Irrigation makes agriculture possible in areas previously unsuitable for intensive crop production. Irrigation transports water to crops to increase yield, keep crops cool under excessive heat conditions and prevent freezing.
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:
- Improving crop quality (most noticeable for vegetable crops)
- Significantly increasing crop yields, particularly on sandy soils which have low moisture-holding capacities
- Increasing opportunities for double cropping (planting soybeans after wheat in the same year)
- Providing a means of liquid fertilizer application
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 Timing
Irrigation 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.
There are four primary types of irrigation:
- Surface irrigation,
- Sprinkler irrigation,
- Drip or trickle irrigation, and
- Subsurface irrigation (or "subirrigation").
Surface Irrigation - With surface irrigation, water flows directly over the surface of the soil. The entire surface can be flooded (most often used for crops that are sown, drilled, or seeded) or the water can be applied through furrows between the rows (for row crops).
Variable-Flow Irrigation - Variable-flow irrigation sprinkler head improves the precision of water and farm chemical applications.
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 Concerns
Environmental 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.
Best Management Practices
- Minimize water use. Apply only enough water to meet crop needs. This can be determined through regular soil moisture monitoring or through a "checkbook" system to monitor water applied and crop needs.
- Irrigation efficiency. Use efficient irrigation systems such as drip irrigation to minimize evaporation.
- Apply at rate the soil can absorb. Runoff due to excess irrigation can cause soil erosion.
- Uniform Irrigation. Make sure water is applied uniformly. This makes the water more efficient, and reduces the chance of runoff and leaching in certain areas where water may be overapplied.
- Provide good drainage. Salinization in areas of low rainfall can be minimized by providing good drainage along with the irrigation, to leach salts down through the soil profile.