water flow, which removes the salt from the soil. China, Egypt and Uzbekistan specifically mention extra water requirements for leaching, the last two quantitatively (Gillham, 1995). However, by returning to the rivers, the salt-contaminated drainage water has a severe impact on rivers and wetlands (Gillham, 1995). It can be assumed that drainage water contains not only salt but also pesticide residues and fertiliser, which enter rivers and lakes directly.
Application of pesticides
Even when pesticides are applied properly, affecting non-target organism can not be prevented. Beneficial insects in and around the cotton fields can be killed and other animals eating these insects can be injured or even poisoned. In one case, although 20 years ago, a breeding colony of laughing gulls near Corpus Christi, Texas, was poisoned by a parathion application designed to kill bollworms on a cotton field about three miles away. The gulls were killed by ingesting poisoned insects from the cotton field (White, 1979; White, 1983).
In addition to run-off contaminated with pesticide, by certain application methods, surface water and even ground water can be directly polluted with pesticides and fertilisers. Pesticide application by aeroplane, for example, can lead to spray drift, i.e. pesticides do not hit the targeted field but adjacent fields, rivers or wetlands instead, this leads to direct poisoning of freshwater species.
Ground water, on the other hand, is impacted by deep percolation, which can also be contaminated by pesticides and fertilisers, by faulty equipment (e.g. leakage) or improper handling of equipment (e.g. cleaning of equipment in surface water).
The depletion of the Aral Sea is the most drastic consequence of extensive freshwater withdrawal for irrigation purposes (Gillham, 1995). The two rivers Amu-Darja and Syr-Darja were over-used for producing cotton and other corps and vegetables. The surface level of the Aral Sea decreased, leading to the extinction of a range of fish species, i.e. 20 of 24 native fish species disappeared (Krever et al., 1998 (?)). Furthermore, wide areas of acreage and former lake area suffer from surface salination.
In China's Yellow River Valley, where cotton is grown under irrigated and rain-fed conditions, a shortage of irrigation water due to falling water tables was also reported (Gillham, 1995).
Investigations conducted in Australia concluded that irrigated cotton cropping can lead to increased run-off into ground water (deep percolation). The consequence of this are rising ground water tables and eventually the establishment of shallow water tables (Willis, 1996). This does not only decrease agricultural productivity but also leads, in dry climates, to the salination of soils (Zilberman, 1998).
In regions where evapotranspiration exceeds, both rain-fall and the amount of freshwater used for irrigation, a salinisation of soil is inevitable. This is especially true of all countries in the broad belt of irrigated cotton, which have desert and Mediterranean climates. In Uzbekistan, for example, 50% of the irrigated area is affected by salinity and in Pakistan 15% is affected. Brazil, with its small area of irrigated crop, also reports problems with salinity (Gillham, 1995).
Dam construction for irrigation
In addition to habitats and ecosystems which are directly destroyed by dammed water, the reduced and regulated water flow also affects freshwater ecosystems which lay downstream of the dam. Freshwater ecosystems are adapted to a certain water flow and any alteration in water amount or its temporal distribution can affect either single species or whole freshwater ecosystems.
In only 30 years, from the 1950s to the late 1980s more than 35,000 large dams were built world-wide (WWF, 1999). Whereas the bigger dams are used mainly for hydropower, the smaller dams were primarily built for irrigation purposes. Most dams however can be used for irrigation, hydropower, flood control and other purposes (ICOLD, 1998). Because of the lack of appropriate data, the significance of cotton production on dam building can not be estimated at this point of the study.
The increase in arable land leads directly to a change from natural landscape to agricultural area. In particular, flood plains and wetlands with their flat shape and usually fertile soil are preferable areas for agriculture and irrigation schemes. However, due to drainage of the soil and to the monocultural cultivation of cotton, the farmland no longer provides a habitat for its original plants and animals. Besides, the remaining natural habitats are fragmented into isolated pieces which are too small to secure the continued existence of the natural ecosystem. Even though this initially concerns terrestrial ecosystems and wetlands, freshwater ecosystems in rivers or lakes are affected by the interrupted links between ecosystems.
Even though the area of cotton cultivation has remained constant since 1930, there has been a need for land reclamation due to a gradual change from over-used farmland to newly cultivated areas. One reason for this change is the salinisation of soil taking place through inappropriate irrigation and water logging. Because of this change, the area affected by cotton planting over the last 50 years is much larger than the recently cultivated area. In the top 6 cotton producing countries, between 12% and 36% of the irrigated area is damaged through salinisation (Dinar, 1998).
Consequences of the use of pesticides and fertilisers can affect irrigated cotton as well as rain-fed cotton. There are indications however, that less pesticides and fertilisers are used on rain-fed cotton. On the other hand, irrigated cotton has a higher yield than rain-fed cotton. A concluding comparison between irrigated and rain-fed cotton is not yet possible because more data is necessary.
In areas like river deltas where agricultural, industrial and urban areas are often mixed, the impact on freshwater ecosystems can not be allocated to any one of the three sectors in particular.
Impacts on freshwater ecosystems can also be caused indirectly by human use of other resources than freshwater, such as land, vegetation and air. For example, reduction of vegetation cover, increased soil compaction and surface sealing reduce infiltration and increase run-off and soil erosion, thus altering the water balance of a catchment.
Freshwater ecosystems are linked to land ecosystems. Therefore, a severe impact on one can influence the other. The poisoning of birds and insects in a land ecosystem through pesticides for example can alter the food chain and thus impact the neighbouring freshwater system.
Loss and fragmentation of habitats is caused not only by the agricultural area itself but also the surrounding infrastructures. This also applies to roads, buildings and a population migration into newly developed areas as well as dams and their effects. Besides the resultant direct loss of land, further impacts like nutrient turn-over and siltation will be accentuated.