By David Pimentel
Water is essential for maintaining an adequate food supply and a productive environment for the human population, plus the natural plants, animals and microbes on Earth. Humans obtain all their nutrients from crops and livestock and these nutrient sources require water, land and energy for production.
A major problem is that nearly 60 percent of the world’s 6.6 billion inhabitants is seriously malnourished. In addition, freshwater demand worldwide is increasing rapidly as population and economies grow. Population growth, accompanied by increased water use, will severely reduce water availability per person and will stress all biodiversity throughout the global ecosystem.
In this article, water utilization by individuals and especially the agricultural systems is analyzed. Interrelationships exist among population growth, water use and distribution, the status of biodiversity and the natural environment. Additionally, the impacts of water borne human diseases are reported.
(Author’s note: Water from different resources is withdrawn both for use and consumption in diverse human activities. The term use refers to all human activities for which some of the withdrawn water is returned for reuse, e.g., cooking water, wash water and waste water.
In contrast, consumption means that the withdrawn water is non-recoverable. For example, evapotranspiration of water from plants is released into the atmosphere and is considered non-recoverable, though it will fall as rain again elsewhere.)
Water resources availability
Although water is considered a renewable resource because it is replenished by rainfall, its availability is finite in terms of the amount available per unit of time in any one region. The average precipitation for most continents is about 700 mm/yr (seven million liters/ha/yr) or 28 inches per year (760,300 gallons per acre per yr), but varies among and within them. In general, a nation is considered water scarce when the availability of water drops below one million liters/capita/yr. Thus Africa, despite having an average of 640 mm/yr of rainfall, is relatively arid since its high temperatures and winds foster rapid evaporation.
Substantial withdrawals from lakes, rivers, groundwater and reservoirs used to meet the needs of individuals, cities, farms and industries already stress the availability of water in some parts of the US. When managing water resources, the total agricultural, societal and environmental system must be considered.
Approximately 30 percent (11 x 1015 m 3) of all freshwater on Earth is stored as groundwater. The amount held as groundwater is more than 100 times the amount collected in rivers and lakes. Most groundwater has accumulated over millions of years in vast aquifers located below the surface of the planet. Aquifers are replenished slowly by rainfall, with an average recharge rate that ranges from 0.1 percent to three percent per year. 
Based on an average one percent recharge rate, only 110 x 1012 m3 of water per year is available for sustainable use worldwide. At present, world groundwater aquifers provide approximately 23 percent of all water used throughout the world. Irrigation for US agriculture relies heavily upon groundwater, with 65 percent being pumped from aquifers.
The capacity of the US Ogallala aquifer (which underlies parts of Nebraska, South Dakota, Colorado, Kansas, Oklahoma, New Mexico and Texas), has decreased 33 percent since about 1950; withdrawal is three times faster than its recharge rate. Other aquifers are being withdrawn more than 10-times faster than the recharge rate for aquifers in parts of Arizona.
Stored water and use
Many US dams were built during the early 20th century in arid regions in an effort to increase the available quantities of water. Although the demand for large construction dams has slowed down in America, it continues in many developing countries worldwide.
Current US freshwater withdrawals, including that from irrigation, total about 1,600 billion liters/day or about 5,700 liters of water/person/day. About 80 percent comes from surface water and 20 percent is withdrawn from groundwater resources.
Worldwide, the average withdrawal is 1,970 liters/person/day for all purposes. Approximately 70 percent of the water withdrawn is consumed and is non-recoverable.
Agriculture and water crop production
Water used by plants is non-recoverable, because some water becomes a part of the plant chemically and the remainder is released into the atmosphere. Carbon dioxide fixation and temperature control require plants to transpire enormous amounts of water. Various crops transpire water at rates between 600 to 2,000 liters per kilogram of dry matter of crops produced (see Table 1). The average global transfer of water into the atmosphere from the terrestrial ecosystems by vegetation transpiration is estimated to be about 64 percent of all precipitation that falls to Earth.
