Irrigation in the United States can be traced back over 4,000 years. Irrigation has evolved from farmers digging ditches by hand along field borders, to fully automated systems that operate on GPS and can be monitored wirelessly via cell phone or computer. In 2008, producers spent $2.1 billion on irrigation equipment, facilities, land improvements, and computer technology. Of those expenses, 50% was for replacement of existing equipment, 35% for new expansion, and 15% for water conservation.
Studies conducted by the USDA have shown irrigation can increase average yields for certain crops by more than 128%, compared to the average yield of the same nonirrigated crop. In research performed by The University of California, improper management of irrigation can lead to infertile soils, depleted water resources, and reduced water quality. Given rising global food demand, the U.S. can ill afford to have agricultural production decrease, while producers need to be aware of potential dangers caused by mismanagement and invest in new technology that allows them to become more efficient and able to conserve natural resources.
Irrigation Overview in the United States
As previously mentioned, irrigation in the U.S. can be traced back over 4,000 years to archaeological excavations near the Santa Cruz River in Tucson, Arizona. Archaeologists have concluded the floodplain of the Santa Cruz River was farmed during the Early Agricultural Period, circa 1200 BC to AD 150. These people constructed irrigation canals and grew corn, beans, and other crops. Irrigation remained simple prior to the middle of the 21st century, with the advent of diesel and electric motors. These inventions led to new and more efficient ways for producers to irrigate their crops. For the first time, irrigation systems could pump water out of aquifers faster than it could be replenished.
In 2008, the USDA’s Farm & Ranch Irrigation Survey (FRIS) reported total U.S. irrigated farmland acres of 54.9 million (excluding institutional, research, and experimental farms and farms with horticulture). This was an increase of 2.4 million acres, or 4.6%, from the last FRIS report in 2003. In 2008, irrigated acres represented 17.7% of the total harvested acres in the U.S.
The five states with the highest amount of irrigated farmland acres are; Nebraska, California, Texas, Arkansas, and Idaho. These states account for 53.4% of the total irrigated acres in the country. Conversely, the five states with the least amount of irrigated farmland only represent 8% of the total irrigated acres.
Types of Irrigation
The two most common methods of distribution are sprinkler systems and gravity flow systems; together they account for 96.3% of U.S. farmland irrigation. Between 2003 and 2008, sprinkler system usage by acre has increased 15% to 30.9 million acres, while gravity usage has declined 5% to 22 million acres. The other most commonly used forms of farmland irrigation in the U.S. include drip or trickle and subirrigation.
There are four different types of sprinkler irrigation, center pivot, mechanical-move, hand move, and solid set. Center pivot irrigation is a form of sprinkler irrigation consisting of several segments of pipe (usually galvanized steel or aluminum) joined together and supported by trusses, mounted on wheeled towers with sprinklers positioned along its length. The system moves in a circular pattern and is fed with water from the pivot point at the center of the arc. Center pivots have evolved to run on GPS, be activated remotely from wireless electronic equipment, and to utilize sprinkler heads that are positioned only a few feet above the crop, all to limit energy usage and evaporative losses.
Mechanical-move systems are classified as linear move, side roll, wheel move, or big gun where the sprinkler device is moved across the field either by self-propelled methods or by tractor. Hand move systems include distribution systems laid out in the spring, moved through the field as needed during the season by hand labor, and removed at the end of the season. Finally, solid set or permanent systems are sprinklers placed in the ground permanently; these are mostly for perennial crops such as alfalfa, mint, and different grasses.
Gravity fed irrigation is a very simple system, consisting of an elevated reservoir with a pipe coming out the bottom that feeds water to crops and is controlled either by hand or a battery powered timer that controls the rate at which the crop is watered. The limiting factor for gravity irrigation is the climate in which crops are being irrigated. Gravity irrigation requires enough precipitation to consistently keep reservoirs full.
