6.0 Tutor Marked Assignments
1. Discuss the concept of virtual water.
2. Discuss the concept of water footprints.
7.0 References and other Resources
Hanemann, W.H. (2006). ―The economic conception of water‖, in P.P. Rogers, M.R. Llamas and L. Martinez-Cortina (eds), Water Crisis: Myth or Reality, Taylor and Francis, London, United Kingdom.
Molle, F. and erkoff, J. (2007a), ―Water pricing in irrigation: Mapping the debate in the light of experience‖, Chapter 2 in F. Molle and J. Berkoff (eds), Irrigation water pricing: The gap between theory and practice, CABI, Wallingford, United Kingdom.
Molle, F. and erkoff, J. (2007b), ―Water pricing in irrigation: The lifetime of an idea‖, Chapter 1 in F. Molle and J. Berkoff (eds), Irrigation water pricing: The gap between theory and practice, CABI, Wallingford, United Kingdom.
Organization for Economic Co-Operation and Development [OECD] (2010). Sustainable Management of Water Resources in Agriculture. Pp. 1-118.
Thompson, M. (2006). ―National Water Initiative – The Economics of Water Management in Australia – An Overview‖, pp. 81-93 in OECD, Paris, Water and Agriculture:
Sustainability, Markets and Policies, www.oecd.org/tad/env.
Wichelns, D. (2010), An Economic Analysis of the Virtual Water Concept in Relation to the Agri-food Sector, OECD consultant report available at www.oecd.org/water.
UNIT 3: WATER AND AGRICULTURE
6.0 Tutor Marked Assignments 7.0 References and other Resources 1.0 Introduction
Plants require water for photosynthesis, growth, and reproduction. The water used by plants is non-recoverable, because some water becomes a part of the chemical makeup of the plant and the remainder is released into the atmosphere (Pimentel et al. 2004b). The processes of carbon dioxide fixation and temperature control require plants to transpire enormous amounts of water.
The processes of carbon dioxide fixation and temperature control require plants to transpire enormous amounts of water. Various crops use water at rates between 300 and 2000 L per kilogram (kg) dry matter of crops produced (Pimentel et al. 2004a). The average global transfer of water into the atmosphere by vegetation transpiration from terrestrial ecosystems is estimated to be about 64% of all precipitation that falls to Earth (Pimentel et al. 2004a).
The minimum soil moisture essential for crop growth varies. For instance, US potatoes require soil moisture levels of 25% to 50%; alfalfa, 30% to 50%; and corn, 50% to 70% (Pimentel et al.
2004a). Rice in China is reported to require at least 80% soil moisture (Pimentel et al. 2004a).
Rainfall patterns, temperature, vegetative cover, high levels of soil organic matter, active soil biota, and water runoff all affect the percolation of rainfall into the soil, where it is used by plants.
The water required by food and forage crops ranges from about 300 to 2000 L per kg dry crop yield (Pimentel et al. 2004a). . For instance, in the United States, 1 ha of corn, with a yield of approximately 9000 kg per ha, transpires about 6 million L water per ha during the growing season (Pimentel et al. 2004a), while an additional 1 million to 2.5 million L per ha of soil moisture evaporate into the atmosphere (Pimentel et al. 2004a). This means that the growing season for corn production requires about 800 mm rainfall (8 million L per ha). Even with annual rainfall of 800 to 1000 mm in the US Corn Belt, corn frequently suffers from insufficient water during the critical summer growing period (Pimentel et al. 2004a).
A hectare of high-yielding rice requires approximately 11 million L water per ha for an average yield of 7 metric tons (t) per ha (Pimentel et al. 2004a). On average, soybeans require about 6 million L water per ha for a yield of 3.0 t per ha (Pimentel et al. 2004a). In contrast, wheat, which produces less plant biomass than either corn or rice, requires only about 2.4 million L per ha of water for a yield of 2.7 t per ha. Under semiarid conditions, yields of non-irrigated crops, such as corn, are low (1.0 to 2.5 t per ha) even when ample amounts of fertilizers are applied (Pimentel et al. 2004a).