The water required by food and forage crops ranges from 500 to 3,000 liters per kilogram (dry) of crop yield. About 800 mm (eight million liters/ha) of rainfall are required during the growing season for corn production. (Even with 800 to 1,000 mm of annual rainfall in the US corn belt region, corn frequently suffers from insufficient water during the critical summer growing period.)
A hectare of high-yielding rice requires approximately 11 million liters/ha22 of water for an average yield, soybeans require about 5.8 million liters/ha.23 Wheat produces less plant biomass than either corn or rice and requires only about 2.4 million liters/ha.24 Note that under semi-arid conditions, yields of non-irrigated crops, such as corn, are low even when ample amounts of fertilizer are applied.25
Irrigated crops and land use
World agriculture consumes approximately 70 percent of freshwater withdrawn per year.26 Approximately 17 percent of the world’s cropland is irrigated but produces 40 percent of the world’s food.27 Globally, the amount of irrigated land is slowly expanding, even though salinization, water logging and siltation continue to decrease its productivity.28
Global irrigation per capita has declined nearly 10 percent during the past decade,29 while in the US, irrigated land per capita has remained constant at about 0.08 ha.30 US irrigated agricultural production accounts for about 40 percent of freshwater withdrawn31 and more than 80 percent of the water consumed32. California agriculture accounts for three percent of the state’s economic production, but consumes 85 percent of the water withdrawn. 33
Energy use in irrigation
Irrigation requires a significant expenditure of fossil energy both for pumping and water delivery. An estimated 15 percent of the total energy expended for all annual crop production in the US is used to pump irrigation water.34 The overall amount of energy consumed is substantially greater than that expended for rainfed crops.
Irrigated wheat requires more than three times the energy expenditure than rainfed wheat. Large quantities of energy required to pump irrigation water are significant considerations both from the standpoint of energy and water resource management.
If corn crops were fully irrigated, total energy inputs would rise to nearly 25 million kcal/ha (2,500 liters of oil equivalents).35 Future energy dependency will influence both the overall economics of irrigated crops and the selection of specific crops worth irrigating36 (see Table 1). While a low value crop, like alfalfa, may be uneconomical, other crops might use less water plus have a higher market value.
The most common irrigation methods (flood and sprinkler irrigation) frequently waste water. Use of more focused application methods, such as drip or micro-irrigation, have found favor because of their increased water efficiency. Drip irrigation uses from 30 to 50 percent less water than surface irrigation and reduces the problems of salinization and waterlogging.37 Although more efficient, they are expensive, may be energy intensive and require clean water to prevent the clogging of the fine plastic delivery tubes.38
Soil salinization and waterlogging
Salinization is not a problem because with rainfed crops the salts are naturally flushed away. When irrigation water is applied to crops and returns to the atmosphere via plant transpiration and evaporation, dissolved salts concentrate in the soil, where they inhibit plant growth. Applying about 10 million liters of irrigation water per hectare each year results in approximately five t/ha of salts being added to the soil; these salt deposits can be flushed away with added fresh water, but at a significant cost.39 Worldwide, almost half of all existing irrigated soils are adversely affected by salinization.40 Each year, the amount of world agricultural land destroyed by salinized soil is estimated to be 10 million hectares.41
Runoff and erosion
More than 99 percent of world food supply comes from the land.42 Erosion adversely affects productivity by reducing availability of water, diminishing soil nutrients, soil biota and soil organic matter and also decreasing soil depth.43 Reduction of the amount of water available to growing plants is considered the most harmful effect of erosion, as eroded soil absorbs 87 percent less water by infiltration than uneroded soils.44 Soybean and oat plantings intercept approximately 10 percent of the rainfall, and tree canopies intercept 15 to 35 percent.45 Thus, the removal of trees increases water runoff and reduces water availability.