Drip or trickle irrigation is a slow, precise application of water and nutrients directly to the plants' root zones in a predetermined pattern. A drip irrigation design can be customized to meet specific needs while maintaining an optimum moisture level within the root zones. This assists farmers in conserving water that might otherwise be lost to non-growth areas, runoff, sun or wind, and providing the proper balance of water and air needed for successful plant growth. Drip irrigation is most commonly used in arid and drought-plagued regions.
Subirrigation or seepage irrigation is mostly used in areas with high water tables, the process artificially raises the water table to allow the soil to be moistened below the plants' root zone. Often these systems are located on permanent grasslands in lowlands or river valleys and combined with drainage infrastructure. A system of pumping stations, canals, weirs and gates allows it to increase or decrease the water level in a network of ditches and thereby control the water table. Advantages are water and nutrient conservation, and labor-saving through lowered system maintenance and automation.
In 2008, the FRIS estimated, 91.2 million acre-feet of water was applied via farmland irrigation compared to only 109 thousand acre-feet of water to horticulture operations. This is an increase of 4.4 million acre-feet of water, or 5.1%, from 2003. In 2008, each irrigated farmland acre received an average of 1.7 acre-feet of water, compared to 2.5 acre-feet for every horticulture acre. An acre-foot of water is the quantity of water required to cover one acre to a depth of one foot. This is equivalent to 43,560 cubic feet or 325,851 gallons.
The FRIS also reported during this same time period, subirrigation systems were the most efficient form of irrigation. The average amount of water per acre applied through subirrigation decreased from 1.2 to 0.7 acre-feet. Water usage for sprinkler systems also decreased from 1.3 to 1.2 acre-feet per acre. Compared to gravity fed and drip systems, which increased their average water usage by 0.2 and 0.4 acre-feet respectively to 2.2 acre feet per acre.
Irrigation provides producers with the availability to apply water to their crops when they need it most. In 2008, (Exhibit 1) average yields for all agricultural crops increased dramatically with the availability of irrigation. Compared to vegetables and other nontraditional agriculture crops, corn, soybeans, and wheat require less acre-feet of water on average to produce maximum yields (Exhibit 2).
Irrigation Uses in the United States
If not mandated by the government, deciding when to irrigate can be a very difficult for producers. According to the FRIS (respondents could select more than one answer), 78% of respondents irrigate based on crop condition, 53% irrigate based on soil characteristics, 25% use their personal calendar, and 20% have their scheduling controlled by the government.
Irrigation is an efficient way to apply specific forms of fertilizer in a safe and controlled manner. Producers use this application method because timely nutrient uptake by the plant is crucial when striving for maximum yields. Furthermore, this allows producers to avoid the risk of nutrients moving through the soil profile faster than the plant can absorb it, also called leaching. Soil pH is also extremely important when producing high yielding crops. Producers will irrigate soils with elevated levels of salt in an attempt to push salt through the soil profile and away from plant roots.
Citrus and other fruit producers will irrigate their crops prior and during prolonged periods of cold weather to form an ice barrier on the exterior of their product, which acts as insulation for the crop during cold temperatures. Conversely, during extended periods of heat, producers will irrigate their crops to reduce heat stress and attempt to delay early budding or blooming.
Food processors and livestock producers also use irrigation as a means to dispose of waste producers. This not only provides them with a source of disposal, but provides valuable nutrients to growing crops. Though not as common in areas with excess water, producers will use irrigation to pump water away from fields to create wildlife habitats.
As in all of agriculture, there is clearly no perfect strategy for crop irrigation. Producers have to make decisions based on not wanting to waste money by over irrigating crops; however they also remain conscience about losing yield if they do not provide their crops with timely applications of water and nutrients.