2.0 Objectives
By the end of this unit, student should be able to understand:
i. energy use in irrigation;
ii. soil salinization and waterlogging in irrigation and iii. water runoff and soil erosion.
3.0 Main Content
3.1 Irrigated Crops and Land Use
World agriculture consumes approximately 70% of the fresh water withdrawn per year (UNESCO 2001). Only about 17% of the world‘s cropland is irrigated, but this irrigated land produces 40% of the world‘s food (FAO 2002).Worldwide, the amount of irrigated land is slowly expanding, even though salinization, waterlogging, and siltation continue to decrease its productivity (Gleick 2002). Despite a small annual increase in total irrigated area, the irrigated area per capita has been declining since 1990 because of rapid population growth (Postel 1999, Gleick 2002).
3.2 Energy Use in Irrigation
Irrigation requires a significant expenditure of fossil energy both for pumping and for delivering water to crops. Overall, the amount of energy consumed in irrigated crop production is substantially greater than that expended for rainfed crops (Pimentel et al. 2004b). For example, irrigated wheat requires the expenditure of more than three times the energy needed to produce rainfed wheat. Rainfed wheat requires an energy input of only about 4.2 million kilocalories (kcal) per ha per year, while irrigated wheat requires 14.3 million kcal per ha per year to supply an average of 5.5 million L water (Pimentel et al. 2004a). Delivering 10 million L water from surface water sources to irrigate 1 ha of corn requires the expenditure of about 880 kilowatt-hours (kWh) of fossil fuel per ha. In contrast, when irrigation water must be pumpedfrom a depth of 100 m, the energy cost increases to 28,500 kWh per ha, or more than 32 times the cost of surface water (Gleick 1993).
The costs of irrigation for energy and capital are significant. The average cost to develop irrigated land ranges from $3800 to $7700 per ha (Postel 1999). Thus, farmers must not only evaluate the costs of developing irrigated land but also consider the annual costs of irrigation pumping. For example, delivering 7 million to 10 million L water per ha costs $750 to $1000 (Pimentel et al. 2004a). About 150,000 ha of agricultural land in the United States have already been abandoned because of high pumping costs (Pimentel et al. 2004a).
3.3 Soil Salinization and Waterlogging in Irrigation
With rainfed crops, salinization is not a problem because the salts are naturally flushed away.
But when irrigation water is applied to crops and returns to the atmosphere through plant transpiration and evaporation, dissolved salts concentrate in the soil, where they inhibit plant growth. The practice of applying about 10 million L irrigation water per ha each year results in approximately 5 t salts per ha being added to the soil (Bouwer 2002). The salt deposits can be flushed away with added fresh water, but at a significant cost (Bouwer 2002). Worldwide, approximately half of all existing irrigated soils are adversely affected by salinization (Hinrichsen et al. 1998). The amount of world agricultural land destroyed by salinized soil each
year is estimated to be 10 million ha (Pimentel et al. 2004a). In addition, drainage water from irrigated cropland contains large quantities of salt.
Waterlogging is another problem associated with irrigation. Over time, seepage from irrigation canals and irrigated fields causes water to accumulate in the upper soil levels (Pimentel et al.
2004b). Because of water losses during pumping and transport, approximately 60% of the water intended for crop irrigation never reaches the crop (Wallace 2000). In the absence of adequate drainage, water tables rise in the upper soil levels, including the plant root zone, and crop growth is impaired. Such irrigated fields are sometimes referred to as ―wet deserts‖ because they are rendered unproductive (Pimentel et al. 2004a). For example, in India, waterlogging adversely affects 8.5 million ha of cropland and results in the loss of as much as 2 million tons grain every year (Pimentel et al. 2004a). To prevent both salinization and waterlogging, sufficient water and adequate soil drainage must be available to ensure that salts and excess water are drained from the soil.