A runoff rate of about 30 percent of total rainfall (800 mm/yr) causes significant water shortages for growing crops and ultimately lowers crop yields.46 In addition, water runoff (which carries sediments, nutrients and pesticides from agricultural fields into surface and ground waters) is the leading cause of non-point source pollution in the US.47 As erosion removes topsoil and organic matter, runoff is intensified and crop yields decrease. The cycle is repeated again with even greater intensity during subsequent rains.
Reducing runoff is an important step toward increasing water availability to crops, conserving resources, decreasing non-point source pollution – ultimately, decreasing water shortages.48 If soil and water conservation measures are not implemented, the loss of water for crops via soil erosion can amount to as much as five million liters per hectare per year.49
Water use in livestock production
The production of animal protein requires significantly more water than the production of plant protein.50 Although US livestock directly use only two percent of the total agricultural water used,51 water inputs for livestock production are substantial because water is required for the forage and grain crops. Worldwide, grain production specifically for livestock requires nearly three times the amount of grain that is fed to US livestock (and three times the amount of water used as well).52
Animal products vary in the amounts of water required for their production. Producing one kg of chicken requires 3,500 liters while producing one kg of sheep requires approximately 51,000 liters.53 One kg of beef requires about 43,000 liters of under feedlot conditions.54 For open rangeland beef, from 120 kg to 200 kg of forage are required, necessitating 120,000 liters to 200,000 liters.55
Water pollution and human diseases
At present, approximately 20 percent of the world’s population lack safe drinking water and nearly half the world population lack adequate sanitation.56 Overall, waterborne infections account for 90 percent of all human infectious diseases in developing countries.57 Lack of sanitary conditions contributes to approximately 12 million deaths each year, primarily among infants and young children.58
Approximately 40 percent of US fresh water is deemed unfit for recreational or drinking water uses because of contamination with dangerous microorganisms, pesticides and fertilizers.59 Waterborne infections account for approximately 940,000 infections and 900 deaths each year.60 In recent decades, more livestock production systems have moved closer to urban areas, causing water and foods to be contaminated with manure.61
In the US, an estimated 1.5 billion tons of livestock manure and other wastes are produced each year.62 The CDC reports that more than 76 million Americans are infected each year with pathogenic E. coli and related foodborne pathogens, resulting in about 5,000 deaths per year.63
The incidence of schistosomiasis (which is also associated with contaminated freshwater) is expanding worldwide and each year infects more than 200 million people.64 It currently causes an estimated 20,000 deaths per year.65 Its spread is associated with an increase in habitats, suitable for the snail intermediate-host population and human accessibility to come the infected water.66
Mosquito-borne malaria is also associated with water bodies. Worldwide this disease presently infects more than 2.4 billion people67 and kills about 2.7 million each year.68 Environmental changes, including polluted water, have fostered this high incidence and increase in malaria.