States such as Nebraska, which has more ground water than any other state in the country, have setup regulatory bodies to govern the states’ natural resources. In 1972, Nebraska Legislature enacted laws to consolidate 154 special purpose entities into 23 Natural Resource District’s (NRD’s). NRD’s are local government entities and are unique to Nebraska; they are charged under state law with 12 areas of oversight and responsibility:
• Erosion prevention and control
• Prevention of damages from flood water and sediment
• Flood prevention and control
• Soil conservation
• Water supply for any beneficial uses
• Development, management, utilization, and conservation of groundwater and surface water
• Pollution control
• Solid waste disposal and drainage
• Drainage improvement and channel rectification
• Development and management of fish and wildlife habitat
• Development and management of recreational and park facilities
• Forestry and range management
Many NRD projects have permanent results such as dams, terraces, drainage ditches, windbreaks, reservoirs, and recreational trails. However, NRD’s also have local responsibility to actively protect ground water from overuse and pollution. NRD’s can mandate that producers not irrigate crops during prolonged dry periods, which can dramatically reduce yields. However, most districts try to be proactive and encourage stewardship by providing financial assistance to landowners for irrigation water management and best-management practices to apply water more efficiently.
For example, in 2002, the Lower Loup Natural Resources District's Ground Water Management Area (GWMA) became effective. Early phases required permitting for new construction of wells that pump more than 50 gallons of water per minute and required expanded educational programs for producers. Under the most recent phase, producers are required to pass a nitrogen certification class and monitor water applications to better manage fertilizer applications and control leaching of nitrates. This district also mandates the timeframe, the type and quantity of fertilizer that can be applied via irrigation, and requires producers to submit annual water and deep soils analysis for nitrogen content.
Spending on Irrigation Infrastructure
According to FRIS, between 2003 and 2008 producers improved over 30.8 million of the 54.9 million irrigated farmland acres. The number one reason producers listed for improving their existing systems was to increase yield potential, while the largest issue for doing nothing was simply investing in improvements was not a priority. However, there are many other reasons why producers chose to or not to upgrade their existing irrigation infrastructure, (Exhibits 3 & 4).
Discontinued Irrigation Acres
The USDA estimates each year approximately 0.5% of all irrigated acres cease production. The two largest known reasons producers remove irrigation from their farmland are, soils develop sufficient moisture capacity 23% and irrigation becomes uneconomical 7%. Discontinuances caused by irrigation account for only 4.6% of the total acres taken out of irrigation, (Exhibit 5).
Public Concerns over Irrigation
There is no federal governing body that oversees farmland irrigation. The Clean Water Act of 1977 explicitly preserves the authority of each state to allocate water among users; water use and water quality issues remain inextricably linked. Farmland irrigation is a substantial user of surface and ground water resources in the United States. During periods of limited precipitation, agriculture producers rely on irrigation to sustain crop yields thereby intensifying the struggle for the remaining water resources among competing users. Because of this, crop producers continue to be challenged to improve irrigation efficiency and to conserve as much water as possible.
The main environmental concerns related to farmland irrigation include:
• Build-up of soil salinity
• Depletion of water sources
• Soil erosion caused by over-application
• Runoff and leaching of chemicals and fertilizer
Build-up of salinity refers to the excess amount of salt in the soil or water. Under-irrigation or irrigation that provides just enough water to the plant (i.e. drip irrigation) offers poor soil salinity control. This can lead to increased soil salinity with consequent build up of toxic salts on the soils surface in areas with high evaporation. Excessive soil salinity reduces the crops ability to absorb water, which has a negative effect on yield. The inability to absorb water can also lead to wilting, which makes the crop more vulnerable to pests.
Most water used for irrigation is of good to excellent quality, but does contain a mixture of naturally occurring salts. Soils irrigated with this water will contain a similar mix but usually at higher concentrations. Salt accumulation is dependent on the irrigation water quality, irrigation management, evaporation, and the adequacy of drainage. To prevent yield loss, salt levels in the soil must be controlled to concentrations below tolerance levels of growing crops. One management technique to help combat soil salinity is to irrigate crops at night when evaporation risk is at its minimum.
Salinity control becomes more difficult as water quality becomes poorer. As water salinity increases, greater care must be taken to leach salts out of the root zone before their accumulation reaches a concentration that affects yields. Alternatively, steps can be taken to manage around high levels of salinity. Producers can plant crops that have tolerances to high levels of root zone salinity or slightly over-irrigate crops to leach salts below a plants root zone.
