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Irrigation
Less than 15% of U.S. cropland is irrigated, although irrigation is essential for crop production in some of the most productive areas of the country. For instance, in Arizona, home to some the highest corn yields in the country (208 bushel per acre state average in 2001 compared to 152 for Illinois), much of the crop is under continuous irrigation from planting until harvest. The need to irrigate is usually driven by the necessity to meet the water needs of the crop from year to year (some areas of the country simply receive too little rainfall during the growing season to support economical crop growth). In other situations, irrigation is viewed as insurance against occasional drought. In areas where rainfall is plentiful in most years, irrigation can bring benefits by reducing risk, meaning that a farmer is better able to control income fluctuation. Other benefits include:
In 1997 there were about 55 million irrigated crop acres in the U.S. Irrigation is concentrated in certain areas like central California, Nebraska and the Great Plains, and the lower Mississippi valley. Although irrigation has always been most common in the West, U.S. irrigated
acreage in the East has also grown from 11 percent of acres in 1969 to
22 percent of acres in 1997. Irrigation water is obtained from either ground water or surface water. Wells drilled on the farm are a common source of water in many areas, and are usually the only source used in the Great Plains. Offsite sources such as rivers, pipelines, canals operated by irrigation districts and private water companies, are also used, mainly in western states. The percentage of water source used for irrigation varies across the U.S. Operations and TimingIrrigation water is applied throughout the growing season to meet crop needs. Moisture needs depend on the type of crop and its stage of development. In the Eastern Corn Belt, for example, it takes 20-22 inches to produce an optimal corn crop, 18-20 inches for a soybean crop, 12-13 inches for small grain, and 24-26 inches for alfalfa. Irrigation can reduce crop stress if rainfall does not provide this amount of moisture during the growing season. It is not only total moisture, but also the timing of moisture application (or rain) that is necessary for optimum crop yields. Crops have critical periods during the growing season when soil moisture must be maintained to ensure optimal yields. For corn, the most critical period is from just before tasseling through silking. For small grain, it is from boot to heading stage, for alfalfa, the start of flowering and after cutting, and for pasture, after grazing. Equipment UsedThere are four primary types of irrigation:
With sprinkler irrigation, water is sprayed through the air from pressurized nozzles, and falls like rain on the crop.
Irrigation types can be further distinguished by whether the equipment is permanently installed in one place (stationary system) or whether it is used until the necessary amount of water is applied, then moved to a different area (traveling system). Stationary systems such as permanent spray installations or trickle systems require less labor, but usually cost much more to install. Traveling systems such as center pivot sprinkler irrigation, linear-move, or cable-tow require more labor but less capital expense. Potential Environmental ConcernsEnvironmental concerns related to irrigation include depletion of the water source (falling water tables or reduced water levels in streams or reservoirs), soil erosion due to over-application, runoff and leaching of chemicals, and salinization of the soil (salt-buildup) and minerals and nutrients in the irrigation return flow that drains from the irrigated area. Back to TopBest Management Practices
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