In addition, more people are living in close proximity to mosquito-infested aquatic ecosystems. Concurrently, the mosquito vectors are evolving resistance to insecticides that pollute their aquatic ecosystems, while protozoan pathogens are evolving resistance to the over-used anti-malarial drugs. Together these factors are reducing the effectiveness of many malaria control efforts.69
Tuberculosis (TB) is another serious water-borne infectious disease that can be transmitted via air, water and food. Approximately two billion people are currently infected with TB, with the number increasing each year.70 Globally, about two billion people are infected with one or more helminth species, either by direct penetration or by use of contaminated water or food.71
In addition to helminthes and microbe pathogens, many chemicals contaminate water and have negative impacts on human health as well as natural biota. For example, an estimated three billion kg of pesticides are applied worldwide each year in agriculture.72 US EPA also allowed the application of sludge to agricultural land and this sludge is contaminated with heavy metals and other toxics.73
Many of these agricultural chemicals, including nitrogen fertilizer, contaminate aquatic ecosystems by leaching and runoff and result in eutrophication of aquatic ecosystems and other environmental problems.74 Worldwide, pesticides alone contribute to an estimated 26 million human poisonings and 220,000 deaths each year.75
Limits to water use
Increases in pollution of surface and groundwater resources pose a threat to public and environmental health and also contribute to the high costs of water treatment, thus further limiting the availability of water for use. Depending on water quality and the purification treatments used, potable water costs an average of $0.50 (USD)/1,000 liters in the US and ranges up to $1.91 (USD)/1000 liters in Germany.76 Appropriate water pricing is important for improved water demand and conservation of water.77
The cost of treating US sewage for release into streams and lakes ranges from $0.55 (USD)/1000 liters for small plants to $0.30 (USD)/1000 liters for large plants.78 When properly treated to make it safe for use as potable water, sewage effluent is relatively expensive and ranges in costs from $1.00 to $2.65 (USD)/1000 liters.79
Purifying and reducing the number of polluting microbes in water, as measured by biological oxygen demand (BOD), is energy costly. Removing one kg of BOD requires one kWh.80 In this process, most of the cost for pumping and delivering water is for energy and equipment. Delivering one m3 (1,000 liters) of water in the US requires the expenditure of about 1.3 kWh.
Excluding only the energy for pumping sewage, the cost and amount of energy required to process 1,000 liters of sewage in a technologically advanced wastewater treatment plant is about $0.65 (USD) and requires about 0.44 kWh of energy.81 Looking to the future, the costs of water treatment and the energy required to purify water will increase.
Dependence on the oceans for freshwater has major problems. When brackish water is desalinized, the energy costs are high, ranging from $0.25 to $0.60 (USD)/1,000 liters, while seawater desalinization ranges from $0.75 to $3.00 (USD)/1,000 liters.82 In addition, transporting large volumes of desalinized water adds to the costs.
Cost of water subsidies
The relatively high cost of treating and delivering water has led many world governments to subsidize the cost of water for agriculture and household use. For example, some US farmers pay as little as $0.01 to $0.05 (USD)/1,000 liters they use in irrigation, while the public pays from $0.30 to $0.80 (USD) per 1,000 liters of treated water for personal use.83
Farmers in the Imperial Irrigation District of California pay $15.50 (USD) in delivery fees for 1.2 million liters of water.84 Some investigators suggest that if US farmers paid the full cost of water, they would have to conserve and manage irrigation water more effectively.85
The construction cost subsidy for federally subsidized western US irrigated cropland amounts to about $5,000 (USD) per hectare,86 representing an annual construction cost subsidy of about $440 (USD) per ha/yr over the life of the project.87 The total annual government subsidy is estimated to range from $2.5 billion to $4.4 billion (USD) for the 4.5 million hectares of irrigated land in the western US.88
Worldwide, from 1994 to 1998, governmental water subsidies totaled $45 billion (USD) per year for non-Organization for Economic Cooperation and Development (OECD) countries and $15 billion (USD) for OECD counties. During the same period, agricultural subsidies per year total $65 billion (USD) for non-OECD and $355 billion (USD) for OECD countries.89
According to the World Bank in 2003, the objectives of fair water pricing are to seek revenue to pay for the operations and maintenance of water availability, to improve water-use efficiency and to recover the full costs of pumping and treatment. In general, however, there appear to be problems with some private, for profit companies operating water systems for communities and regions, which often operate as monopolies, leading to pricing problems.90
If US prices of gasoline and diesel energy increase significantly, it follows that irrigation costs will also escalate from the current nearly $4 billion (USD) per year.91 Since vegetable and fruit crops return more per dollar invested in irrigation water than field crops, farmers may have to reassess the crops they grow.
