Water from different resources is withdrawn both for use and consumption in diverse human activities. The term use refers to all human activities for which some of the withdrawn water is returned for reuse, e.g., cooking water, wash water, and waste water. In contrast, consumption means that the withdrawn water is non-recoverable.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, transportation of water from plants is released into the atmosphere and is considered non-recoverable.
Human Water Usage
The water content of living organisms ranges from 60% to 95%; humans are about 60% water. To sustain health, humans should drink from 1.5 to 2.5 liters of water/person/day. In addition to drinking water, Americans use about 400 liters water/person/day for cooking, washing, disposing of wastes, and other personal uses. Compare this amount to the 83 other countries that report an average below 100 liters/person/day of water for personal use.
In the US to The World
Currently the U.S. freshwater withdrawals, including that from irrigation, total about 1,600 billion liters/day or about 5,700 liters of water/person/day. Of this amount about 80% comes from surface water and 20% is withdrawn from groundwater resources. Worldwide, the average withdrawal is 1,970 liters/person/day for all purposes. Approximately 70% of the water withdrawn is consumed and is non-recoverable worldwide.
Water in Crop Production
Plants require water for photosynthesis, growth, and reproduction. Water used by plants is non-recoverable, because some water becomes a part of the plant chemically and remainder is released into the atmosphere. The processes of carbon dioxide fixation and temperature control require plants to transpire enormous amounts of water. Various crops transpire water at rates between 600 to 2000 liters of water per kilogram of dry matter of crops produced. The average global transfer of water into the atmosphere from the terrestrial ecosystems by vegetation transpiration is estimated to be about 64% of all precipitation that falls to Earth.
The minimum soil moisture essential for crop growth varies. For instance, U.S. potatoes require 25% to 50%, alfalfa 30% to 50%, and corn 50% to 70%, while rice in China is reported to require at least 80% soil moisture. Rainfall patterns, temperature, vegetative cover, high levels of soil organic matter, active soil bio ta, and water runoff all effect the percolation of rainfall into the soil where it will be used by plants.
The water required by food and forage crops ranges from 600 to 3,000 liters of water per kilogram (dry) of crop yield. For instance, a hectare of U.S. corn, with a yield of approximately 9,000 kg/ha, transpires about 6 million liters per hectare of water during the growing season, while an additional 1 to 2.5 million liters/ha of soil moisture evaporate into the atmosphere. This means that about 800 mm (8 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 U.S. Corn-Belt region, corn frequently suffers from insufficient water during the critical summer growing period.
Hectares’s Required Water
A hectare of high yielding rice requires approximately 11 million liters/ha of water for an average yield of 7 t/ha (metric tons per hectare) (Snyder, 2000). On average, soybeans require about 5.8 million liters/ha of water for a yield of 3 t/ha. In contrast, wheat that produces less plant biomass than either corn or rice, requires only about 2.4 million liters/ha of water for a yield of 2.7 t/ha. Note, under semi-arid conditions, yields of non-irrigated crops, such as corn, are low (1 to 2.5 t/ha) even when ample amounts of fertilizers are applied.
World agriculture consumes approximately 70% of freshwater withdrawn per year. Approximately 17% of the world’s cropland is irrigated but produces 40% of the world’s food. Worldwide, the amount of irrigated land is slowly expanding, even though initialization, water logging, and situation continue to decrease its productivity. Despite a small annual increase in total irrigated areas, the per capital irrigated area has been declining since 1990, due to rapid population growth. Specifically, global irrigation per capital has declined nearly 10% during the past decade, while in the U.S. irrigated land per capital has remained constant at about 0.08 ha.
Irrigated Crop in The US
Irrigated U.S. agricultural production accounts for about 40% of freshwater withdrawn, and more than 80% of the water consumed. California agriculture accounts for 3% of the state’s economic production, but consumes 85% of the water withdrawn.
Energy Use in Irrigation
Irrigation requires a significant expenditure of fossil energy both for pumping and delivering water to crops. Annually in the U.S., we estimate that 15% of the total energy expended for all crop production is used to pump irrigation water. Overall the amount of energy consumed in irrigated crop production is substantially greater than that expended for rained crops. For example, irrigated wheat requires the expenditure of more than 3 times more energy than rainfed wheat. Specifically, about 4.2 million kcal/ha/yr are the required energy input for rained wheat.On the other hand, irrigated wheat requires 14.3 million kcal/ha/yr to apply an average of 5.5 million liters of water.
Delivering the 10 million liters of irrigation water needed by a hectare of irrigated corn from surface water sources. It requires the expenditure of about 880 kWh/ha of fossil fuel. In contrast, irrigation water must be pumped from a depth of 100 m. It leads to the energy cost increases up to 28,500 kWh/ha. In other word, it is more than 32 times the cost of surface water.
Cost of Irrigation
The costs of irrigation for energy and capital are significant. The average cost to develop irrigated land ranges from $3,800/ha to $7,700/ha. Thus, farmers must not only evaluate the dollar cost of developing irrigated land. Moreover, they must also consider the annual costs of irrigation pumping. For example, delivering 7 to 10 million liters/ha of water costs from $750 to $1,000. About 150,000 ha of agricultural land have already been abandoned in the U.S. due to high pumping costs.
The Large Quantity
The large quantities of energy required to pump irrigation water are significant considerations. The causes both from the standpoint of energy and water resource management. For example, approximately 8 million kcal of fossil energy are expended. It is all for machinery, fuel, fertilizers, pesticides, and partial (15%) irrigation. This figure also to produce one hectare of rained U.S. corn. In contrast, if the corn crop were fully irrigated, the total energy inputs would rise to nearly 25 million kcal/ha (2,500 liters of oil equivalents). In the future, this energy dependency will influence the overall economics of irrigated crops. Similarly, they will also be the selection of specific crops worth irrigating. While a low value crop, like alfalfa, may be uneconomical, other crops might use less water plus have a higher market value.
The Efficiency Varies
The efficiency varies with irrigation technologies. The most common irrigation methods, flood irrigation and sprinkler irrigation, frequently waste water. In contrast, the use of more focused application methods. For instance, such as “drip” or “micro-irrigation” have found favor because of their increased water efficiency. Drip irrigation delivers water to individual plants by plastic tubes and uses from 30% to 50% less water than surface irrigation. In addition to conserving water, drip irrigation reduces the problems of initialization and water-logging. Although drip systems achieve up to 95% water efficiency, they are expensive. Moreover, they may be energy intensive, and require clean water to prevent the clogging of the fine delivery tubes.