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 via plant transpiration and evaporation, dissolved salts concentrate in the soil where they inhibit plant growth. The practice of applying about 10 million liters of irrigation water per hectare each year, results in approximately 5 t/ha of salts being added to the soil. The salt deposits can be flushed away with added fresh water but at a significant cost. Worldwide, approximately half of all existing irrigated soils are adversely affected by salinization. Each year the amount of world agricultural land destroyed by salinized soil is estimated to be 10 million hectares.
In addition, drainage water from irrigated cropland contains large quantities of salt. For instance, as the Colorado River flows through Grand Valley, Colorado, it picks up 580,000 tons of salts per year. Based on the drainage area of 20,000 ha, the water returned to the Colorado River contains an estimated 30 t/ha of salts per year. In Arizona, the Salt River and Colorado River deliver a total of 1.6 million tons of salt into south-central Arizona each year.
Waterlogging is another problem associated with irrigation. Over time, seepage from irrigation canals and irrigated fields cause water to accumulate in the upper soil levels. Due to 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. For example in India, waterlogging adversely affects 8.5 million hectares of cropland and results in the loss of as much as 2 million tons of grain every year. To prevent both salinization and waterlogging, sufficient water along with adequate soil drainage must be available to ensure salts and excess water are drained from the soil.
Because more than 99% of world food supply comes from the land, an adequate world food supply depends on the continued availability of productive soils. Erosion adversely affects crop productivity by reducing the availability of water, diminishing soil nutrients, soil biota, and soil organic matter, and also decreasing soil depth. The reduction in the amount of water available to the growing plants is considered the most harmful effect of erosion, because eroded soil absorbs 87% less water by infiltration than uneroded soils. Soybean and oat plantings intercept approximately 10% of the rainfall, whereas tree canopies intercept 15% to 35%. Thus, deforestation increases water runoff and reduces water availability.
Water Runoff Rate Compared to Rainfall Rate
A water runoff rate of about 30% of total rainfall of 800 mm/yr causes significant water shortages for growing crops, like corn, and ultimately lowering crop yields. 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 U.S. Thus, soil erosion is a self-degrading cycle on agricultural land. As erosion removes topsoil and organic matter, water runoff is intensified and crop yields decrease. The cycle is repeated again with even greater intensity during subsequent rains.
Increasing soil organic matter by applying manure or similar materials can improve the water infiltration rate by as much as 150%. In addition, using vegetative cover, such as inter-cropping and grass strips, helps slow both water runoff and erosion. For example, when silage corn is inter-planted with red clover, water runoff can be reduced by as much as 87% and soil loss can be reduced by 78%. Reducing water runoff in these and other ways is an important step in increasing water availability to crops, conserving water resources, decreasing non-point source pollution, and ultimately decreasing water shortages.
Planting trees to serve as shelter belts between fields reduces evaporate transpiration from the crop ecosystem by up to 20% during the growing season, thereby reducing non-point source pollution, and increases some crop yields, such as potatoes and peanuts. If soil and water conservation measures are not implemented, the loss of water for crops via soil erosion can amount to as much as 5 million liters per hectare per year.
Water Use Livestock Production
The production of animal protein requires significantly more water than the production of plant protein. Although U.S. livestock directly use only 2% of the total water used in agriculture, the water inputs for livestock production are substantial because water is required for the forage and grain crops.
US livestock to Worldwide
Each year the total of 253 million tons of grain are fed to U.S. livestock requiring a total of about 250 x 1012 liters of water. Worldwide grain production specifically for livestock requires nearly 3 times the amount of grain that is fed U.S. livestock and 3 times the amount of water used in the U.S. to produce the grain feed.
Animal products vary in the amounts of water required for their production. For example, producing 1 kg of chicken requires 3,500 liters of water while producing 1 kg of sheep requires approximately 51,000 liters of water in order to produce the required 21 kg of grain and 30 kg of forage to feed these animals. For open rang-eland, from 120 kg to 200 kg of forage are required to produce 1 kg of beef. This amount of forage requires 120,000 liters to 200,000 liters of water per kilogram of beef. Beef cattle can be produced on rang-eland, but a minimum of 200 mm per year of rainfall are needed.
U.S. agricultural production is projected to expand in order to meet the increased food needs of a U.S. population that is projected to double in the next 70 years. The food situation is expected to be more serious in developing countries, such as Egypt and Kenya, because of rapidly growing populations. Increasing crop yields necessitates a parallel increase in freshwater utilization in agriculture. Therefore, increased crop and livestock production during the next 5 to 7 decades will significantly increase the demand on all water resources, especially in the western, southern, and central United States, as well as in many regions of the world with low rainfall.