Loss of biodiversity
World biodiversity is adversely affected when water resources are reduced and/or polluted. The drastic drainage of more than half of US wetlands92 that contain 45 percent of our federally endangered and threatened species has seriously disrupted these ecosystems.93 In 2002, approximately 33,000 salmon perished in the Klamath River when farmers were allowed to withdraw increased volumes of water for irrigation.94
Pear farmers in the Rogue Valley of Oregon use significant amounts of the water before it reaches Klamath Lake, leaving only 616 million m3 of water per year for wildlife and other farmers downstream.95 Similarly, overpumping and upstream removal of water have reduced biodiversity in the Colorado River and the Rio Grande River.96 The major alteration of the natural water flow in the lower portion of the US Colorado River has been responsible for 45 species of plants and animals to be listed as federally endangered or threatened.97
Conserving water resources
Estimates of water resources and their future availability can only be based on present world climate patterns. Conserving world water must be a priority of individuals, communities and countries. An important approach is to find ways to facilitate the percolation of rainfall into the soil instead of allowing it to run off into streams and rivers. For example, the increased use of trees and shrubs make it possible to catch and slow water runoff by 10 to 20 percent, thereby conserving water before it reaches streams, rivers and lakes.98 This approach also reduces flooding.
Maintaining crop, livestock and forest production requires conserving all water resources available, including rainfall.99 Some practical strategies that support water conservation for crop production include:
- monitoring soil water content;
- adjusting water application needs to specific crops;
- applying organic mulches to prevent water loss and improve water percolation, through reduced water runoff and evaporation;
- using crop rotations and cover crops that reduce water runoff;
- preventing the removal of biomass from land;
- increasing use of trees and shrubs to slow water runoff, and
- employing precision irrigation in water delivery systems, such as drip irrigation, that will result in efficient crop watering 100
In forest areas, it will be necessary to avoid clear cutting and to employ sound forest management. Trees also benefit urban areas with high runoff rates. Since water runoff is rapid from roofs, driveways, roads and parking lots, water can be collected in cisterns and constructed ponds. Estimated runoff rates from urban area were 72 percent higher than areas with forest cover.101
Given that many aquifers are being over drafted, government efforts are needed to limit pumping to sustainable withdrawal levels or to the known recharge rate. Integrated water resource management programs offer many opportunities to conserve water resources for everyone, farmers and the public.102
Using water wisely in the future
Providing adequate quantities of pure freshwater for humans and their diverse activities is a major problem worldwide. If competition for water resources within regions and between countries continues to escalate and remain unresolved, this, too, will have negative impacts on essential freshwater supplies for personal and agricultural use.
Even now, freshwater resources for food production and other human needs are declining because of increasing demand103 and becoming outright scarce in arid regions. In these dry areas, where groundwater resources are the primary sources of water, future irrigation, industrial and urban water use must be carefully managed to prevent exhausting the aquifers. Priorities for using water wisely are as follows:
- Because agriculture consumes 70 percent of world’s freshwater, farmers should be the prime target for conservation incentives.
- Implement water-conserving irrigation practices, like drip irrigation, to reduce water waste.
- Implement water/soil conservation practices, like cover crops and crop rotation, to minimize rapid water runoff related to soil erosion.
- Reduce and/or eliminate water subsidies that encourage the wasteful use of water by farmers and others.
- Implement World Bank policies for the fair pricing of freshwater.
- Protect forests, wetlands and natural ecosystems to enhance the conservation of water.
- Control water pollution to protect public health, agriculture and the environment.
About the author
David Pimentel, Ph.D., is a professor in the College of Agriculture and Life Sciences at Cornell University, Ithaca, NY, where he earned his Ph.D. His research spans the fields of energy, ecological and economic aspects of pest control, biological control, biotechnology, sustainable agriculture, land and water conservation, and environmental policy. Pimentel has published more than 600 scientific papers and 28 books and has served on many national and government committees including the National Academy of Sciences; President’s Science Advisory Council; US Department of Agriculture; US Department of Energy; US Department of Health, Education and Welfare; Office of Technology Assessment of the US Congress and the US State Department. He can be contacted at 5126 Comstock Hall, Cornell University, Ithaca, NY 14853, or by email at email@example.com.
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