3.4 Water Runoff and Soil Erosion
ecause more than 99% of the world‘s food comes from the land, an adequate global food supply depends on the continued availability of productive soils (FAO 1998). Erosion adversely affects crop productivity by reducing the availability of water; by diminishing soil nutrients, soil biota, and soil organic matter; and by decreasing soil depth (Pimentel et al. 2004a). The reduction in the amount of water available to growing plants is considered the most harmful effect of erosion, because eroded soil absorbs 87% less water through infiltration than uneroded soil (Pimentel et al. 2004a). Soybeans and oats intercept approximately 10% of the rainfall in areas where they are planted, whereas tree canopies intercept 15% to 35% (Pimentel et al.
2004a). Thus, the removal of trees increases water runoff and reduces water availability. Given a total rainfall of 800 mm per year, a water runoff rate of about 30% causes significant water shortages for growing crops such as corn, ultimately lowering crop yields (Pimentel et al.
2004a).
4.0 Summary
In this unit we have leant that:
i. The reduction in the amount of water available to growing plants is considered the most harmful effect of erosion.
ii. Irrigation requires a significant expenditure of fossil energy both for pumping and for delivering water to crops.
iii. The processes of carbon dioxide fixation and temperature control require plants to transpire enormous amounts of water.
5.0 Conclusion
Rainfall patterns, temperature, vegetative cover, high levels of soil organic matter, active soil biota, and water runoff all affect the percolation of rainfall into the soil, where it is used by
plants. With rainfed crops, salinization is not a problem because the salts are naturally flushed away.
6.0 Tutor Marked Assignments
1. Explain energy use in irrigation.
2. Why is waterlogging a problem that is associated with irrigation?
7.0 References and other Resources
Bouwer, H. (2002). Integrated water management for the 21st century: Problems and solutions.
Journal of Irrigation and Drainage Engineering, 128: 193–202.
Food and Agriculture Organization (2002). Crops and Drops: Making the Best Use of Water for Agriculture. Food and Agriculture Organization, Rome, Italy. Rome.
Food and Agriculture Organization (1998). Food Balance Sheets. Food and Agriculture Organization, Rome, Italy.
Gleick, P.H., (1993). Water in Crisis: A Guide to the World‘s Fresh Water Resources. New York: Oxford University Press.
Gleick, P.H, Wolff, E.L, Chalecki, R.R. (2002). The New Economy of Water: The Risks and Benefits of Globalization and Privatization of Freshwater. Oakland (CA): Pacific Institute for Studies in Development, Environment, and Security.
Hinrichsen, D., Robey, B., Upadhyay, U.D. (1998). Solutions for a Water-short World.
Baltimore: Johns Hopkins School of Public Health, Population Information Program.
Population Reports, series M, no. 14.
Pimentel, D., Berger, B., Filiberto, D., Newton, M., Wolfe, B., Karabinakis, E., Clark, S., Poon, E., Abbett, E., Nandaopal, S. (2004a). Water Resources, Agriculture, and the Environment. Ithaca (NY): New York State College of Agriculture and Life Sciences, Cornell University. Environmental Biology Report 04-1.
Pimentel, D., Berger, B., Filiberto, D., Newton, M., Wolfe, B., Karabinakis, E., Clark, S., Poon, E., Abbett, E., Nandaopal, S. (2004b). Water Resources, Agriculture, and the Environment. Ithaca (NY): New York State College of Agriculture and Life Sciences, Cornell University. Water Resources: Agricultural and Environmental Issues. American Institute of Biological Sciences, Vol. 54, No. 10
Postel, S. (1999). Pillar of Sand: Can the Irrigation Miracle Last? New York: W. W. Norton.
Wallace, J.S. (2000). Increasing agricultural water use efficiency to meet future food production.
Agriculture, Ecosystems and Environment, 82: 105–119.
[UNESCO] United Nations Educational, Scientific and Cultural Organization (2001). Securing the Food Supply. Paris: UNESCO.
UNIT 4: WATER, CLIMATE CHANGE AND CONFLICTS OVER WATER USE