Depletion of water sources occurs when more water is removed from the source (i.e. aquifers, the water table, or other bodies of water) than replaced. Depletion is critical in parts of the world such as Africa. In the United States, new methods of groundwater management such as artificial recharge and injection of surface waters during seasonal wet periods have extended the lives of many freshwater sources.
Two examples of aquifer depletion in the United States are the Ogallala and Edwards Aquifers. The Ogallala Aquifer underlies portions of eight states and contains primarily fossil water from the last glaciers. Water levels in the Ogallala continue to be depleted because of growing municipal use and continuing agricultural use. It is estimated that annual recharge in arid regions of the aquifer are only 10% of annual withdrawals. On the other hand, the Edwards Aquifer in central Texas is labeled as a significant and sustainable aquifer. Even though the aquifer provides water to nearly 2 million people, it remains completely full because of tremendous recharge from a number of area streams, rivers and lakes.
The majority of irrigated farmland in the U.S. occurs in arid climates where rainfall is limited, on ground that is generally flat, or on soils with poor water holding characteristics, leaving soil erosion a non issue. However, producers who irrigate farmland that is more prone to soil erosion need to recognize the dangers to soil erosion and take preventative measures to protect their soil.
Over-irrigation on soils that cannot fully absorb the amount of water being applied to it can experience pooling or run-off problems. Producers can easily combat this problem by studying their soil types to better understand the amount of water it can handle. With a general knowledge of their soil structure, producers can invest in precision irrigation systems that will preserve their time, money, and soil fertility. In fields with undulating terrain, producers can utilize lower pressure sprinkler heads to provide soils an opportunity to fully absorb water before it can erode soil.
Runoff and leaching of fertilizer and chemicals is a serious issue not only on irrigated land, but in all of agriculture. There is no standardized plan to deal with fertilizer runoff. Compared to most forms of nitrogen, fertilizers such as potassium and phosphorous move slow through the soil structure and remain available for plant uptake for many years, making them less prone to runoff and leaching.
Producers are aware of nitrogen’s tendencies to move through the soil with moisture and will apply the majority of their nitrogen via irrigation. Applying fertilizer through irrigation provides efficient and timely application of valuable nutrients, and limits the amount of nitrogen lost through runoff or leaching. Timely application of nutrients not only saves producers money, but helps sustain the environment by limiting contamination of underground resources.
Irrigation has been around of thousands for years and can be directly attributed to helping produce the quantities of food necessary to feed the worlds growing population. Irrigated agriculture (farmland & horticulture) is responsible for approximately 70% of all the freshwater withdrawn in the world. It is estimated this number will continue to grow while tillable acres will continue to decline, as food demand continues to expand. The challenge presenting itself to the agricultural industry is to produce more food with fewer resources.
There are environmental and sustainability concerns tied to farmland irrigation. However, farmland irrigation is not the only reason for these concerns; municipalities, businesses, and individuals are also responsible for these problems. Producers still need to act responsible and adhere to proper management techniques to help reduce and prevent these problems from occurring.
The following are management practices to assist producers in properly irrigating their farmland:
• 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
• Irrigation Equipment Efficiency – Use efficient irrigation systems such as sprinkler style irrigation, GPS,
and automatic timers to minimize runoff, leaching, and evaporation.
• Apply Water at Rates the Soil can Absorb – Runoff due to excess irrigation can cause soil erosion.
• Uniform Irrigation – Make sure water is applied uniformly. This makes water availability more efficient,
reduces operating costs, and reduces the chance of runoff and leaching in certain areas where water
may be over-applied.
• Provide Adequate Drainage – High levels of salinity in areas of low rainfall can be minimized by
providing good drainage along with the irrigation, to leach salts down through the soil profile.
We expect irrigation to continue to be a driving force in the world's quest to produce more food and improve food security; however, producers will have to evolve their operations to incorporate more efficient, cleaner, and integrated uses of irrigated water.
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